CHAPTER 1

INTRODUCTION

11 Brief Description of the Excitation System

The static excitation equipment regulates the voltage (andor the flow of reactive

power during parallel operation) from the synchronous machine (generator) by direct

control of the rotor current (field current) using (static) Thyristor converters

The entire unit can be broken down into four major groups

bull The excitation transformer T01

bull The control unit REG

bull Thyristor converter TY

bull The field breaker field flashing and de-excitation equipment FF amp F

In excitations with shunt-connected supply there is no enough remnant voltage in

the rotating generator to build up the generator voltage autonomously via the converter

To accomplish this special field flashing equipment is needed When the field flashing

equipment is being supplied with power from a DC power source (power station battery)

resistor is used to limit the field flashing current When it is being supplied from an AC

power grid a transformer serves as the adapter needed

Excitation of the generator is started by closing the field circuit-breaker and the field

flashing breaker This supplies current to the field which excites the generator up to 15

30 of Generator voltage The generator then supplies voltage to the converter voltage

the firing electronics and the converters are able to continue the voltage build-up so that

the field flashing circuit is relieved of current Once the voltage exceeds approx 70 of

Generator voltage the field flashing breaker is opened having no current The diode

bridge at the input to the field flashing breaker prevents a backflow of current to the field

flashing source

1

To accomplish this special field flashing equipment is needed When the field

flashing equipment is being supplied with power from a DC power source (power station

battery) a resistor is used to limit the field flashing current When it is being supplied

from an AC power grid a transformer serves as the adapter needed Excitation of the

generator is started by closing the field circuit-breaker Q1 and the field flashing breaker

Q2 This supplies current to the field which excites the generator up to 15 30 U The

generator then supplies voltage to the converter via the excitation transformer Starting

from approx10 of the generator voltage the firing electronics and the converter are

able to continue the voltage build-up so that the field flashing circuit is relieved of

current Once the voltage exceeds approx 25 of U the field flashing breaker is finally

opened having no current The diode bridge at the input to the field flashing breaker

prevents a back-flow of current to the field flashing source The converter TY has Final

Pulse Stage cooling and monitoring of the elements Redundancy in the regulator section

is ensured by means of two fully separate channels with independent measuring inputs

and extensive monitoring (ldquoSUPERVISIONrdquo)

Channel 1 (AUTOMATIC channel) is built as voltage regulators and is ON during

normal operation In addition to the voltage regulator which has a PID control algorithm

2

the AUTOMATIC channel also contains various limiters and corrective control circuits

to ensure the use and stable operation of the synchronous machine up to its operating

limits This channel possesses a Gate Control Unit with a subsequent Intermediate Pulse

Stage to generate the firing pulses for the Thyristor converter During normal operation

the Intermediate Pulse Stage of AUTOMATIC Channel is active and transmits the firing

pulses galvanically separated to the common pulse bus at the input to the Final Pulse

Stage Various monitoring functions of the AUTOMATIC channel and pulse monitoring

on the common pulse bus initiate an automatic switch-over to stand by Channel

(MANUAL)in case of a malfunction

Channel 2 (the MANUAL channel) is built as a simple field-current regulator with a PI

control algorithm It serves as a back-up channel in case of a malfunction on the

AUTOMATIC channel Manual channel performs valuable service for testing

commissioning and preventive maintenance The MANUAL channel has its own Gate

Control Unit (the software for the If regulator is also implemented therein) and its own

intermediate pulse Stage During normal operation (AUTOMATIC) the output pulses

from Intermediate Pulse Stage are blocked from reaching the pulse bus Various

monitoring on the MANUAL channel initiate an alarm in case of a malfunction while the

MANUAL channel is on stand-by If the MANUAL channel suffers a malfunction while

it is in operation the excitation is switched off (TRIP) Both channels are equipped with

tracking equipment so that the inactive channel always generates the same control

variable as the active channel during steady-state operation

This ensures smooth switch-over from Automatic to Manual channel and vice

versa To ensure that the MANUAL channel will in a switch-over initiated by a

malfunction take over the operating point of the machine as it was prior to the problem

the response of the tracking for the MANUAL channel is set relatively slow In addition

to the pulse monitors (ldquoSUPERVISIONrdquo) shown in the basic circuit diagram the

excitation system has an autonomous Excitation Monitoring As one of its functions this

equipment monitors for field currents that exceed acceptable maximum limits It initiates

an emergency switch-over to the MANUAL channel whenever the field current exceeds

the preset limit If even after such a switch-over the field current does not drop back to

3

the permissible level the excitation is switched off by Excitation Protection The most

important measuring inputs for the excitation system (If Ug Usyn) are redundant (2-

fold) The Excitation Monitoring checks these measuring inputs for discrepancy and

plausibility An alarm is always initiated in case of malfunction In certain cases a

switch-over to MANUAL channel is also initiated The excitation system contains an

Excitation Protection to protect the excitation transformer the converters and the

synchronous machine The protection system can detect short-circuits in the excitation

circuit and keep secondary damage within acceptable limits by a quick tripping of the

excitation and an opening of the generator breaker An overheating of the excitation

transformer first sets off an alarm (at a given preset limit) and then likewise initiates a

protective shut-down at an even higher limit The over voltage protection in the de-

excitation equipment provides an autonomous protective function for the rotor and the

rectifier This protection system monitors the field voltage in both polarities for over

voltage and if necessary de-energizes the field via the de-excitation resistor

12 Principle of primary power supply

In the shunt excitation system the excitation transformer also provides

the power supply for the electronic equipment and the converter fans So a failure of the

auxiliary supply to the converter fans does not cause a shutdown of the excitation When

the auxiliary supply fails the supply to converter fans is switched over to Excitation

transformer OP with a contactor A station battery supply is absolutely necessary for the

control of the field circuit breaker It is the power source for the electronic devices till the

generator is able to supply voltage Auxiliary power to the field flashing equipment must

be present in order to build up the generator excitation The power supply for standstill

heating and the cubicle lighting is also from Station Auxiliary Power Supply and is of

secondary importance for operation of the plant Power supply to rotor earth fault

detection circuit too is from Station Auxiliary Supply The two synchronous voltages

Usyn are each supplied to the AUTOMATIC channel and the MANUAL channel

separately across transformers The Gate Control Units need these voltages to enable

4

them to issue the pulses at a given firing angle relative to the input voltage of the

converter

5

CHAPTER 2

Digital Automatic Voltage Regulator (DAVR)

21 Principle of Operation of the Regulator (DAVR)

To regulate the voltage and the reactive power of a synchronous machine the

field voltage must be adjusted quickly to the changes in the operating conditions (with a

response time that does not exceed a few ms) To accomplish this analog control systems

include amplifiers which make continuous comparison of the actual values against the

reference values and vary the control variable to the converter with almost no delay Most

of the delay that occurs originates in the converter since the firing pulses for changing

the rectifier phase angle are only issued periodically (every 33 ms)

The DVR digital voltage regulator calculates the control variable from the

measured and reference data in very short time intervals This results outwardly in a

quasi-continuous behavior with a negligible delay time (as in an analog regulator) The

calculations are made in the binary number system Analog measurement signals such as

those for generator voltage and generator current are converted into binary signals in

analogdigital converters The set-points and limit values have already been defined in

digital (binary) form An understanding of the actual computation processes in the digital

voltage regulator is not necessary for operation preventive maintenance or

troubleshooting Like the operator of a pocket calculator or a personal computer all the

operator needs is to know how to operate the instrument and the programming for this

working tool For that reason we will explain below only the principle division of work

among the various modules and the flow of data processing The purpose is above all to

make clear how the processor system has been integrated into the rest of the power

electronics system

6

22 Basic Structure of the Processor Systems

7

The signal processors 25 analog inputoutput modules Each of these processor

systems has a common bus circuit and output and the control lines There is a specific

range of addresses assigned to each assignment Board including the power supply bus

the address lines the two data lines to the input calculates the reactive current (I sin φ)

and the active current ( Icos φ) With these two channel processor Synchronized with

these interrupts (ie with the phase positions of current Ig the field current If and the

synchronous voltage Usyn From the exchange data with the microprocessor card across

the two data lines generator voltage Ug) this processor measures the generator current

Ig and then hardwired connections or multi-conductor cables Binary and analog

inputoutput modules ie for galvanic isolation and adaptation to the electronics level

The most important input interrupts per period to trigger the cycles for processing actual

values in the AUTOMATIC module on the processor bus) for filtering and further

processing

Monitoring each consist of the central microprocessor module and binary and

parameters to the AUTOMATIC channel are the generator voltage Ug the generator

peripheral unit Ug Ig and Usyn are sent to the Interrupt Generator (plug-in peripheral

units (wall-mounted units) peripheral units are used for preprocessing signals from

external measurement circuits power supply units Signals are exchanged among these

processor systems via processed across separate peripheral units for each channel These

processor working on the bus (a house address that can be adjusted using a switch)

systems The AUTOMATIC channel the programmable controls and the Excitation The

actual values measured from AUTOMATIC channel and MANUAL channel are The

AUTOMATIC channel and the MANUAL channel each have their own The digital

voltage regulator is broken down into several autonomous microprocessor The inputs and

outputs of the processor systems are directed across voltage-isolating The Interrupt

Generator also uses the 3-phase Ug signal to generate the 12 themselves contain a limited

number of hardware inputs and outputs with fixed equipment Whenever addresses from

this range are called up the signal processing module can results the processor is then

able to derive further operating parameters such as the load angle the active power etc

The functions of all microprocessor systems other than the programmable controls

have been accomplished in firmware The non-varying standard function modules can be

configured to the design desired for plant-specific purposes using software switches

(KFlags) Thus for example the stored status of a K-Flag determines whether or not a

Limiter is active and whether the de-excitation or the excitation limiters take precedence

Because these K-flags determine the software Scope of Supply for the installation they

cannot be changed permanently via the Micro-Terminal In this way they differ from

such setting data as the values of the parameters for the PID filter of the voltage regulator

or the set-points for the limiters These values can be permanently changed using the

Micro-Terminal Communication is possible with each of the processor systems via the

Micro-Terminal by plugging on the connecting cable In this way signals within the

processor and setting parameters can be viewed analog signals can be issued and the set

parameters can be altered temporarily (F range) or permanently (C range) Unlike the

other processor systems the programmable controls do not include any firmware for

realization of the functions They have been designed so that the designer can adapt and

change their functions easily using the ldquoFunctional Block Programming Language P10

Digital and analog functions can be implemented in practically any degree of complexity

desired using the P10 functional blocks The control variable of the voltage regulator

(AUTOMATIC channel) and the control variable of the field current regulator

(MANUAL channel) are each processed in separate Gate Control Unit and formed into a

chain of pulses at the appropriate firing angle The pulses of the active channel are

directed to the pulse bus via the associated Intermediate Pulse Stage The pulses for each

converter block are amplified sufficiently in Final Pulse Stage to fire the Thyristor

231 General Information

The functions of the automatic voltage regulator AVR are

1 to regulate the generator voltage

2 to regulate the effect of the reactive andor active current on the voltage

3 to limit VoltHz

4 to limit max and min field current

5 to limit inductive stator current

6 to limit capacitive stator current

7 to limit the load angle

8 to stabilize the power system

Block Diagram shows the software structure of AUTOMATIC channel The

generator limiters not provided for the installation in question (optional equipment) are

identified in this overview as ldquoNot Suppliedrdquo The parameter values signal values and

software switches (flags) marked with addresses (hexadecimal numbers) can be viewed

and altered via the Micro-Terminal The values selected are displayed in sec pu Hz

etc and can where necessary be changed directly in these formats The plant-specific

settings of the variables and the flags can be obtained from the Test and Commissioning

Report This block diagram provides information about the important functions and

possible settings of the AUTOMATIC channel For the sake of clarity no detailed

presentation has been given of special functions such as tracking circuits initializations

etc The page heading cross-refers this overview to the various sheets of the schematic

diagram Binary signals are shown in broken lines analog signals in solid lines The

corresponding text designations in the schematic diagram can be used for identification of

the input signals (hardware inputs) The only analog output signal from the automatic

voltage regulator control variable Ucontr is sent via the data bus (CRU bus) to the Gate

Control Unit Most of the binary messages (outputs) from the AVR are of no interest

functionally and they have been omitted for the sake of clarity The basic structure of the

digital voltage regulator and the limiters is simple This is necessary in order that the

behavior of the regulatorslimiters will remain calculable and understandable in all

operating situations and that there will be no problem in adjusting and optimizing them

The central PID filter in the digital voltage regulator defines the dynamic response of the

closed-loop controls both in the voltage regulator mode and after limiters have

intervened The ldquocontrol deviationrdquo at the input to the PID filter is either the control

deviation for voltage the control deviation of a de-excitation limiter (the value

determined by minimum value selection) or the control deviation of an excitation limiter

(the value determined by maximum value selection) Flag F730 (ldquoPRIORrdquo) is used to

determine whether the exciting (Min value) or the de-exciting signal takes precedence on

the minmax value limiter (normally F730 = 1111 ie the de-exciting signal takes

precedence) With the exception of the Minimum Field Current Limiter all other limiters

have variable factoring multipliers of the signal outputs so that they can be adjusted

individually together with the common PID filter which has been optimized for voltage

regulation The setting parameters for this PID filter are as follows

Vo = KR Static amplification

1

Ta = ---- Integration time constant

Tc1

Vp Proportional amplification

1

Tb = ---- Differential time constant

Tc2

Vinfin Amplification of high frequencies

The BODE diagram below shows the assignment of settings in accordance with

DINIEC standards based on a typical example

The PID filter amplifications Vo Vp and Vinfin can be adjusted in pu values But

the ldquoceiling factorrdquo pl+ must be adjusted correctly with parameter F310 if the total

amplification (circuit amplification) of the control circuit is actually to conform to the

pu settings This factor must agree with the ldquoexternalrdquo amplification ie with the

ceiling value of the transformer- converter circuit

Ceiling factor(pl+) = Ufmax Ufo

in which Ufmax = ceiling field voltage

Ufo = no-load field voltage

To attain a suitable response of the AVR when starting excitation

(ldquoEXCITATION ONrdquo) it may be necessary to change the proportional amplification of

the regulator during this phase Vp2 (transiently activated) and Vp1 (permanently

activated) can be adjusted for this purpose For example the value of Vp2 takes effect

immediately once the excitation is switched on and remains effective for a period as set at

F30C Once the period F30C (eg 5 sec) has expired Vp shifts over to Vp1 (becomes

the steady-state Vp) at the rate of change set The standard operating mode for the PID

filter is voltage regulation for which the discrepancy between the voltage set-point and

the current value for generator voltage Ug (the control deviation) is supplied at the input

To compensate for the voltage drop in the block transformer or whenever several

generators are operating to the same distributing bus the generator voltage must be

varied in proportion to the measured generator current (droop influence) To accomplish

this the voltage set-point is varied as a function of the measured reactive current IX

andor active current IR Flag F712 enables the IX droop Flag F710 the IR droop The

desired compensation is set in F282 and F286 respectively Flags F284 and F288 are used

to select whether this droop influence is to increase the voltage or to reduce it

(compensation) Combined influence of the active and reactive currents is attained by

enabling both droops IX and IR Flag F716 activates a so-called ldquoSoft-Startrdquo at the

starting of excitation This ldquoSoft-Startrdquo ensures that the voltage set-point integrates from

0 to 100 within the time set on F290 when the excitation is switched on

(ldquoEXCITATION ONrdquo) A ldquosmoothrdquo excitation of the generator can be achieved in this

way whenever there is no demand for a quick excitation

232 Voltage Set-Point

Various signals and settings control and limit the voltage set-point F270 For

example the values of F254 and F252 define the normal operating range possible for set-

point adjustment (eg 90 110) using external control commands (control room local

operatorrsquos panel superposed control system) The effective set-point adjustment rate is

governed The set-point can be set at the values of F250 and F256 by activating

appropriate control commands for ldquoSETrdquo input Enabling Flag F71A and activating a

binary input prior to switching on the excitation (ldquoEXCITATION OFFrdquo) sets the Ug set-

point at the value of UAUX This makes it possible for example to ensure that the

generator voltage will agree exactly with the network voltage after the voltage build-up

An external value with variable amplification can be added to the Ug set point by

enabling F724 (for example for stability tests)

233 Regulator Tracking in MANUAL Operation

Whenever the AUTOMATIC channel is not in operation (the MANUAL channel

is ON) a follow-up equipment ensures a smooth switch-back to the AUTOMATIC

mode will always be possible To track the voltage set-point is shifted by means of

RAISELOWER pulses from the Gate Control Unit so that control variable Ucontr at the

output from the PID filter is held steady and identical to the control variable Ucontr from

the MANUAL channel Because this tracking must react slowly resultant transient

control deviations resulting from the amplification in the PID filter might cause severe

interference with control variable Ucontr

To prevent this the follow-up equipment intervenes on the regulators mixing

point with a corresponding compensation signal

234 Ugf Limiter

At under frequency the Ugf Limiter reduces the generator voltage so as to

prevent saturation effects in the supply and measuring transformers To adjust this

limiter the max permissible generator voltage at rated frequency is defined and set

When any under-frequency occurs the generator voltage is thus reduced in proportion to

that setting

235 Field Current Maximum Limiter

The Field Current Maximum Limiter is provided to protect the generator rotor

from s occurring in steady-state and transient operation High field currents are normally

the result of a sharp drop in network voltage or of an improper raising of the voltage set-

point by the operating staff The field current is held steady at the value TH1 ie at the

maximum thermal value permissible for the excitation circuit and the rotor In order that

the generator can support the power network with its transient overload capacity during

brief collapses in voltage a temporary switch-over is made to the transient limit MAX1

(a higher setting) When the generator or the converter is operating at a reduced capacity

These limits TH1MAX1 can be switched over to the lower settings TH2MAX2 by

activating the corresponding binary signals The switch-over from the thermal limit

TH12 to the transient limit MAX12 can be configured in one of three ways

a) Depending on the over current with -dUdt ENABLE

Flag programming F418 = any setting desired F41A = 0000

This variant enables the transient value MAX12 whenever a collapse of voltage

in the network is detected The ENABLE time is fixed and can be set The example

below shows the typical behavior of the limiter configured in this way

b) Dependent on the time integral with -dUdt ENABLE

Flag programming F418 = inactive F41A = 1111

This variant likewise enables the transient value only when a collapse of network

voltage has been detected However the switch-back to the thermal limit is not made

dependent upon the time itself but on the calculated time integral intisup2dt of the The setting

on Parameter F414 in spu takes into account the time the rotor needs to cool down ie

the rate of temperature change in the case of intermittent operation The example below

shows how the timing of the switch-back to the thermal limit depends on the present

value for intisup2dt

The time integral is based on the formula

Example The setting of =isup2dt equivalent to Version a (F416) at a constant 16 times the

nominal field current for 10 seconds (with TH12 = 105) is

c) Dependent on the time integral without any preconditions

Flag programming F418 = 1111 F41A = 1111

In this variant the transient becomes available without any prior conditions

(without a -dUdt ENABLE) with the time integral intisup2dt

237 Inductive Stator Current Limiter

The Inductive Stator Current Limiter holds the stator current Ig within permissible

limits while the generator is in the ldquoover-excitedrdquo operating range by reducing the field

current accordingly The setting TH (thermal limit) provides the limit against stationary s

that might occur To take advantage of the generatorrsquos transient overload capacity a

switch-over is made to the higher setting MAX The principle of operation of this switch-

over to the value MAX permissible only transiently is identical to that employed for the

field current limiter (refer to the description above) When the drive output from the

turbine is very high stator current may exceed permissible limits even while inductive

loading of the generator is low In this case if the stator current limiter is not kept from

influencing the field current the control circuit will oscillate back and forth between the

Inductive Stator Current Limiter (de-

exciting) and the Capacitive Stator Current Limiter (exciting)The output signal of that

function then dominates the control variable of the Ig-dependent limiter via a maximum

value selection

238 Capacitive Stator Current Limiter

239 Load Angle Limiter

The Load Angle Limiter prevents the synchronous machine from slipping out of

phase due to slippage of the rotor The load angle δ the difference in phase between the

rotor and the stator rotating field results mainly from the driving torque (active power P)

acting on the generator and the level of rotor current (field current) If the driving torque

remains constant a increase in the field current reduces the load angle δ The current load

angle δ at any moment is obtained from the generator current and generator voltage based

on a simplified model of the generator Whenever this calculated load angle δ exceeds the

preset limit angle the limiter increases the field current until the load angle has dropped

back to its permissible value The quadrature reactance Xq of the generator and the

network reactance Xe during normal operation must be adjusted on the regulator in order

to obtain the load angle δ The graph below shows the Power Chart for a salient-pole

machine with typical limiter characteristics

The purpose of a Power System Stabilizer is to use the generator excitation to

damp electromechanical oscillations between the network and the generator Depending

on the design of the generator and the requirements imposed for network stability its

main function will be either to damp the oscillations originating in the machine or those

from the network A synchronous generator working in a combined power network is in

principle an oscillating structure In order to produce a torque the magnetic field of the

rotor and the stator must form a given angle (referred to as the rotor displacement or load

angle δ) The electrical torque ME increases as the angle δ increases just as with a

torsion spring Because the ME of the generator and the mechanical driving torque MA

from the turbine are in equilibrium during steady-state operation the angle δ remains in a

given position Whenever this state of equilibrium between MA and ME is disturbed the

load angle slips of this rest position and change thereby the electrical torque ME The

torque attempts to restore the load angle to a stationary position Due to the mass inertia

of the turbinegenerator rotor however this can only take place aperiodically It does so

in the form of more or less effectively damped oscillations (again similar to the effect of

mass inertia on a torsion spring) In order to damp the oscillations there must be a

damping torque produced depending not on the electrical torque ME associated with the

angle but on the difference in frequency (Df) between the rotor and the stator rotating

field ie on the slippage This torque is produced mainly by the so-called damper

winding in the rotor but the dimensioning of this is subject to limits imposed by

considerations of design and economy Some further action is therefore needed to

increase the damping effect The following drastically simplified formula shows the

parameters upon which the amount of active power PE supplied by the generator

depends

PE = active power

It can be seen from the above relationship that the active power that the generator

transfers depends not only on the load angle δ but also on the field current If That means

that a transient change can be made in the active power PE and with that in the effective

electrical torque ME by varying the field current The principle of operation of the DVR

Power System Stabilizer becomes clear from a consideration of the oscillations in power

output and frequency (ΔPE Δf) and the vector diagram If it is assumed that oscillations

in the network frequency generates load oscillations with the mass inertia of the rotor

then the active load of the generator (eg MW-measured) is influenced with a sinusoidal

value -ΔPE (ME-MA = -ΔPE) By inversion of -ΔPE one obtains the fluctuation in

power provided by the rotor +ΔPE As is known the slip signal Δf follows +ΔPE with a

phase delayed by 90deg The +ME produced by the periodic changes in the load angle δ is

in phase with +ΔPE A good damping is attained if ME is varied in phase with the slip

Δf However this signal must also be advanced somewhat to compensate for the time

constants in the excitation circuit and the generator

As mentioned above the electrical torque ME can be influenced by varying the

field current To accomplish this a suitable control signal referred to as variable

disturbance compensation must be imposed upon the voltage set-point or the converter

control variable Ucontr As can be seen from the vector diagram by applying proper

weighting factors (K1 K2) and then adding together the signals -ΔPE and Δf an overall

stabilization signal can be produced that rotates in advance of the Df signal by any angle

desired between 0deg and 90deg Because the amplitude of -DPE remains proportional to the

amplitude of Δf a constant angle in advance of Δf results for the compensation of the

time constants referred to above The optimum weighting factors K1 and K2 for a

synchronous generator working to a power network depend on its operating point at any

moment and the external reactance of the network Normally the selection of a

compromise setting is good enough to attain stability in all operating points and for all

external reactance For special demands these settings must be parameterized as a

function of the external reactance (which means optional equipment Xe-Identification)

The Power System Stabilizer PSS is a section of the AVR computer program and is

processed once per network cycle The voltage at the generator terminals and the

generator current are measured in order to define the signals ΔPE and Δf The calculated

signals for _P_ (=PE) and Δf are then sent across DC filters ldquoDrdquo (real differentiators) that

transmit only the dynamic portion of the signals The ΔPE and Δf signals obtained in this

way are then weighted (multiplied by) with the factors K1 and K2 and sent to the

summing point of the voltage regulator

The PSS stabilization signal is imposed on the automatic voltage regulator only if

the following prerequisites are met

bull Generator on line

bull Generator power output gt the value F338

bull Generator voltage in a range between F33C and F33A

The stabilization signal is limited at the output from the PSS to the lower and

upper limits Flag defines whether the stabilization signal is introduced before or after the

PID filter (usually before the filter) Because the PID filter as noted above already takes

the ceiling factor Vp1 into account the PSS signal needs to be multiplied by Vp1 if it is

added to the voltage regulator following the PID filter (divider at the input to the

minmax limiter) This precaution prevents the DC filter ldquoDrdquo in the P-channel from

producing an unnecessary ldquostabilizationrdquo effect in the case of rapid changes in turbine

load As an alternative for the AVRrsquos Power System Stabilizer a stabilization signal from

an outside system can be imposed by activating the binary input ldquoPSS-SIGNEXTrdquo Flag

F340 can be used to select between an analog and a 12-bit signal and F33E to select the

polarity desired for that signal

24 The MANUAL Channel

241 Summary

The MANUAL channel (Channel 2) has been built as a simple field current

regulator

without additional limiters Its main function is to maintain the excitation of the generator

even if the AUTOMATIC channel becomes non-operational The MANUAL channel

also performs valuable service for purposes of testing commissioning and preventive

maintenance Its measurements regulator generation of firing pulses and power supply

are physically separate from those on the AUTOMATIC channel

242 Principle of Operation

All the functions of the MANUAL channel including the generation of firing

pulses have been implemented in a single electronic module the Gate Control Unit The

control variable Ucontr of voltage regulator is used as the reference value for generating

firing pulses on the principle known as ldquoramp controlrdquo (Comparison of Ucontr with

Usynsynchronous sawtooth signal) For further processing in the UN 0096 Intermediate

Pulse Stage the Gate Control Unit supplies six firing pulses at its output whose phase

position with respect to the synchronous voltage Usyn is in accordance with control

variable Ucontr An internal linearization ensures that the field voltage produced via the

firing pulses remains proportional to the control variable Ucontr throughout the entire

range As a result the circuit amplification of the control remains constant over the entire

range Whenever excitation is switched ON the set-point for Generator Voltage is set

automatically at the preset - ref Value This provision ensures that the generator voltage

always attains approximately its nominal value after the field flashing The Gate Control

Unit can be refunctioned ( by pre-selection with a switch ) for purposes of testing to act

as a purely firing pulse control In this case the control variable Ucontr is adjusted

directly using the RAISELOWER push buttons on the front of the module In this way

for example the relationship between the phase position of the firing pulses and the

control variable Ucontr can be checked easily

CHAPTER 3

PULSE SECTION

31 Pulse Generation and Amplification

The Gate Control Units of both AUTOMATIC channel and MANUAL channel

each supply six firing pulses for operating the 6-pulse thyristor bridges The low-power

pulse signals from these Gate Control Units are then amplified in the Intermediate Pulse

Stage galvanically isolated and then sent to the common pulse bus On the output end

the Intermediate Pulse Stage of the non-active channel) is always blocked The Gate

Control Units generate the pulses based on microprocessor control The reference voltage

used for the firing pulse phase location is the output voltage from the excitation

transformer (Usyn1 Usyn2) The commutation spikes of the synchronous voltage caused

by the converter are calculated prior to use of the voltage as a reference value and are

deliberately filtered out The lower limit for the firing pulses (double pulses) which are

offset from one another by 60deg is defined by the limit rectifier position (αmin) and the

upper limit by the limit inverter position (αmax) for the firing angle αmin and αmax can

be adjusted on the Gate Control Units using BCD (Binary Coded Decimal) switches

αmin ensures that the firing pulses will not be issued (premature firing) until there is

sufficient positive phase voltage on the thyristor involved αmax prevents a dangerous

ldquotippingrdquo of the thyristor bridge into the rectifier mode if the firing angle α is too large

(ldquolate firingrdquo) The critical factors determining αmax are the overlap time uumlmax (max

commutation time) and the ldquorecovery timerdquo of the thyristors (αmax lt 180deg - uumlmax - γ )

An external control signal can force the firing pulses into their inverter limit position

Other binary inputs can block or direct the firing pulses of the Gate Control Units so as to

produce freewheeling on the thyristor bridge During freewheeling the firing pulses for

the thyristor pair R and S are blocked and the pulse signals T+T- are engaged with

chains of pulses Both Gate Control Units (for the MANUAL amp AUTOMATIC channels)

contain a field current monitor that blocks the firing pulses immediately whenever the

current exceeds a preset threshold level In this case the field circuit-breaker is also

tripped via an output contact The purpose of these provisions is to prevent damage to

thyristors and thyristor fuses in case of a slip-ring short-circuit or to keep any damage

that does occur to a minimum The pulse signals are galvanically separated at the outputs

from the Intermediate Pulse Stage (with pulse transmitters) and are then directed to the

common pulse bus This transmission of the pulse signals to the pulse bus via passive

transmitters ensures a high degree of active channel autonomy Practically no possible

malfunctions on the inactive channel (including for example sustained pulses) affect the

active channel

32 Pulse Monitoring

The ldquoPulse Busrdquo and the pulse signals of the AUTOMATIC channel are

monitored This monitoring device consists of potential isolating stages and the common

monitor If the pulse monitoring of the ldquoPulse Busrdquo responds a switch-over is made to

MANUAL channel The function of the potential isolating stages is to couple the pulse

monitoring device to the pulse circuits without any feedback effect The pulse monitoring

checks the six pulse lines for the following malfunctions continuous or periodic failure

of one or more pulses Periodic occurrence of synchronous or asynchronous false pulses

Continuous pulses the pulse monitoring device can be tested while the machine is in

operation

CHAPTER 4

CONVERTER

Thyristor

The term thyristor usually refers to a family of four layer solid state device having

turn on characteristics that can be externally controlled by either current or voltage They

are also referred to as breakdown device because their working depends on avalanche

breakdown Thyristors have only two stages OFF and ON Thyristors have a similar

function to Uni-junctions they act as switches Thyristors use current flow as a switch

Thyristors have three states

1 Reverse blocking mode mdash Voltage is applied in the direction that would be

blocked by a diode

2 Forward blocking mode mdash Voltage is applied in the direction that would cause

a diode to conduct but the thyristor has not yet been triggered into conduction

3 Forward conducting mode mdash The thyristor has been triggered into conduction

and will remain conducting until the forward current drops below a threshold value

known as the holding current Converter is a semiconductor device which converts ac

input voltage into a constant dc output voltage In present excitation system three phase

fully controlled thyristor converter is used

Because of the following advantages thyristor converters are used

a) Thyristors are used for high power applications ie up to 10Kv3500A1KHz

b) Having high reliability and low losses

c) Uni-directional device like diode

d) Itrsquos operation as a rectifier which are low resistance in forward conduction

mode and high resistance in reverse conduction mode

PROTECTION OF THYRISTORS

For reliable operation of a thyristor demands that its specified ratings are not

exceeded When Subjected to or over voltages During the turn - on of SCR didt

prohibitively large False triggering of SCR by high value of dvdt andSpurious signals

between gate and cathode may leads to unwanted turn ndash on

DIDT AND PROTECTION

When thyristor starts conducting in forward conduction mode and is turned on by

gate pulse The anode current increases rapidly whole area of the gate to Cathode

junction then hot spots will be formed near the gate connection this locality of heating

destroys the thyristor Thyristor thermal time is constant The causes due to faults and

short circuits or surge currents Electronic crowbar protection is used against the over

voltages The rate rise of anode current must be kept at the time of turn on below the

rated or specified limiting value The didt value maintained below limited value by using

a inductor also called ldquodidt inductorrdquo in series with anode circuit The locality of heating

is avoided by applying gate current but not greater the maximum gate current

DVDT AND OVER VOLTAGE PROTECTION

With forward voltage across the anode and cathode of a thyristor the two outer

junctions are forward biased but the inner junction is reverse biased This reverse biased

junction J2 has the characteristics of a capacitor due to charges existing across the

junction In other words space-charges exist in the depletion region around junction J2

and therefore junction J2 behaves like a capacitance If the entire anode to cathode

forward voltage Va appears across J2 junction and the charge is denoted by Q then a

charging current i given by Eq (46) follows

i = dQdt =d(Cj Va )dt

= Cj (d Va dt) + Va(d Cj dt) helliphelliphelliphellip(46 a)

As Cj the capacitance of junction J2 is almost constant the current is given by

i = Cj (d Va dt) helliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip(46 b)

If the rate of rise of forward voltage dVadt is high the charging current i will be

more This charging current plays the role of gate current and turns on the SCR even

when gate signal is zero Such phenomena of turning-on a thyristor called dvdt turn-on

must be avoided as it leads to false operation of the thyristor circuit

For controllable operation of the thyristor the rate of rise of forward anode to

cathode voltage dVadt must be kept below the specified rated limit Typical values of

dvdt are 20 ndash 500 Vμsec False turn-on of a thyristor by large dvdt can be prevented by

using a snubber circuit in parallel with the device thyristor are very sensitive for over

voltage than the semiconductor devices

Over voltage transients are perhaps the main cause of thyristor failure

In thyristor there are mainly two types

1 Internal over voltages

Due to the commutation of the thyristors large voltages are generated internally

Because of the series inductance of the SCR circuit the large transient voltages L didt

produced This voltage several times the break over voltage of the device then thyristor

destroys permanently

2 External over voltages

External over voltages are caused due to the interruptions of current flow in an

inductive circuit and also due to the lightening strokes on the lines feeding the thyristor

system For the reliable operation of thyristor the over voltages must be suppressed by

adopting suitable techniques

Suppression of over voltages

The RC circuit called snubber circuit is connected across the device to protect In

order to keep the protective components to a minimum the thyristors are chosen with

their peak voltages ratings are 25 to 3 times of the normal peak working voltage

ldquoselenium thyrector diodes metal oxide varistors or avalanche diode suppressers are

commonly employed for protecting the thyristor circuit against the over voltages

Gate protection

Gate circuit should also be protected against the over voltages and surges Over

voltage at gate circuit can cause false triggering of the SCR may rises the junction

temperature behind specified limit leading to its damage Protection against over

voltage can be achieved by connecting a ZD across the gate circuit and a resister is

connected in series with gate circuit to protect against the s A capacitor and resister are

connected across gate to cathode to by pass the noise

41 Final Pulse Stages

The Final Pulse Stages adapt the output pulses from the Intermediate Pulse Stage

(pulses on the pulse bus) to the gate currents needed for the thyristors Each thyristor

bridge is equipped with its own Final Pulse Stage Each Final Pulse Stages is provided

with a power supply module The amplified output pulses from the Final Pulse Stages

start as a short strong steep pulse with an amplitude approx 2frac12 times that of the main

pulse This initial pulse edge assures proper firing of the thyristors being triggered

Subsequently the weaker part of main pulse keeps firing conditions steady As already

mentioned the Final Pulse Stages and their associated thyristor bridges form single units

All six pulse outputs from a Final Pulse Stage can be blocked by an external control

signal so that all thyristors in the associated thyristor bridge will block the current A

blocking of the pulses is initiated whenever there is a malfunction in the associated

thyristor bridge

42 Converter Power Section

The thyristor converter consists of three independent parallel rectifier blocks TY1

to TY3 which are all in service Even if one block fails the remaining blocks take over

automatically the full design current of the excitation circuit During normal operation

(with ideal current share) and all three bridges in operation each of these blocks has to

carry only (n-2)n (ie33)of its design current If 2 thyristor bridges fail the excitation

is limited Only when all three bridges fail the excitation is switched off Each thyristor

bridge arm is equipped with current flow monitoring CTrsquos Failure of conduction in any

arm is identified by a Current flow monitoring module

43 Converter Cooling

A cooling system is needed to dissipate heat losses in the converter blocks and

electronics Each converter block has therefore been equipped with a fan supplied with

power from the converterrsquos primary voltage (via transformer ndashT8 in field flashing

cubicle) The fans are protected with motor protection circuit breakers An air flow

monitoring unit is provided for monitoring the air flow through the thyristor bridge If a

circuit breaker failure is detected or if the air flow monitor drops off at one of the

thyristor bridges the bridge involved is immediately set out of operation by blocking its

firing pulses

44 Thyristor Converter Monitoring

A thyristor bridge in which defects occur that could threaten the safety of

operation or cause secondary damage is switched off automatically ie its firing pulses

are blocked This happens whenever A thyristor fuse is blown The fuses are monitored

individually with micro switches The Final Pulse Stage fails which is detected by

internal monitors (supply voltage sustained pulse short-circuit on the output end) The

power supply to the fan fails fan air flow as monitored by the Air flow monitor fails or

is insufficient Isolator on ACDC side is open

CHAPTER 5

Field Current Circuit Excitation Transformer

51 Field Circuit Breaker

The circuit-breaker in the field circuit is used to isolate the field circuit from the

converter It is capable of switching off the synchronous machine from full load under the

maximum conditions of a 3-phase short-circuit In addition to its main contacts the field

circuit-breaker also has a de-excitation contact with which the field energy stored in the

field can be dissipated across the de-excitation resistor The de-excitation contact closes

shortly before the main contacts open so as to ensure proper commutation of the field

current from the main contacts to the de-excitation contact when the breaker is switched

off The field circuit-breaker is switched on by electromagnetic force and is kept switched

on by a mechanical latch When the latch is released by a trip coil the circuit-breaker

opens The circuit-breaker also has auxiliary contacts that report its status

52 Field Flashing

In shunt supplied excitation circuits (excitation transformer connected to the

generator terminals) the generator does not have enough remnant voltage for a generator

voltage build-up via the converter In this case a field flashing circuit is provided It

consists of the field flashing contactor the diode bridge and a transformer used to adapt

the auxiliary input voltage to the voltage needed for field flashing when power is

supplied from the auxiliaries network

Fig Field Flashing

Because the field flashing contactor is not able to switch off the energy stored in

the field the control ensures that the contactor can only reopen if the field circuit breaker

has already been opened (generating the TRIP order) or in a normal field flashing

sequence when the converter has taken over the field current Field flashing occurs in the

following stages

1048729The excitation is switched on closing the field flashing contactor ( Field

Circuit Breaker is already closed )

1048729The start-up excitation current flows through the rotor driving the generator

voltage up to approx 15 U

1048729After about 10 U the firing pulses to the converter are released and it begins

to excite the generator to its rated voltage

1048729After about 30 U the field flashing contactor opens (with no current since

the converter is now supplying the current)

The diode bridge at the input to the field flashing contactor prevents a feed-back

from the converter to the source of field flashing while the contactor is still closed

53 De-excitation

When malfunctions occur the stored field energy must be dissipated as quickly

and safely as possible to protect the generator This is done by the converter the field

circuit-breaker and the de-excitation (discharge) resistor

De-excitation (with opening of the field circuit-breaker) takes place in the following

stages

1048729The converter drives to its inverter limit position (negative ceiling voltage)

recovers a portion of the field energy into the network A trip command is given to the

field circuit breaker

1048729The de-excitation contact closes diverting the field voltage to the de-excitation

resistor

1048729Then immediately the main contacts open building voltage The field voltage

commutates to the de-excitation resistor

1048729The current diminishes at a given time constant TE

(With linear resistance TE = Lf (Rf + Re))

Due to the reversal of the field voltage by the converter the field current

commutates from the main contacts of the field circuit-breaker to the de-excitation

resistor in a very early phase This reversal of the field voltage prevents burn-off on the

main contacts and provides effective protection for the field circuit-breaker Depending

on the operating policy an operational shut-down of the excitation can also be effected

with the field circuit-breaker closed This method is useful mainly when the excitation is

switched on and off frequently In this case the converter is merely driven into the

inverter limit position so that the field energy is recovered into the network The

converter then blocks since it is supplying positive current only

54 Excitation Transformer

The excitation transformer matches the generator voltage to the field voltage

(required ceiling voltage) It also serves as a commutation reactance for the thyristor

converter and as a potential isolator between the network and the excitation circuit In

addition the transformer functions as a current limiter in that it makes it possible to keep

any short circuits in the excitation circuit under better control The excitation transformer

is equipped with temperature monitoring probes which set off an alarm when the

temperature exceeds a first max limit and then trips the excitation if the temperature

continues rising to a second (higher)limit

CHAPTER 6

Monitoring and Protection

61 Excitation Monitoring

611 General Information

The main goal of Excitation Monitoring is to make optimum use of the

redundancies provided in the excitation system and to give alarm whenever a malfunction

makes these redundancies unavailable The field current is monitored to see that it does

not exceed a maximum level and if necessary a switch-over to the MANUAL channel is

initiated In addition the criterion for switching off the field flashing is generated The

excitation Monitoring consists of an autonomous processor system

612 over current Alarms

In the Excitation Monitoring the limits for are set at higher levels than the

settings on the Field Current Maximum Limiter Whenever the current exceeds 110 of

the nominal field current contact R1 and the binary output associated with it are

activated immediately If field current remains gt 110 then after a preset inverse-time

has lapsed relay R2 and - after a further delay - relay R0 and the binary outputs

associated with them are activated Parameters match the measurements for If1 and If2 to

the nominal value for field current so that the internal values can be processed and read as

pu values It can be used to falsify the actual value of the field current If (to raise it) so

as to cause a response from the alarm limits for purposes of testing The processed If

signal is always taken from on the active channel (CHANNEL I OR CHANNEL II) As

long as the field current If is above the threshold value 11 Ifn its peak value is

measured This is stored (until RESET) and can be read at any time on the Micro-

Terminal Once the value of If exceeds 11 Ifn integration of this value starts Whenever

the integrated time-current value (intisup2dt) exceeds the preselected reference value the

alarm OVER CURRENT INVERSE-TIME is set off and a command is simultaneously

issued to switch over to the stand by AUTO channel Software switch F758 enables the

three over current alarm functions (R0 R1 R2) and selects one of three possible inverse-

time curves T1 T2 or T3 Within the characteristic curve (T1 T2 T3) selected the

desired limit curve for response is set using the factor F216

613 Switch-Off Criterion for Field Flashing

The Excitation Monitoring supplies the criterion for switching off the field

flashing Whether this criterion is activated based on the actual value for generator voltage

Ug or for field current If or both depends on the settings of the two threshold values

F200F202 (0 setting means that the output is always ldquological 1rdquo) The measurements

Ug12 and If12 are switched over depending on the present status of the channels

(Channel 1 or Channel 2 ON) Whenever Flag F750 is not activated the binary output is

fixed at ldquological 1rdquo

614 Storage of Alarm Status

The outputs of the over current alarms (R1 R2 R0) and the messages NO

FAILURE MONITORING PARAMETERS CHANGED are stored messages can be

erased by activating the input ldquoGENERAL RESETrdquo or by using the RESET button on the

front of the module Erasure with the input ldquoGENERAL RESETrdquo is effective only if the

situation causing the alarm or the malfunction is no longer present Whenever the self-

diagnosis equipment in the processor detects a malfunction the output NO FAILURE of

MONITORING is set at ldquological 0rdquo (= alarm) The alarm ldquoPARAMETERS CHANGEDrdquo

is activated whenever parameters or settings of software switches have been changed via

the Micro-Terminal

615 Actual Value Monitoring

The actual values for generator voltage Ug synchronous voltage Usyn and field

current If are monitored for malfunctions This monitoring is active regardless of whether

or not the generator is in operation Essentially when the generator is in operation the

measurements are monitored by comparing the signals (the smaller signal reading is

detected as incorrect) When the generator is not in operation the measured data are

monitored for extreme values The percentage of deviation permissible in the

measurement signals being compared

(Ug1ampUsyn1 Ug1ampUg2 Ug2ampUsyn2 If1ampIf2) is defined by parameters F208 and

F20AIf the excitation transformer is being supplied from an auxiliary power source (no

shunt operation) the values of Ug and Usyn will be different in some operational

conditions

In that case Flag F75A can be used to deactivate comparative measurements Ug1 Usyn1

and Ug2 Usyn2 The ldquoprimary monitoringrdquo compares Ug1 with Ug2 and generates the

messages that Channel 1 or Channel 2 has suffered a malfunction Whenever Ug1 lt Ug2

and the binary message from CH1 reports no malfunction a malfunction on

Measurement Channel 1 is reported (Ug1Usyn1 FAILURE) A similar malfunction is

also present whenever the binary message CH1 DISTURBANCE is reported and a

discrepancy is detected between Ug1ampUg2 The generation of the alarm ldquoUg2 Usyn2

FAILURErdquo is analogous to that for Channel 1 The ldquosecondary monitoringrdquo compares

Ug1 with Usyn1 amp Ug2 with Usyn2 This is enabled whenever the binary message of the

comparison channel reports a malfunction or whenever both binary messages report no

malfunction - but both secondary monitors report a malfunction As long as the secondary

monitoring is blocked the differences Ug1neUsyn1 or Ug2neUsyn2 trigger malfunction

signals for the measurement channel involved (suspicion that there is a corresponding

error in Usyn) The measurement channel malfunctions are enabled operationally

whenever after excitation has been switched on generator voltage Ug exceeds the value

set on F204 The voltages Ug1 and Ug2 are checked 16 seconds after the excitation is

switched off to see that they do not exceed the limit value F210 that applies to both of

them At the same time g1Ug2Usyn1Usyn2 are checked for extreme values (gt or lt

the operating range) Monitoring for extreme values is likewise enabled during normal

operation ( Excitation ON and Ug gt F204 ) Flag F754 is used to enable or block the

malfunction signals to the binary outputs Basically the monitoring of the actual values

for If1If2 functions like that of the Ug1Ug2 monitoring

62 Excitation Protection

621 General Information

The Excitation Protection switches off the excitation (and de-excites the machine

rapidly) whenever a danger arises that threatens the excitation transformer the converter

or the generator Generally limiter or monitoring functions precede the emergency trips

and these normally respond before the Excitation Protection must initiate a trip

Protective trip commands are issued directly to the field circuit-breaker from potential

free contacts of the board via the trip relays They are directed redundantly to the

operative field circuit-breaker ldquoOFFrdquo command

622 Protection against Excitation Transformer Overheating

This equipment monitors the excitation transformer for overheating in the

windings that could result from over current short-circuits or inadequate cooling The

monitoring uses temperature monitoring modules in conjunction with temperature

sensors built into the windings Normally the temperature is monitored in two stages the

first stage sets off an alarm the second causes a trip of the excitation

623 Rotor Over voltage Protection

Malfunctions in the generator circuit (eg terminal short-circuit failed

synchronization asynchronous operation) cause induced negative field currents that

produce high voltages in the field circuit These must be restricted to a level with a

sufficient safety margin below the insulation capacity of the field winding (test voltage)

and also below the peak blocking voltage of the converter thyristors The crow bar

employs spark gap elements to detect over voltages in the field circuit Whenever they

respond the associated thyristors are fired immediately switching the de-excitation

resistor parallel to the field The de-excitation current generated thereby initiates an

excitation trip via a supervision circuit causing an immediate opening of the field circuit-

breaker The malfunction isets off an alarm and an internal malfunction is indicated at the

cubicle

TEST VALUES OBTAINED WHEN EXCITATION IS RAISED

TEST

SNO PARTICULARS ACTUAL VALUE OBTAINED VALUE

1 VREF 996 100

2 VACT 997 1003

3 IFACT 735 765

4 IGACT 703 707

5 ACTIVE

POWER

703 705

6 REACTIVE

POWER

101 142

7 POWER

FACTOR

099 IND 098 IND

8 ACTIVE

CURRENT(IR)

705 703

9 REACTIVE

CURRENT(IX)

102 138

10 POWER

ANGLE

566 546

11 FIRING

ANGLE

640 632

VALUES OBTAINED WHEN EXCITATION IS LOWERED

SNO PARTICULARS ACTUAL

VALUE

OBTAINED VALUE

1 VREF 100 997

2 VACT 100 997

3 IFACT 787 761

4 IGACT 839 837

5 ACTIVE

POWER

839 830

6 REACTIVE

POWER

155 110

7 POWER

FACTOR

098 IND 099 IND

8 ACTIVE

CURRENT(IR)

835 830

9 REACTIVE

CURRENT(IX)

140 94

10 POWER

ANGLE

613 DEG 631 DEG

11 FIRING

ANGLE

601 DEG 599 DEG

CONCLUSION

For generating the EMF in stator winding excitation is required to the rotor of a

generator There are two types of excitation

1 Static excitation system

2 Brushless excitation system

A certain disadvantage in brushless excitation system is the slow response time of

the field in case of fast load changes specified No slip-rings and brushes direct

measurements of the field parameters not possible

To avoid all loses static excitation is used Since it does not have any rotating

parts mechanical loses and windage loses This system has fast response and speed

control While preferring this excitation system there are no limitations for the

redundancy of Thyristor bridge circuits

Static excitation has fast field discharge by resistor and inverter operation direct

measurement of field quantity is possible The meaning of excitation is nothing but

continuous supply of DC current (ie field current) to the rotor to buildup required

output voltage in the stator

Field current is changed with respect to the change of load so the digital

automatic voltage regulator (DAVR) is used to regulate the output voltage according to

the load variations

So we conclude that static excitation system with DAVR is preferred since it is

having excellent dynamic performance and better options for R amp M

To accomplish this special field flashing equipment is needed When the field

flashing equipment is being supplied with power from a DC power source (power station

battery) a resistor is used to limit the field flashing current When it is being supplied

from an AC power grid a transformer serves as the adapter needed Excitation of the

generator is started by closing the field circuit-breaker Q1 and the field flashing breaker

Q2 This supplies current to the field which excites the generator up to 15 30 U The

generator then supplies voltage to the converter via the excitation transformer Starting

from approx10 of the generator voltage the firing electronics and the converter are

able to continue the voltage build-up so that the field flashing circuit is relieved of

current Once the voltage exceeds approx 25 of U the field flashing breaker is finally

opened having no current The diode bridge at the input to the field flashing breaker

prevents a back-flow of current to the field flashing source The converter TY has Final

Pulse Stage cooling and monitoring of the elements Redundancy in the regulator section

is ensured by means of two fully separate channels with independent measuring inputs

and extensive monitoring (ldquoSUPERVISIONrdquo)

Channel 1 (AUTOMATIC channel) is built as voltage regulators and is ON during

normal operation In addition to the voltage regulator which has a PID control algorithm

2

the AUTOMATIC channel also contains various limiters and corrective control circuits

to ensure the use and stable operation of the synchronous machine up to its operating

limits This channel possesses a Gate Control Unit with a subsequent Intermediate Pulse

Stage to generate the firing pulses for the Thyristor converter During normal operation

the Intermediate Pulse Stage of AUTOMATIC Channel is active and transmits the firing

pulses galvanically separated to the common pulse bus at the input to the Final Pulse

Stage Various monitoring functions of the AUTOMATIC channel and pulse monitoring

on the common pulse bus initiate an automatic switch-over to stand by Channel

(MANUAL)in case of a malfunction

Channel 2 (the MANUAL channel) is built as a simple field-current regulator with a PI

control algorithm It serves as a back-up channel in case of a malfunction on the

AUTOMATIC channel Manual channel performs valuable service for testing

commissioning and preventive maintenance The MANUAL channel has its own Gate

Control Unit (the software for the If regulator is also implemented therein) and its own

intermediate pulse Stage During normal operation (AUTOMATIC) the output pulses

from Intermediate Pulse Stage are blocked from reaching the pulse bus Various

monitoring on the MANUAL channel initiate an alarm in case of a malfunction while the

MANUAL channel is on stand-by If the MANUAL channel suffers a malfunction while

it is in operation the excitation is switched off (TRIP) Both channels are equipped with

tracking equipment so that the inactive channel always generates the same control

variable as the active channel during steady-state operation

This ensures smooth switch-over from Automatic to Manual channel and vice

versa To ensure that the MANUAL channel will in a switch-over initiated by a

malfunction take over the operating point of the machine as it was prior to the problem

the response of the tracking for the MANUAL channel is set relatively slow In addition

to the pulse monitors (ldquoSUPERVISIONrdquo) shown in the basic circuit diagram the

excitation system has an autonomous Excitation Monitoring As one of its functions this

equipment monitors for field currents that exceed acceptable maximum limits It initiates

an emergency switch-over to the MANUAL channel whenever the field current exceeds

the preset limit If even after such a switch-over the field current does not drop back to

3

the permissible level the excitation is switched off by Excitation Protection The most

important measuring inputs for the excitation system (If Ug Usyn) are redundant (2-

fold) The Excitation Monitoring checks these measuring inputs for discrepancy and

plausibility An alarm is always initiated in case of malfunction In certain cases a

switch-over to MANUAL channel is also initiated The excitation system contains an

Excitation Protection to protect the excitation transformer the converters and the

synchronous machine The protection system can detect short-circuits in the excitation

circuit and keep secondary damage within acceptable limits by a quick tripping of the

excitation and an opening of the generator breaker An overheating of the excitation

transformer first sets off an alarm (at a given preset limit) and then likewise initiates a

protective shut-down at an even higher limit The over voltage protection in the de-

excitation equipment provides an autonomous protective function for the rotor and the

rectifier This protection system monitors the field voltage in both polarities for over

voltage and if necessary de-energizes the field via the de-excitation resistor

12 Principle of primary power supply

In the shunt excitation system the excitation transformer also provides

the power supply for the electronic equipment and the converter fans So a failure of the

auxiliary supply to the converter fans does not cause a shutdown of the excitation When

the auxiliary supply fails the supply to converter fans is switched over to Excitation

transformer OP with a contactor A station battery supply is absolutely necessary for the

control of the field circuit breaker It is the power source for the electronic devices till the

generator is able to supply voltage Auxiliary power to the field flashing equipment must

be present in order to build up the generator excitation The power supply for standstill

heating and the cubicle lighting is also from Station Auxiliary Power Supply and is of

secondary importance for operation of the plant Power supply to rotor earth fault

detection circuit too is from Station Auxiliary Supply The two synchronous voltages

Usyn are each supplied to the AUTOMATIC channel and the MANUAL channel

separately across transformers The Gate Control Units need these voltages to enable

4

them to issue the pulses at a given firing angle relative to the input voltage of the

converter

5

CHAPTER 2

Digital Automatic Voltage Regulator (DAVR)

21 Principle of Operation of the Regulator (DAVR)

To regulate the voltage and the reactive power of a synchronous machine the

field voltage must be adjusted quickly to the changes in the operating conditions (with a

response time that does not exceed a few ms) To accomplish this analog control systems

include amplifiers which make continuous comparison of the actual values against the

reference values and vary the control variable to the converter with almost no delay Most

of the delay that occurs originates in the converter since the firing pulses for changing

the rectifier phase angle are only issued periodically (every 33 ms)

The DVR digital voltage regulator calculates the control variable from the

measured and reference data in very short time intervals This results outwardly in a

quasi-continuous behavior with a negligible delay time (as in an analog regulator) The

calculations are made in the binary number system Analog measurement signals such as

those for generator voltage and generator current are converted into binary signals in

analogdigital converters The set-points and limit values have already been defined in

digital (binary) form An understanding of the actual computation processes in the digital

voltage regulator is not necessary for operation preventive maintenance or

troubleshooting Like the operator of a pocket calculator or a personal computer all the

operator needs is to know how to operate the instrument and the programming for this

working tool For that reason we will explain below only the principle division of work

among the various modules and the flow of data processing The purpose is above all to

make clear how the processor system has been integrated into the rest of the power

electronics system

6

22 Basic Structure of the Processor Systems

7

The signal processors 25 analog inputoutput modules Each of these processor

systems has a common bus circuit and output and the control lines There is a specific

range of addresses assigned to each assignment Board including the power supply bus

the address lines the two data lines to the input calculates the reactive current (I sin φ)

and the active current ( Icos φ) With these two channel processor Synchronized with

these interrupts (ie with the phase positions of current Ig the field current If and the

synchronous voltage Usyn From the exchange data with the microprocessor card across

the two data lines generator voltage Ug) this processor measures the generator current

Ig and then hardwired connections or multi-conductor cables Binary and analog

inputoutput modules ie for galvanic isolation and adaptation to the electronics level

The most important input interrupts per period to trigger the cycles for processing actual

values in the AUTOMATIC module on the processor bus) for filtering and further

processing

Monitoring each consist of the central microprocessor module and binary and

parameters to the AUTOMATIC channel are the generator voltage Ug the generator

peripheral unit Ug Ig and Usyn are sent to the Interrupt Generator (plug-in peripheral

units (wall-mounted units) peripheral units are used for preprocessing signals from

external measurement circuits power supply units Signals are exchanged among these

processor systems via processed across separate peripheral units for each channel These

processor working on the bus (a house address that can be adjusted using a switch)

systems The AUTOMATIC channel the programmable controls and the Excitation The

actual values measured from AUTOMATIC channel and MANUAL channel are The

AUTOMATIC channel and the MANUAL channel each have their own The digital

voltage regulator is broken down into several autonomous microprocessor The inputs and

outputs of the processor systems are directed across voltage-isolating The Interrupt

Generator also uses the 3-phase Ug signal to generate the 12 themselves contain a limited

number of hardware inputs and outputs with fixed equipment Whenever addresses from

this range are called up the signal processing module can results the processor is then

able to derive further operating parameters such as the load angle the active power etc

The functions of all microprocessor systems other than the programmable controls

have been accomplished in firmware The non-varying standard function modules can be

configured to the design desired for plant-specific purposes using software switches

(KFlags) Thus for example the stored status of a K-Flag determines whether or not a

Limiter is active and whether the de-excitation or the excitation limiters take precedence

Because these K-flags determine the software Scope of Supply for the installation they

cannot be changed permanently via the Micro-Terminal In this way they differ from

such setting data as the values of the parameters for the PID filter of the voltage regulator

or the set-points for the limiters These values can be permanently changed using the

Micro-Terminal Communication is possible with each of the processor systems via the

Micro-Terminal by plugging on the connecting cable In this way signals within the

processor and setting parameters can be viewed analog signals can be issued and the set

parameters can be altered temporarily (F range) or permanently (C range) Unlike the

other processor systems the programmable controls do not include any firmware for

realization of the functions They have been designed so that the designer can adapt and

change their functions easily using the ldquoFunctional Block Programming Language P10

Digital and analog functions can be implemented in practically any degree of complexity

desired using the P10 functional blocks The control variable of the voltage regulator

(AUTOMATIC channel) and the control variable of the field current regulator

(MANUAL channel) are each processed in separate Gate Control Unit and formed into a

chain of pulses at the appropriate firing angle The pulses of the active channel are

directed to the pulse bus via the associated Intermediate Pulse Stage The pulses for each

converter block are amplified sufficiently in Final Pulse Stage to fire the Thyristor

231 General Information

The functions of the automatic voltage regulator AVR are

1 to regulate the generator voltage

2 to regulate the effect of the reactive andor active current on the voltage

3 to limit VoltHz

4 to limit max and min field current

5 to limit inductive stator current

6 to limit capacitive stator current

7 to limit the load angle

8 to stabilize the power system

Block Diagram shows the software structure of AUTOMATIC channel The

generator limiters not provided for the installation in question (optional equipment) are

identified in this overview as ldquoNot Suppliedrdquo The parameter values signal values and

software switches (flags) marked with addresses (hexadecimal numbers) can be viewed

and altered via the Micro-Terminal The values selected are displayed in sec pu Hz

etc and can where necessary be changed directly in these formats The plant-specific

settings of the variables and the flags can be obtained from the Test and Commissioning

Report This block diagram provides information about the important functions and

possible settings of the AUTOMATIC channel For the sake of clarity no detailed

presentation has been given of special functions such as tracking circuits initializations

etc The page heading cross-refers this overview to the various sheets of the schematic

diagram Binary signals are shown in broken lines analog signals in solid lines The

corresponding text designations in the schematic diagram can be used for identification of

the input signals (hardware inputs) The only analog output signal from the automatic

voltage regulator control variable Ucontr is sent via the data bus (CRU bus) to the Gate

Control Unit Most of the binary messages (outputs) from the AVR are of no interest

functionally and they have been omitted for the sake of clarity The basic structure of the

digital voltage regulator and the limiters is simple This is necessary in order that the

behavior of the regulatorslimiters will remain calculable and understandable in all

operating situations and that there will be no problem in adjusting and optimizing them

The central PID filter in the digital voltage regulator defines the dynamic response of the

closed-loop controls both in the voltage regulator mode and after limiters have

intervened The ldquocontrol deviationrdquo at the input to the PID filter is either the control

deviation for voltage the control deviation of a de-excitation limiter (the value

determined by minimum value selection) or the control deviation of an excitation limiter

(the value determined by maximum value selection) Flag F730 (ldquoPRIORrdquo) is used to

determine whether the exciting (Min value) or the de-exciting signal takes precedence on

the minmax value limiter (normally F730 = 1111 ie the de-exciting signal takes

precedence) With the exception of the Minimum Field Current Limiter all other limiters

have variable factoring multipliers of the signal outputs so that they can be adjusted

individually together with the common PID filter which has been optimized for voltage

regulation The setting parameters for this PID filter are as follows

Vo = KR Static amplification

1

Ta = ---- Integration time constant

Tc1

Vp Proportional amplification

1

Tb = ---- Differential time constant

Tc2

Vinfin Amplification of high frequencies

The BODE diagram below shows the assignment of settings in accordance with

DINIEC standards based on a typical example

The PID filter amplifications Vo Vp and Vinfin can be adjusted in pu values But

the ldquoceiling factorrdquo pl+ must be adjusted correctly with parameter F310 if the total

amplification (circuit amplification) of the control circuit is actually to conform to the

pu settings This factor must agree with the ldquoexternalrdquo amplification ie with the

ceiling value of the transformer- converter circuit

Ceiling factor(pl+) = Ufmax Ufo

in which Ufmax = ceiling field voltage

Ufo = no-load field voltage

To attain a suitable response of the AVR when starting excitation

(ldquoEXCITATION ONrdquo) it may be necessary to change the proportional amplification of

the regulator during this phase Vp2 (transiently activated) and Vp1 (permanently

activated) can be adjusted for this purpose For example the value of Vp2 takes effect

immediately once the excitation is switched on and remains effective for a period as set at

F30C Once the period F30C (eg 5 sec) has expired Vp shifts over to Vp1 (becomes

the steady-state Vp) at the rate of change set The standard operating mode for the PID

filter is voltage regulation for which the discrepancy between the voltage set-point and

the current value for generator voltage Ug (the control deviation) is supplied at the input

To compensate for the voltage drop in the block transformer or whenever several

generators are operating to the same distributing bus the generator voltage must be

varied in proportion to the measured generator current (droop influence) To accomplish

this the voltage set-point is varied as a function of the measured reactive current IX

andor active current IR Flag F712 enables the IX droop Flag F710 the IR droop The

desired compensation is set in F282 and F286 respectively Flags F284 and F288 are used

to select whether this droop influence is to increase the voltage or to reduce it

(compensation) Combined influence of the active and reactive currents is attained by

enabling both droops IX and IR Flag F716 activates a so-called ldquoSoft-Startrdquo at the

starting of excitation This ldquoSoft-Startrdquo ensures that the voltage set-point integrates from

0 to 100 within the time set on F290 when the excitation is switched on

(ldquoEXCITATION ONrdquo) A ldquosmoothrdquo excitation of the generator can be achieved in this

way whenever there is no demand for a quick excitation

232 Voltage Set-Point

Various signals and settings control and limit the voltage set-point F270 For

example the values of F254 and F252 define the normal operating range possible for set-

point adjustment (eg 90 110) using external control commands (control room local

operatorrsquos panel superposed control system) The effective set-point adjustment rate is

governed The set-point can be set at the values of F250 and F256 by activating

appropriate control commands for ldquoSETrdquo input Enabling Flag F71A and activating a

binary input prior to switching on the excitation (ldquoEXCITATION OFFrdquo) sets the Ug set-

point at the value of UAUX This makes it possible for example to ensure that the

generator voltage will agree exactly with the network voltage after the voltage build-up

An external value with variable amplification can be added to the Ug set point by

enabling F724 (for example for stability tests)

233 Regulator Tracking in MANUAL Operation

Whenever the AUTOMATIC channel is not in operation (the MANUAL channel

is ON) a follow-up equipment ensures a smooth switch-back to the AUTOMATIC

mode will always be possible To track the voltage set-point is shifted by means of

RAISELOWER pulses from the Gate Control Unit so that control variable Ucontr at the

output from the PID filter is held steady and identical to the control variable Ucontr from

the MANUAL channel Because this tracking must react slowly resultant transient

control deviations resulting from the amplification in the PID filter might cause severe

interference with control variable Ucontr

To prevent this the follow-up equipment intervenes on the regulators mixing

point with a corresponding compensation signal

234 Ugf Limiter

At under frequency the Ugf Limiter reduces the generator voltage so as to

prevent saturation effects in the supply and measuring transformers To adjust this

limiter the max permissible generator voltage at rated frequency is defined and set

When any under-frequency occurs the generator voltage is thus reduced in proportion to

that setting

235 Field Current Maximum Limiter

The Field Current Maximum Limiter is provided to protect the generator rotor

from s occurring in steady-state and transient operation High field currents are normally

the result of a sharp drop in network voltage or of an improper raising of the voltage set-

point by the operating staff The field current is held steady at the value TH1 ie at the

maximum thermal value permissible for the excitation circuit and the rotor In order that

the generator can support the power network with its transient overload capacity during

brief collapses in voltage a temporary switch-over is made to the transient limit MAX1

(a higher setting) When the generator or the converter is operating at a reduced capacity

These limits TH1MAX1 can be switched over to the lower settings TH2MAX2 by

activating the corresponding binary signals The switch-over from the thermal limit

TH12 to the transient limit MAX12 can be configured in one of three ways

a) Depending on the over current with -dUdt ENABLE

Flag programming F418 = any setting desired F41A = 0000

This variant enables the transient value MAX12 whenever a collapse of voltage

in the network is detected The ENABLE time is fixed and can be set The example

below shows the typical behavior of the limiter configured in this way

b) Dependent on the time integral with -dUdt ENABLE

Flag programming F418 = inactive F41A = 1111

This variant likewise enables the transient value only when a collapse of network

voltage has been detected However the switch-back to the thermal limit is not made

dependent upon the time itself but on the calculated time integral intisup2dt of the The setting

on Parameter F414 in spu takes into account the time the rotor needs to cool down ie

the rate of temperature change in the case of intermittent operation The example below

shows how the timing of the switch-back to the thermal limit depends on the present

value for intisup2dt

The time integral is based on the formula

Example The setting of =isup2dt equivalent to Version a (F416) at a constant 16 times the

nominal field current for 10 seconds (with TH12 = 105) is

c) Dependent on the time integral without any preconditions

Flag programming F418 = 1111 F41A = 1111

In this variant the transient becomes available without any prior conditions

(without a -dUdt ENABLE) with the time integral intisup2dt

237 Inductive Stator Current Limiter

The Inductive Stator Current Limiter holds the stator current Ig within permissible

limits while the generator is in the ldquoover-excitedrdquo operating range by reducing the field

current accordingly The setting TH (thermal limit) provides the limit against stationary s

that might occur To take advantage of the generatorrsquos transient overload capacity a

switch-over is made to the higher setting MAX The principle of operation of this switch-

over to the value MAX permissible only transiently is identical to that employed for the

field current limiter (refer to the description above) When the drive output from the

turbine is very high stator current may exceed permissible limits even while inductive

loading of the generator is low In this case if the stator current limiter is not kept from

influencing the field current the control circuit will oscillate back and forth between the

Inductive Stator Current Limiter (de-

exciting) and the Capacitive Stator Current Limiter (exciting)The output signal of that

function then dominates the control variable of the Ig-dependent limiter via a maximum

value selection

238 Capacitive Stator Current Limiter

239 Load Angle Limiter

The Load Angle Limiter prevents the synchronous machine from slipping out of

phase due to slippage of the rotor The load angle δ the difference in phase between the

rotor and the stator rotating field results mainly from the driving torque (active power P)

acting on the generator and the level of rotor current (field current) If the driving torque

remains constant a increase in the field current reduces the load angle δ The current load

angle δ at any moment is obtained from the generator current and generator voltage based

on a simplified model of the generator Whenever this calculated load angle δ exceeds the

preset limit angle the limiter increases the field current until the load angle has dropped

back to its permissible value The quadrature reactance Xq of the generator and the

network reactance Xe during normal operation must be adjusted on the regulator in order

to obtain the load angle δ The graph below shows the Power Chart for a salient-pole

machine with typical limiter characteristics

The purpose of a Power System Stabilizer is to use the generator excitation to

damp electromechanical oscillations between the network and the generator Depending

on the design of the generator and the requirements imposed for network stability its

main function will be either to damp the oscillations originating in the machine or those

from the network A synchronous generator working in a combined power network is in

principle an oscillating structure In order to produce a torque the magnetic field of the

rotor and the stator must form a given angle (referred to as the rotor displacement or load

angle δ) The electrical torque ME increases as the angle δ increases just as with a

torsion spring Because the ME of the generator and the mechanical driving torque MA

from the turbine are in equilibrium during steady-state operation the angle δ remains in a

given position Whenever this state of equilibrium between MA and ME is disturbed the

load angle slips of this rest position and change thereby the electrical torque ME The

torque attempts to restore the load angle to a stationary position Due to the mass inertia

of the turbinegenerator rotor however this can only take place aperiodically It does so

in the form of more or less effectively damped oscillations (again similar to the effect of

mass inertia on a torsion spring) In order to damp the oscillations there must be a

damping torque produced depending not on the electrical torque ME associated with the

angle but on the difference in frequency (Df) between the rotor and the stator rotating

field ie on the slippage This torque is produced mainly by the so-called damper

winding in the rotor but the dimensioning of this is subject to limits imposed by

considerations of design and economy Some further action is therefore needed to

increase the damping effect The following drastically simplified formula shows the

parameters upon which the amount of active power PE supplied by the generator

depends

PE = active power

It can be seen from the above relationship that the active power that the generator

transfers depends not only on the load angle δ but also on the field current If That means

that a transient change can be made in the active power PE and with that in the effective

electrical torque ME by varying the field current The principle of operation of the DVR

Power System Stabilizer becomes clear from a consideration of the oscillations in power

output and frequency (ΔPE Δf) and the vector diagram If it is assumed that oscillations

in the network frequency generates load oscillations with the mass inertia of the rotor

then the active load of the generator (eg MW-measured) is influenced with a sinusoidal

value -ΔPE (ME-MA = -ΔPE) By inversion of -ΔPE one obtains the fluctuation in

power provided by the rotor +ΔPE As is known the slip signal Δf follows +ΔPE with a

phase delayed by 90deg The +ME produced by the periodic changes in the load angle δ is

in phase with +ΔPE A good damping is attained if ME is varied in phase with the slip

Δf However this signal must also be advanced somewhat to compensate for the time

constants in the excitation circuit and the generator

As mentioned above the electrical torque ME can be influenced by varying the

field current To accomplish this a suitable control signal referred to as variable

disturbance compensation must be imposed upon the voltage set-point or the converter

control variable Ucontr As can be seen from the vector diagram by applying proper

weighting factors (K1 K2) and then adding together the signals -ΔPE and Δf an overall

stabilization signal can be produced that rotates in advance of the Df signal by any angle

desired between 0deg and 90deg Because the amplitude of -DPE remains proportional to the

amplitude of Δf a constant angle in advance of Δf results for the compensation of the

time constants referred to above The optimum weighting factors K1 and K2 for a

synchronous generator working to a power network depend on its operating point at any

moment and the external reactance of the network Normally the selection of a

compromise setting is good enough to attain stability in all operating points and for all

external reactance For special demands these settings must be parameterized as a

function of the external reactance (which means optional equipment Xe-Identification)

The Power System Stabilizer PSS is a section of the AVR computer program and is

processed once per network cycle The voltage at the generator terminals and the

generator current are measured in order to define the signals ΔPE and Δf The calculated

signals for _P_ (=PE) and Δf are then sent across DC filters ldquoDrdquo (real differentiators) that

transmit only the dynamic portion of the signals The ΔPE and Δf signals obtained in this

way are then weighted (multiplied by) with the factors K1 and K2 and sent to the

summing point of the voltage regulator

The PSS stabilization signal is imposed on the automatic voltage regulator only if

the following prerequisites are met

bull Generator on line

bull Generator power output gt the value F338

bull Generator voltage in a range between F33C and F33A

The stabilization signal is limited at the output from the PSS to the lower and

upper limits Flag defines whether the stabilization signal is introduced before or after the

PID filter (usually before the filter) Because the PID filter as noted above already takes

the ceiling factor Vp1 into account the PSS signal needs to be multiplied by Vp1 if it is

added to the voltage regulator following the PID filter (divider at the input to the

minmax limiter) This precaution prevents the DC filter ldquoDrdquo in the P-channel from

producing an unnecessary ldquostabilizationrdquo effect in the case of rapid changes in turbine

load As an alternative for the AVRrsquos Power System Stabilizer a stabilization signal from

an outside system can be imposed by activating the binary input ldquoPSS-SIGNEXTrdquo Flag

F340 can be used to select between an analog and a 12-bit signal and F33E to select the

polarity desired for that signal

24 The MANUAL Channel

241 Summary

The MANUAL channel (Channel 2) has been built as a simple field current

regulator

without additional limiters Its main function is to maintain the excitation of the generator

even if the AUTOMATIC channel becomes non-operational The MANUAL channel

also performs valuable service for purposes of testing commissioning and preventive

maintenance Its measurements regulator generation of firing pulses and power supply

are physically separate from those on the AUTOMATIC channel

242 Principle of Operation

All the functions of the MANUAL channel including the generation of firing

pulses have been implemented in a single electronic module the Gate Control Unit The

control variable Ucontr of voltage regulator is used as the reference value for generating

firing pulses on the principle known as ldquoramp controlrdquo (Comparison of Ucontr with

Usynsynchronous sawtooth signal) For further processing in the UN 0096 Intermediate

Pulse Stage the Gate Control Unit supplies six firing pulses at its output whose phase

position with respect to the synchronous voltage Usyn is in accordance with control

variable Ucontr An internal linearization ensures that the field voltage produced via the

firing pulses remains proportional to the control variable Ucontr throughout the entire

range As a result the circuit amplification of the control remains constant over the entire

range Whenever excitation is switched ON the set-point for Generator Voltage is set

automatically at the preset - ref Value This provision ensures that the generator voltage

always attains approximately its nominal value after the field flashing The Gate Control

Unit can be refunctioned ( by pre-selection with a switch ) for purposes of testing to act

as a purely firing pulse control In this case the control variable Ucontr is adjusted

directly using the RAISELOWER push buttons on the front of the module In this way

for example the relationship between the phase position of the firing pulses and the

control variable Ucontr can be checked easily

CHAPTER 3

PULSE SECTION

31 Pulse Generation and Amplification

The Gate Control Units of both AUTOMATIC channel and MANUAL channel

each supply six firing pulses for operating the 6-pulse thyristor bridges The low-power

pulse signals from these Gate Control Units are then amplified in the Intermediate Pulse

Stage galvanically isolated and then sent to the common pulse bus On the output end

the Intermediate Pulse Stage of the non-active channel) is always blocked The Gate

Control Units generate the pulses based on microprocessor control The reference voltage

used for the firing pulse phase location is the output voltage from the excitation

transformer (Usyn1 Usyn2) The commutation spikes of the synchronous voltage caused

by the converter are calculated prior to use of the voltage as a reference value and are

deliberately filtered out The lower limit for the firing pulses (double pulses) which are

offset from one another by 60deg is defined by the limit rectifier position (αmin) and the

upper limit by the limit inverter position (αmax) for the firing angle αmin and αmax can

be adjusted on the Gate Control Units using BCD (Binary Coded Decimal) switches

αmin ensures that the firing pulses will not be issued (premature firing) until there is

sufficient positive phase voltage on the thyristor involved αmax prevents a dangerous

ldquotippingrdquo of the thyristor bridge into the rectifier mode if the firing angle α is too large

(ldquolate firingrdquo) The critical factors determining αmax are the overlap time uumlmax (max

commutation time) and the ldquorecovery timerdquo of the thyristors (αmax lt 180deg - uumlmax - γ )

An external control signal can force the firing pulses into their inverter limit position

Other binary inputs can block or direct the firing pulses of the Gate Control Units so as to

produce freewheeling on the thyristor bridge During freewheeling the firing pulses for

the thyristor pair R and S are blocked and the pulse signals T+T- are engaged with

chains of pulses Both Gate Control Units (for the MANUAL amp AUTOMATIC channels)

contain a field current monitor that blocks the firing pulses immediately whenever the

current exceeds a preset threshold level In this case the field circuit-breaker is also

tripped via an output contact The purpose of these provisions is to prevent damage to

thyristors and thyristor fuses in case of a slip-ring short-circuit or to keep any damage

that does occur to a minimum The pulse signals are galvanically separated at the outputs

from the Intermediate Pulse Stage (with pulse transmitters) and are then directed to the

common pulse bus This transmission of the pulse signals to the pulse bus via passive

transmitters ensures a high degree of active channel autonomy Practically no possible

malfunctions on the inactive channel (including for example sustained pulses) affect the

active channel

32 Pulse Monitoring

The ldquoPulse Busrdquo and the pulse signals of the AUTOMATIC channel are

monitored This monitoring device consists of potential isolating stages and the common

monitor If the pulse monitoring of the ldquoPulse Busrdquo responds a switch-over is made to

MANUAL channel The function of the potential isolating stages is to couple the pulse

monitoring device to the pulse circuits without any feedback effect The pulse monitoring

checks the six pulse lines for the following malfunctions continuous or periodic failure

of one or more pulses Periodic occurrence of synchronous or asynchronous false pulses

Continuous pulses the pulse monitoring device can be tested while the machine is in

operation

CHAPTER 4

CONVERTER

Thyristor

The term thyristor usually refers to a family of four layer solid state device having

turn on characteristics that can be externally controlled by either current or voltage They

are also referred to as breakdown device because their working depends on avalanche

breakdown Thyristors have only two stages OFF and ON Thyristors have a similar

function to Uni-junctions they act as switches Thyristors use current flow as a switch

Thyristors have three states

1 Reverse blocking mode mdash Voltage is applied in the direction that would be

blocked by a diode

2 Forward blocking mode mdash Voltage is applied in the direction that would cause

a diode to conduct but the thyristor has not yet been triggered into conduction

3 Forward conducting mode mdash The thyristor has been triggered into conduction

and will remain conducting until the forward current drops below a threshold value

known as the holding current Converter is a semiconductor device which converts ac

input voltage into a constant dc output voltage In present excitation system three phase

fully controlled thyristor converter is used

Because of the following advantages thyristor converters are used

a) Thyristors are used for high power applications ie up to 10Kv3500A1KHz

b) Having high reliability and low losses

c) Uni-directional device like diode

d) Itrsquos operation as a rectifier which are low resistance in forward conduction

mode and high resistance in reverse conduction mode

PROTECTION OF THYRISTORS

For reliable operation of a thyristor demands that its specified ratings are not

exceeded When Subjected to or over voltages During the turn - on of SCR didt

prohibitively large False triggering of SCR by high value of dvdt andSpurious signals

between gate and cathode may leads to unwanted turn ndash on

DIDT AND PROTECTION

When thyristor starts conducting in forward conduction mode and is turned on by

gate pulse The anode current increases rapidly whole area of the gate to Cathode

junction then hot spots will be formed near the gate connection this locality of heating

destroys the thyristor Thyristor thermal time is constant The causes due to faults and

short circuits or surge currents Electronic crowbar protection is used against the over

voltages The rate rise of anode current must be kept at the time of turn on below the

rated or specified limiting value The didt value maintained below limited value by using

a inductor also called ldquodidt inductorrdquo in series with anode circuit The locality of heating

is avoided by applying gate current but not greater the maximum gate current

DVDT AND OVER VOLTAGE PROTECTION

With forward voltage across the anode and cathode of a thyristor the two outer

junctions are forward biased but the inner junction is reverse biased This reverse biased

junction J2 has the characteristics of a capacitor due to charges existing across the

junction In other words space-charges exist in the depletion region around junction J2

and therefore junction J2 behaves like a capacitance If the entire anode to cathode

forward voltage Va appears across J2 junction and the charge is denoted by Q then a

charging current i given by Eq (46) follows

i = dQdt =d(Cj Va )dt

= Cj (d Va dt) + Va(d Cj dt) helliphelliphelliphellip(46 a)

As Cj the capacitance of junction J2 is almost constant the current is given by

i = Cj (d Va dt) helliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip(46 b)

If the rate of rise of forward voltage dVadt is high the charging current i will be

more This charging current plays the role of gate current and turns on the SCR even

when gate signal is zero Such phenomena of turning-on a thyristor called dvdt turn-on

must be avoided as it leads to false operation of the thyristor circuit

For controllable operation of the thyristor the rate of rise of forward anode to

cathode voltage dVadt must be kept below the specified rated limit Typical values of

dvdt are 20 ndash 500 Vμsec False turn-on of a thyristor by large dvdt can be prevented by

using a snubber circuit in parallel with the device thyristor are very sensitive for over

voltage than the semiconductor devices

Over voltage transients are perhaps the main cause of thyristor failure

In thyristor there are mainly two types

1 Internal over voltages

Due to the commutation of the thyristors large voltages are generated internally

Because of the series inductance of the SCR circuit the large transient voltages L didt

produced This voltage several times the break over voltage of the device then thyristor

destroys permanently

2 External over voltages

External over voltages are caused due to the interruptions of current flow in an

inductive circuit and also due to the lightening strokes on the lines feeding the thyristor

system For the reliable operation of thyristor the over voltages must be suppressed by

adopting suitable techniques

Suppression of over voltages

The RC circuit called snubber circuit is connected across the device to protect In

order to keep the protective components to a minimum the thyristors are chosen with

their peak voltages ratings are 25 to 3 times of the normal peak working voltage

ldquoselenium thyrector diodes metal oxide varistors or avalanche diode suppressers are

commonly employed for protecting the thyristor circuit against the over voltages

Gate protection

Gate circuit should also be protected against the over voltages and surges Over

voltage at gate circuit can cause false triggering of the SCR may rises the junction

temperature behind specified limit leading to its damage Protection against over

voltage can be achieved by connecting a ZD across the gate circuit and a resister is

connected in series with gate circuit to protect against the s A capacitor and resister are

connected across gate to cathode to by pass the noise

41 Final Pulse Stages

The Final Pulse Stages adapt the output pulses from the Intermediate Pulse Stage

(pulses on the pulse bus) to the gate currents needed for the thyristors Each thyristor

bridge is equipped with its own Final Pulse Stage Each Final Pulse Stages is provided

with a power supply module The amplified output pulses from the Final Pulse Stages

start as a short strong steep pulse with an amplitude approx 2frac12 times that of the main

pulse This initial pulse edge assures proper firing of the thyristors being triggered

Subsequently the weaker part of main pulse keeps firing conditions steady As already

mentioned the Final Pulse Stages and their associated thyristor bridges form single units

All six pulse outputs from a Final Pulse Stage can be blocked by an external control

signal so that all thyristors in the associated thyristor bridge will block the current A

blocking of the pulses is initiated whenever there is a malfunction in the associated

thyristor bridge

42 Converter Power Section

The thyristor converter consists of three independent parallel rectifier blocks TY1

to TY3 which are all in service Even if one block fails the remaining blocks take over

automatically the full design current of the excitation circuit During normal operation

(with ideal current share) and all three bridges in operation each of these blocks has to

carry only (n-2)n (ie33)of its design current If 2 thyristor bridges fail the excitation

is limited Only when all three bridges fail the excitation is switched off Each thyristor

bridge arm is equipped with current flow monitoring CTrsquos Failure of conduction in any

arm is identified by a Current flow monitoring module

43 Converter Cooling

A cooling system is needed to dissipate heat losses in the converter blocks and

electronics Each converter block has therefore been equipped with a fan supplied with

power from the converterrsquos primary voltage (via transformer ndashT8 in field flashing

cubicle) The fans are protected with motor protection circuit breakers An air flow

monitoring unit is provided for monitoring the air flow through the thyristor bridge If a

circuit breaker failure is detected or if the air flow monitor drops off at one of the

thyristor bridges the bridge involved is immediately set out of operation by blocking its

firing pulses

44 Thyristor Converter Monitoring

A thyristor bridge in which defects occur that could threaten the safety of

operation or cause secondary damage is switched off automatically ie its firing pulses

are blocked This happens whenever A thyristor fuse is blown The fuses are monitored

individually with micro switches The Final Pulse Stage fails which is detected by

internal monitors (supply voltage sustained pulse short-circuit on the output end) The

power supply to the fan fails fan air flow as monitored by the Air flow monitor fails or

is insufficient Isolator on ACDC side is open

CHAPTER 5

Field Current Circuit Excitation Transformer

51 Field Circuit Breaker

The circuit-breaker in the field circuit is used to isolate the field circuit from the

converter It is capable of switching off the synchronous machine from full load under the

maximum conditions of a 3-phase short-circuit In addition to its main contacts the field

circuit-breaker also has a de-excitation contact with which the field energy stored in the

field can be dissipated across the de-excitation resistor The de-excitation contact closes

shortly before the main contacts open so as to ensure proper commutation of the field

current from the main contacts to the de-excitation contact when the breaker is switched

off The field circuit-breaker is switched on by electromagnetic force and is kept switched

on by a mechanical latch When the latch is released by a trip coil the circuit-breaker

opens The circuit-breaker also has auxiliary contacts that report its status

52 Field Flashing

In shunt supplied excitation circuits (excitation transformer connected to the

generator terminals) the generator does not have enough remnant voltage for a generator

voltage build-up via the converter In this case a field flashing circuit is provided It

consists of the field flashing contactor the diode bridge and a transformer used to adapt

the auxiliary input voltage to the voltage needed for field flashing when power is

supplied from the auxiliaries network

Fig Field Flashing

Because the field flashing contactor is not able to switch off the energy stored in

the field the control ensures that the contactor can only reopen if the field circuit breaker

has already been opened (generating the TRIP order) or in a normal field flashing

sequence when the converter has taken over the field current Field flashing occurs in the

following stages

1048729The excitation is switched on closing the field flashing contactor ( Field

Circuit Breaker is already closed )

1048729The start-up excitation current flows through the rotor driving the generator

voltage up to approx 15 U

1048729After about 10 U the firing pulses to the converter are released and it begins

to excite the generator to its rated voltage

1048729After about 30 U the field flashing contactor opens (with no current since

the converter is now supplying the current)

The diode bridge at the input to the field flashing contactor prevents a feed-back

from the converter to the source of field flashing while the contactor is still closed

53 De-excitation

When malfunctions occur the stored field energy must be dissipated as quickly

and safely as possible to protect the generator This is done by the converter the field

circuit-breaker and the de-excitation (discharge) resistor

De-excitation (with opening of the field circuit-breaker) takes place in the following

stages

1048729The converter drives to its inverter limit position (negative ceiling voltage)

recovers a portion of the field energy into the network A trip command is given to the

field circuit breaker

1048729The de-excitation contact closes diverting the field voltage to the de-excitation

resistor

1048729Then immediately the main contacts open building voltage The field voltage

commutates to the de-excitation resistor

1048729The current diminishes at a given time constant TE

(With linear resistance TE = Lf (Rf + Re))

Due to the reversal of the field voltage by the converter the field current

commutates from the main contacts of the field circuit-breaker to the de-excitation

resistor in a very early phase This reversal of the field voltage prevents burn-off on the

main contacts and provides effective protection for the field circuit-breaker Depending

on the operating policy an operational shut-down of the excitation can also be effected

with the field circuit-breaker closed This method is useful mainly when the excitation is

switched on and off frequently In this case the converter is merely driven into the

inverter limit position so that the field energy is recovered into the network The

converter then blocks since it is supplying positive current only

54 Excitation Transformer

The excitation transformer matches the generator voltage to the field voltage

(required ceiling voltage) It also serves as a commutation reactance for the thyristor

converter and as a potential isolator between the network and the excitation circuit In

addition the transformer functions as a current limiter in that it makes it possible to keep

any short circuits in the excitation circuit under better control The excitation transformer

is equipped with temperature monitoring probes which set off an alarm when the

temperature exceeds a first max limit and then trips the excitation if the temperature

continues rising to a second (higher)limit

CHAPTER 6

Monitoring and Protection

61 Excitation Monitoring

611 General Information

The main goal of Excitation Monitoring is to make optimum use of the

redundancies provided in the excitation system and to give alarm whenever a malfunction

makes these redundancies unavailable The field current is monitored to see that it does

not exceed a maximum level and if necessary a switch-over to the MANUAL channel is

initiated In addition the criterion for switching off the field flashing is generated The

excitation Monitoring consists of an autonomous processor system

612 over current Alarms

In the Excitation Monitoring the limits for are set at higher levels than the

settings on the Field Current Maximum Limiter Whenever the current exceeds 110 of

the nominal field current contact R1 and the binary output associated with it are

activated immediately If field current remains gt 110 then after a preset inverse-time

has lapsed relay R2 and - after a further delay - relay R0 and the binary outputs

associated with them are activated Parameters match the measurements for If1 and If2 to

the nominal value for field current so that the internal values can be processed and read as

pu values It can be used to falsify the actual value of the field current If (to raise it) so

as to cause a response from the alarm limits for purposes of testing The processed If

signal is always taken from on the active channel (CHANNEL I OR CHANNEL II) As

long as the field current If is above the threshold value 11 Ifn its peak value is

measured This is stored (until RESET) and can be read at any time on the Micro-

Terminal Once the value of If exceeds 11 Ifn integration of this value starts Whenever

the integrated time-current value (intisup2dt) exceeds the preselected reference value the

alarm OVER CURRENT INVERSE-TIME is set off and a command is simultaneously

issued to switch over to the stand by AUTO channel Software switch F758 enables the

three over current alarm functions (R0 R1 R2) and selects one of three possible inverse-

time curves T1 T2 or T3 Within the characteristic curve (T1 T2 T3) selected the

desired limit curve for response is set using the factor F216

613 Switch-Off Criterion for Field Flashing

The Excitation Monitoring supplies the criterion for switching off the field

flashing Whether this criterion is activated based on the actual value for generator voltage

Ug or for field current If or both depends on the settings of the two threshold values

F200F202 (0 setting means that the output is always ldquological 1rdquo) The measurements

Ug12 and If12 are switched over depending on the present status of the channels

(Channel 1 or Channel 2 ON) Whenever Flag F750 is not activated the binary output is

fixed at ldquological 1rdquo

614 Storage of Alarm Status

The outputs of the over current alarms (R1 R2 R0) and the messages NO

FAILURE MONITORING PARAMETERS CHANGED are stored messages can be

erased by activating the input ldquoGENERAL RESETrdquo or by using the RESET button on the

front of the module Erasure with the input ldquoGENERAL RESETrdquo is effective only if the

situation causing the alarm or the malfunction is no longer present Whenever the self-

diagnosis equipment in the processor detects a malfunction the output NO FAILURE of

MONITORING is set at ldquological 0rdquo (= alarm) The alarm ldquoPARAMETERS CHANGEDrdquo

is activated whenever parameters or settings of software switches have been changed via

the Micro-Terminal

615 Actual Value Monitoring

The actual values for generator voltage Ug synchronous voltage Usyn and field

current If are monitored for malfunctions This monitoring is active regardless of whether

or not the generator is in operation Essentially when the generator is in operation the

measurements are monitored by comparing the signals (the smaller signal reading is

detected as incorrect) When the generator is not in operation the measured data are

monitored for extreme values The percentage of deviation permissible in the

measurement signals being compared

(Ug1ampUsyn1 Ug1ampUg2 Ug2ampUsyn2 If1ampIf2) is defined by parameters F208 and

F20AIf the excitation transformer is being supplied from an auxiliary power source (no

shunt operation) the values of Ug and Usyn will be different in some operational

conditions

In that case Flag F75A can be used to deactivate comparative measurements Ug1 Usyn1

and Ug2 Usyn2 The ldquoprimary monitoringrdquo compares Ug1 with Ug2 and generates the

messages that Channel 1 or Channel 2 has suffered a malfunction Whenever Ug1 lt Ug2

and the binary message from CH1 reports no malfunction a malfunction on

Measurement Channel 1 is reported (Ug1Usyn1 FAILURE) A similar malfunction is

also present whenever the binary message CH1 DISTURBANCE is reported and a

discrepancy is detected between Ug1ampUg2 The generation of the alarm ldquoUg2 Usyn2

FAILURErdquo is analogous to that for Channel 1 The ldquosecondary monitoringrdquo compares

Ug1 with Usyn1 amp Ug2 with Usyn2 This is enabled whenever the binary message of the

comparison channel reports a malfunction or whenever both binary messages report no

malfunction - but both secondary monitors report a malfunction As long as the secondary

monitoring is blocked the differences Ug1neUsyn1 or Ug2neUsyn2 trigger malfunction

signals for the measurement channel involved (suspicion that there is a corresponding

error in Usyn) The measurement channel malfunctions are enabled operationally

whenever after excitation has been switched on generator voltage Ug exceeds the value

set on F204 The voltages Ug1 and Ug2 are checked 16 seconds after the excitation is

switched off to see that they do not exceed the limit value F210 that applies to both of

them At the same time g1Ug2Usyn1Usyn2 are checked for extreme values (gt or lt

the operating range) Monitoring for extreme values is likewise enabled during normal

operation ( Excitation ON and Ug gt F204 ) Flag F754 is used to enable or block the

malfunction signals to the binary outputs Basically the monitoring of the actual values

for If1If2 functions like that of the Ug1Ug2 monitoring

62 Excitation Protection

621 General Information

The Excitation Protection switches off the excitation (and de-excites the machine

rapidly) whenever a danger arises that threatens the excitation transformer the converter

or the generator Generally limiter or monitoring functions precede the emergency trips

and these normally respond before the Excitation Protection must initiate a trip

Protective trip commands are issued directly to the field circuit-breaker from potential

free contacts of the board via the trip relays They are directed redundantly to the

operative field circuit-breaker ldquoOFFrdquo command

622 Protection against Excitation Transformer Overheating

This equipment monitors the excitation transformer for overheating in the

windings that could result from over current short-circuits or inadequate cooling The

monitoring uses temperature monitoring modules in conjunction with temperature

sensors built into the windings Normally the temperature is monitored in two stages the

first stage sets off an alarm the second causes a trip of the excitation

623 Rotor Over voltage Protection

Malfunctions in the generator circuit (eg terminal short-circuit failed

synchronization asynchronous operation) cause induced negative field currents that

produce high voltages in the field circuit These must be restricted to a level with a

sufficient safety margin below the insulation capacity of the field winding (test voltage)

and also below the peak blocking voltage of the converter thyristors The crow bar

employs spark gap elements to detect over voltages in the field circuit Whenever they

respond the associated thyristors are fired immediately switching the de-excitation

resistor parallel to the field The de-excitation current generated thereby initiates an

excitation trip via a supervision circuit causing an immediate opening of the field circuit-

breaker The malfunction isets off an alarm and an internal malfunction is indicated at the

cubicle

TEST VALUES OBTAINED WHEN EXCITATION IS RAISED

TEST

SNO PARTICULARS ACTUAL VALUE OBTAINED VALUE

1 VREF 996 100

2 VACT 997 1003

3 IFACT 735 765

4 IGACT 703 707

5 ACTIVE

POWER

703 705

6 REACTIVE

POWER

101 142

7 POWER

FACTOR

099 IND 098 IND

8 ACTIVE

CURRENT(IR)

705 703

9 REACTIVE

CURRENT(IX)

102 138

10 POWER

ANGLE

566 546

11 FIRING

ANGLE

640 632

VALUES OBTAINED WHEN EXCITATION IS LOWERED

SNO PARTICULARS ACTUAL

VALUE

OBTAINED VALUE

1 VREF 100 997

2 VACT 100 997

3 IFACT 787 761

4 IGACT 839 837

5 ACTIVE

POWER

839 830

6 REACTIVE

POWER

155 110

7 POWER

FACTOR

098 IND 099 IND

8 ACTIVE

CURRENT(IR)

835 830

9 REACTIVE

CURRENT(IX)

140 94

10 POWER

ANGLE

613 DEG 631 DEG

11 FIRING

ANGLE

601 DEG 599 DEG

CONCLUSION

For generating the EMF in stator winding excitation is required to the rotor of a

generator There are two types of excitation

1 Static excitation system

2 Brushless excitation system

A certain disadvantage in brushless excitation system is the slow response time of

the field in case of fast load changes specified No slip-rings and brushes direct

measurements of the field parameters not possible

To avoid all loses static excitation is used Since it does not have any rotating

parts mechanical loses and windage loses This system has fast response and speed

control While preferring this excitation system there are no limitations for the

redundancy of Thyristor bridge circuits

Static excitation has fast field discharge by resistor and inverter operation direct

measurement of field quantity is possible The meaning of excitation is nothing but

continuous supply of DC current (ie field current) to the rotor to buildup required

output voltage in the stator

Field current is changed with respect to the change of load so the digital

automatic voltage regulator (DAVR) is used to regulate the output voltage according to

the load variations

So we conclude that static excitation system with DAVR is preferred since it is

having excellent dynamic performance and better options for R amp M

the AUTOMATIC channel also contains various limiters and corrective control circuits

to ensure the use and stable operation of the synchronous machine up to its operating

limits This channel possesses a Gate Control Unit with a subsequent Intermediate Pulse

Stage to generate the firing pulses for the Thyristor converter During normal operation

the Intermediate Pulse Stage of AUTOMATIC Channel is active and transmits the firing

pulses galvanically separated to the common pulse bus at the input to the Final Pulse

Stage Various monitoring functions of the AUTOMATIC channel and pulse monitoring

on the common pulse bus initiate an automatic switch-over to stand by Channel

(MANUAL)in case of a malfunction

Channel 2 (the MANUAL channel) is built as a simple field-current regulator with a PI

control algorithm It serves as a back-up channel in case of a malfunction on the

AUTOMATIC channel Manual channel performs valuable service for testing

commissioning and preventive maintenance The MANUAL channel has its own Gate

Control Unit (the software for the If regulator is also implemented therein) and its own

intermediate pulse Stage During normal operation (AUTOMATIC) the output pulses

from Intermediate Pulse Stage are blocked from reaching the pulse bus Various

monitoring on the MANUAL channel initiate an alarm in case of a malfunction while the

MANUAL channel is on stand-by If the MANUAL channel suffers a malfunction while

it is in operation the excitation is switched off (TRIP) Both channels are equipped with

tracking equipment so that the inactive channel always generates the same control

variable as the active channel during steady-state operation

This ensures smooth switch-over from Automatic to Manual channel and vice

versa To ensure that the MANUAL channel will in a switch-over initiated by a

malfunction take over the operating point of the machine as it was prior to the problem

the response of the tracking for the MANUAL channel is set relatively slow In addition

to the pulse monitors (ldquoSUPERVISIONrdquo) shown in the basic circuit diagram the

excitation system has an autonomous Excitation Monitoring As one of its functions this

equipment monitors for field currents that exceed acceptable maximum limits It initiates

an emergency switch-over to the MANUAL channel whenever the field current exceeds

the preset limit If even after such a switch-over the field current does not drop back to

3

the permissible level the excitation is switched off by Excitation Protection The most

important measuring inputs for the excitation system (If Ug Usyn) are redundant (2-

fold) The Excitation Monitoring checks these measuring inputs for discrepancy and

plausibility An alarm is always initiated in case of malfunction In certain cases a

switch-over to MANUAL channel is also initiated The excitation system contains an

Excitation Protection to protect the excitation transformer the converters and the

synchronous machine The protection system can detect short-circuits in the excitation

circuit and keep secondary damage within acceptable limits by a quick tripping of the

excitation and an opening of the generator breaker An overheating of the excitation

transformer first sets off an alarm (at a given preset limit) and then likewise initiates a

protective shut-down at an even higher limit The over voltage protection in the de-

excitation equipment provides an autonomous protective function for the rotor and the

rectifier This protection system monitors the field voltage in both polarities for over

voltage and if necessary de-energizes the field via the de-excitation resistor

12 Principle of primary power supply

In the shunt excitation system the excitation transformer also provides

the power supply for the electronic equipment and the converter fans So a failure of the

auxiliary supply to the converter fans does not cause a shutdown of the excitation When

the auxiliary supply fails the supply to converter fans is switched over to Excitation

transformer OP with a contactor A station battery supply is absolutely necessary for the

control of the field circuit breaker It is the power source for the electronic devices till the

generator is able to supply voltage Auxiliary power to the field flashing equipment must

be present in order to build up the generator excitation The power supply for standstill

heating and the cubicle lighting is also from Station Auxiliary Power Supply and is of

secondary importance for operation of the plant Power supply to rotor earth fault

detection circuit too is from Station Auxiliary Supply The two synchronous voltages

Usyn are each supplied to the AUTOMATIC channel and the MANUAL channel

separately across transformers The Gate Control Units need these voltages to enable

4

them to issue the pulses at a given firing angle relative to the input voltage of the

converter

5

CHAPTER 2

Digital Automatic Voltage Regulator (DAVR)

21 Principle of Operation of the Regulator (DAVR)

To regulate the voltage and the reactive power of a synchronous machine the

field voltage must be adjusted quickly to the changes in the operating conditions (with a

response time that does not exceed a few ms) To accomplish this analog control systems

include amplifiers which make continuous comparison of the actual values against the

reference values and vary the control variable to the converter with almost no delay Most

of the delay that occurs originates in the converter since the firing pulses for changing

the rectifier phase angle are only issued periodically (every 33 ms)

The DVR digital voltage regulator calculates the control variable from the

measured and reference data in very short time intervals This results outwardly in a

quasi-continuous behavior with a negligible delay time (as in an analog regulator) The

calculations are made in the binary number system Analog measurement signals such as

those for generator voltage and generator current are converted into binary signals in

analogdigital converters The set-points and limit values have already been defined in

digital (binary) form An understanding of the actual computation processes in the digital

voltage regulator is not necessary for operation preventive maintenance or

troubleshooting Like the operator of a pocket calculator or a personal computer all the

operator needs is to know how to operate the instrument and the programming for this

working tool For that reason we will explain below only the principle division of work

among the various modules and the flow of data processing The purpose is above all to

make clear how the processor system has been integrated into the rest of the power

electronics system

6

22 Basic Structure of the Processor Systems

7

The signal processors 25 analog inputoutput modules Each of these processor

systems has a common bus circuit and output and the control lines There is a specific

range of addresses assigned to each assignment Board including the power supply bus

the address lines the two data lines to the input calculates the reactive current (I sin φ)

and the active current ( Icos φ) With these two channel processor Synchronized with

these interrupts (ie with the phase positions of current Ig the field current If and the

synchronous voltage Usyn From the exchange data with the microprocessor card across

the two data lines generator voltage Ug) this processor measures the generator current

Ig and then hardwired connections or multi-conductor cables Binary and analog

inputoutput modules ie for galvanic isolation and adaptation to the electronics level

The most important input interrupts per period to trigger the cycles for processing actual

values in the AUTOMATIC module on the processor bus) for filtering and further

processing

Monitoring each consist of the central microprocessor module and binary and

parameters to the AUTOMATIC channel are the generator voltage Ug the generator

peripheral unit Ug Ig and Usyn are sent to the Interrupt Generator (plug-in peripheral

units (wall-mounted units) peripheral units are used for preprocessing signals from

external measurement circuits power supply units Signals are exchanged among these

processor systems via processed across separate peripheral units for each channel These

processor working on the bus (a house address that can be adjusted using a switch)

systems The AUTOMATIC channel the programmable controls and the Excitation The

actual values measured from AUTOMATIC channel and MANUAL channel are The

AUTOMATIC channel and the MANUAL channel each have their own The digital

voltage regulator is broken down into several autonomous microprocessor The inputs and

outputs of the processor systems are directed across voltage-isolating The Interrupt

Generator also uses the 3-phase Ug signal to generate the 12 themselves contain a limited

number of hardware inputs and outputs with fixed equipment Whenever addresses from

this range are called up the signal processing module can results the processor is then

able to derive further operating parameters such as the load angle the active power etc

The functions of all microprocessor systems other than the programmable controls

have been accomplished in firmware The non-varying standard function modules can be

configured to the design desired for plant-specific purposes using software switches

(KFlags) Thus for example the stored status of a K-Flag determines whether or not a

Limiter is active and whether the de-excitation or the excitation limiters take precedence

Because these K-flags determine the software Scope of Supply for the installation they

cannot be changed permanently via the Micro-Terminal In this way they differ from

such setting data as the values of the parameters for the PID filter of the voltage regulator

or the set-points for the limiters These values can be permanently changed using the

Micro-Terminal Communication is possible with each of the processor systems via the

Micro-Terminal by plugging on the connecting cable In this way signals within the

processor and setting parameters can be viewed analog signals can be issued and the set

parameters can be altered temporarily (F range) or permanently (C range) Unlike the

other processor systems the programmable controls do not include any firmware for

realization of the functions They have been designed so that the designer can adapt and

change their functions easily using the ldquoFunctional Block Programming Language P10

Digital and analog functions can be implemented in practically any degree of complexity

desired using the P10 functional blocks The control variable of the voltage regulator

(AUTOMATIC channel) and the control variable of the field current regulator

(MANUAL channel) are each processed in separate Gate Control Unit and formed into a

chain of pulses at the appropriate firing angle The pulses of the active channel are

directed to the pulse bus via the associated Intermediate Pulse Stage The pulses for each

converter block are amplified sufficiently in Final Pulse Stage to fire the Thyristor

231 General Information

The functions of the automatic voltage regulator AVR are

1 to regulate the generator voltage

2 to regulate the effect of the reactive andor active current on the voltage

3 to limit VoltHz

4 to limit max and min field current

5 to limit inductive stator current

6 to limit capacitive stator current

7 to limit the load angle

8 to stabilize the power system

Block Diagram shows the software structure of AUTOMATIC channel The

generator limiters not provided for the installation in question (optional equipment) are

identified in this overview as ldquoNot Suppliedrdquo The parameter values signal values and

software switches (flags) marked with addresses (hexadecimal numbers) can be viewed

and altered via the Micro-Terminal The values selected are displayed in sec pu Hz

etc and can where necessary be changed directly in these formats The plant-specific

settings of the variables and the flags can be obtained from the Test and Commissioning

Report This block diagram provides information about the important functions and

possible settings of the AUTOMATIC channel For the sake of clarity no detailed

presentation has been given of special functions such as tracking circuits initializations

etc The page heading cross-refers this overview to the various sheets of the schematic

diagram Binary signals are shown in broken lines analog signals in solid lines The

corresponding text designations in the schematic diagram can be used for identification of

the input signals (hardware inputs) The only analog output signal from the automatic

voltage regulator control variable Ucontr is sent via the data bus (CRU bus) to the Gate

Control Unit Most of the binary messages (outputs) from the AVR are of no interest

functionally and they have been omitted for the sake of clarity The basic structure of the

digital voltage regulator and the limiters is simple This is necessary in order that the

behavior of the regulatorslimiters will remain calculable and understandable in all

operating situations and that there will be no problem in adjusting and optimizing them

The central PID filter in the digital voltage regulator defines the dynamic response of the

closed-loop controls both in the voltage regulator mode and after limiters have

intervened The ldquocontrol deviationrdquo at the input to the PID filter is either the control

deviation for voltage the control deviation of a de-excitation limiter (the value

determined by minimum value selection) or the control deviation of an excitation limiter

(the value determined by maximum value selection) Flag F730 (ldquoPRIORrdquo) is used to

determine whether the exciting (Min value) or the de-exciting signal takes precedence on

the minmax value limiter (normally F730 = 1111 ie the de-exciting signal takes

precedence) With the exception of the Minimum Field Current Limiter all other limiters

have variable factoring multipliers of the signal outputs so that they can be adjusted

individually together with the common PID filter which has been optimized for voltage

regulation The setting parameters for this PID filter are as follows

Vo = KR Static amplification

1

Ta = ---- Integration time constant

Tc1

Vp Proportional amplification

1

Tb = ---- Differential time constant

Tc2

Vinfin Amplification of high frequencies

The BODE diagram below shows the assignment of settings in accordance with

DINIEC standards based on a typical example

The PID filter amplifications Vo Vp and Vinfin can be adjusted in pu values But

the ldquoceiling factorrdquo pl+ must be adjusted correctly with parameter F310 if the total

amplification (circuit amplification) of the control circuit is actually to conform to the

pu settings This factor must agree with the ldquoexternalrdquo amplification ie with the

ceiling value of the transformer- converter circuit

Ceiling factor(pl+) = Ufmax Ufo

in which Ufmax = ceiling field voltage

Ufo = no-load field voltage

To attain a suitable response of the AVR when starting excitation

(ldquoEXCITATION ONrdquo) it may be necessary to change the proportional amplification of

the regulator during this phase Vp2 (transiently activated) and Vp1 (permanently

activated) can be adjusted for this purpose For example the value of Vp2 takes effect

immediately once the excitation is switched on and remains effective for a period as set at

F30C Once the period F30C (eg 5 sec) has expired Vp shifts over to Vp1 (becomes

the steady-state Vp) at the rate of change set The standard operating mode for the PID

filter is voltage regulation for which the discrepancy between the voltage set-point and

the current value for generator voltage Ug (the control deviation) is supplied at the input

To compensate for the voltage drop in the block transformer or whenever several

generators are operating to the same distributing bus the generator voltage must be

varied in proportion to the measured generator current (droop influence) To accomplish

this the voltage set-point is varied as a function of the measured reactive current IX

andor active current IR Flag F712 enables the IX droop Flag F710 the IR droop The

desired compensation is set in F282 and F286 respectively Flags F284 and F288 are used

to select whether this droop influence is to increase the voltage or to reduce it

(compensation) Combined influence of the active and reactive currents is attained by

enabling both droops IX and IR Flag F716 activates a so-called ldquoSoft-Startrdquo at the

starting of excitation This ldquoSoft-Startrdquo ensures that the voltage set-point integrates from

0 to 100 within the time set on F290 when the excitation is switched on

(ldquoEXCITATION ONrdquo) A ldquosmoothrdquo excitation of the generator can be achieved in this

way whenever there is no demand for a quick excitation

232 Voltage Set-Point

Various signals and settings control and limit the voltage set-point F270 For

example the values of F254 and F252 define the normal operating range possible for set-

point adjustment (eg 90 110) using external control commands (control room local

operatorrsquos panel superposed control system) The effective set-point adjustment rate is

governed The set-point can be set at the values of F250 and F256 by activating

appropriate control commands for ldquoSETrdquo input Enabling Flag F71A and activating a

binary input prior to switching on the excitation (ldquoEXCITATION OFFrdquo) sets the Ug set-

point at the value of UAUX This makes it possible for example to ensure that the

generator voltage will agree exactly with the network voltage after the voltage build-up

An external value with variable amplification can be added to the Ug set point by

enabling F724 (for example for stability tests)

233 Regulator Tracking in MANUAL Operation

Whenever the AUTOMATIC channel is not in operation (the MANUAL channel

is ON) a follow-up equipment ensures a smooth switch-back to the AUTOMATIC

mode will always be possible To track the voltage set-point is shifted by means of

RAISELOWER pulses from the Gate Control Unit so that control variable Ucontr at the

output from the PID filter is held steady and identical to the control variable Ucontr from

the MANUAL channel Because this tracking must react slowly resultant transient

control deviations resulting from the amplification in the PID filter might cause severe

interference with control variable Ucontr

To prevent this the follow-up equipment intervenes on the regulators mixing

point with a corresponding compensation signal

234 Ugf Limiter

At under frequency the Ugf Limiter reduces the generator voltage so as to

prevent saturation effects in the supply and measuring transformers To adjust this

limiter the max permissible generator voltage at rated frequency is defined and set

When any under-frequency occurs the generator voltage is thus reduced in proportion to

that setting

235 Field Current Maximum Limiter

The Field Current Maximum Limiter is provided to protect the generator rotor

from s occurring in steady-state and transient operation High field currents are normally

the result of a sharp drop in network voltage or of an improper raising of the voltage set-

point by the operating staff The field current is held steady at the value TH1 ie at the

maximum thermal value permissible for the excitation circuit and the rotor In order that

the generator can support the power network with its transient overload capacity during

brief collapses in voltage a temporary switch-over is made to the transient limit MAX1

(a higher setting) When the generator or the converter is operating at a reduced capacity

These limits TH1MAX1 can be switched over to the lower settings TH2MAX2 by

activating the corresponding binary signals The switch-over from the thermal limit

TH12 to the transient limit MAX12 can be configured in one of three ways

a) Depending on the over current with -dUdt ENABLE

Flag programming F418 = any setting desired F41A = 0000

This variant enables the transient value MAX12 whenever a collapse of voltage

in the network is detected The ENABLE time is fixed and can be set The example

below shows the typical behavior of the limiter configured in this way

b) Dependent on the time integral with -dUdt ENABLE

Flag programming F418 = inactive F41A = 1111

This variant likewise enables the transient value only when a collapse of network

voltage has been detected However the switch-back to the thermal limit is not made

dependent upon the time itself but on the calculated time integral intisup2dt of the The setting

on Parameter F414 in spu takes into account the time the rotor needs to cool down ie

the rate of temperature change in the case of intermittent operation The example below

shows how the timing of the switch-back to the thermal limit depends on the present

value for intisup2dt

The time integral is based on the formula

Example The setting of =isup2dt equivalent to Version a (F416) at a constant 16 times the

nominal field current for 10 seconds (with TH12 = 105) is

c) Dependent on the time integral without any preconditions

Flag programming F418 = 1111 F41A = 1111

In this variant the transient becomes available without any prior conditions

(without a -dUdt ENABLE) with the time integral intisup2dt

237 Inductive Stator Current Limiter

The Inductive Stator Current Limiter holds the stator current Ig within permissible

limits while the generator is in the ldquoover-excitedrdquo operating range by reducing the field

current accordingly The setting TH (thermal limit) provides the limit against stationary s

that might occur To take advantage of the generatorrsquos transient overload capacity a

switch-over is made to the higher setting MAX The principle of operation of this switch-

over to the value MAX permissible only transiently is identical to that employed for the

field current limiter (refer to the description above) When the drive output from the

turbine is very high stator current may exceed permissible limits even while inductive

loading of the generator is low In this case if the stator current limiter is not kept from

influencing the field current the control circuit will oscillate back and forth between the

Inductive Stator Current Limiter (de-

exciting) and the Capacitive Stator Current Limiter (exciting)The output signal of that

function then dominates the control variable of the Ig-dependent limiter via a maximum

value selection

238 Capacitive Stator Current Limiter

239 Load Angle Limiter

The Load Angle Limiter prevents the synchronous machine from slipping out of

phase due to slippage of the rotor The load angle δ the difference in phase between the

rotor and the stator rotating field results mainly from the driving torque (active power P)

acting on the generator and the level of rotor current (field current) If the driving torque

remains constant a increase in the field current reduces the load angle δ The current load

angle δ at any moment is obtained from the generator current and generator voltage based

on a simplified model of the generator Whenever this calculated load angle δ exceeds the

preset limit angle the limiter increases the field current until the load angle has dropped

back to its permissible value The quadrature reactance Xq of the generator and the

network reactance Xe during normal operation must be adjusted on the regulator in order

to obtain the load angle δ The graph below shows the Power Chart for a salient-pole

machine with typical limiter characteristics

The purpose of a Power System Stabilizer is to use the generator excitation to

damp electromechanical oscillations between the network and the generator Depending

on the design of the generator and the requirements imposed for network stability its

main function will be either to damp the oscillations originating in the machine or those

from the network A synchronous generator working in a combined power network is in

principle an oscillating structure In order to produce a torque the magnetic field of the

rotor and the stator must form a given angle (referred to as the rotor displacement or load

angle δ) The electrical torque ME increases as the angle δ increases just as with a

torsion spring Because the ME of the generator and the mechanical driving torque MA

from the turbine are in equilibrium during steady-state operation the angle δ remains in a

given position Whenever this state of equilibrium between MA and ME is disturbed the

load angle slips of this rest position and change thereby the electrical torque ME The

torque attempts to restore the load angle to a stationary position Due to the mass inertia

of the turbinegenerator rotor however this can only take place aperiodically It does so

in the form of more or less effectively damped oscillations (again similar to the effect of

mass inertia on a torsion spring) In order to damp the oscillations there must be a

damping torque produced depending not on the electrical torque ME associated with the

angle but on the difference in frequency (Df) between the rotor and the stator rotating

field ie on the slippage This torque is produced mainly by the so-called damper

winding in the rotor but the dimensioning of this is subject to limits imposed by

considerations of design and economy Some further action is therefore needed to

increase the damping effect The following drastically simplified formula shows the

parameters upon which the amount of active power PE supplied by the generator

depends

PE = active power

It can be seen from the above relationship that the active power that the generator

transfers depends not only on the load angle δ but also on the field current If That means

that a transient change can be made in the active power PE and with that in the effective

electrical torque ME by varying the field current The principle of operation of the DVR

Power System Stabilizer becomes clear from a consideration of the oscillations in power

output and frequency (ΔPE Δf) and the vector diagram If it is assumed that oscillations

in the network frequency generates load oscillations with the mass inertia of the rotor

then the active load of the generator (eg MW-measured) is influenced with a sinusoidal

value -ΔPE (ME-MA = -ΔPE) By inversion of -ΔPE one obtains the fluctuation in

power provided by the rotor +ΔPE As is known the slip signal Δf follows +ΔPE with a

phase delayed by 90deg The +ME produced by the periodic changes in the load angle δ is

in phase with +ΔPE A good damping is attained if ME is varied in phase with the slip

Δf However this signal must also be advanced somewhat to compensate for the time

constants in the excitation circuit and the generator

As mentioned above the electrical torque ME can be influenced by varying the

field current To accomplish this a suitable control signal referred to as variable

disturbance compensation must be imposed upon the voltage set-point or the converter

control variable Ucontr As can be seen from the vector diagram by applying proper

weighting factors (K1 K2) and then adding together the signals -ΔPE and Δf an overall

stabilization signal can be produced that rotates in advance of the Df signal by any angle

desired between 0deg and 90deg Because the amplitude of -DPE remains proportional to the

amplitude of Δf a constant angle in advance of Δf results for the compensation of the

time constants referred to above The optimum weighting factors K1 and K2 for a

synchronous generator working to a power network depend on its operating point at any

moment and the external reactance of the network Normally the selection of a

compromise setting is good enough to attain stability in all operating points and for all

external reactance For special demands these settings must be parameterized as a

function of the external reactance (which means optional equipment Xe-Identification)

The Power System Stabilizer PSS is a section of the AVR computer program and is

processed once per network cycle The voltage at the generator terminals and the

generator current are measured in order to define the signals ΔPE and Δf The calculated

signals for _P_ (=PE) and Δf are then sent across DC filters ldquoDrdquo (real differentiators) that

transmit only the dynamic portion of the signals The ΔPE and Δf signals obtained in this

way are then weighted (multiplied by) with the factors K1 and K2 and sent to the

summing point of the voltage regulator

The PSS stabilization signal is imposed on the automatic voltage regulator only if

the following prerequisites are met

bull Generator on line

bull Generator power output gt the value F338

bull Generator voltage in a range between F33C and F33A

The stabilization signal is limited at the output from the PSS to the lower and

upper limits Flag defines whether the stabilization signal is introduced before or after the

PID filter (usually before the filter) Because the PID filter as noted above already takes

the ceiling factor Vp1 into account the PSS signal needs to be multiplied by Vp1 if it is

added to the voltage regulator following the PID filter (divider at the input to the

minmax limiter) This precaution prevents the DC filter ldquoDrdquo in the P-channel from

producing an unnecessary ldquostabilizationrdquo effect in the case of rapid changes in turbine

load As an alternative for the AVRrsquos Power System Stabilizer a stabilization signal from

an outside system can be imposed by activating the binary input ldquoPSS-SIGNEXTrdquo Flag

F340 can be used to select between an analog and a 12-bit signal and F33E to select the

polarity desired for that signal

24 The MANUAL Channel

241 Summary

The MANUAL channel (Channel 2) has been built as a simple field current

regulator

without additional limiters Its main function is to maintain the excitation of the generator

even if the AUTOMATIC channel becomes non-operational The MANUAL channel

also performs valuable service for purposes of testing commissioning and preventive

maintenance Its measurements regulator generation of firing pulses and power supply

are physically separate from those on the AUTOMATIC channel

242 Principle of Operation

All the functions of the MANUAL channel including the generation of firing

pulses have been implemented in a single electronic module the Gate Control Unit The

control variable Ucontr of voltage regulator is used as the reference value for generating

firing pulses on the principle known as ldquoramp controlrdquo (Comparison of Ucontr with

Usynsynchronous sawtooth signal) For further processing in the UN 0096 Intermediate

Pulse Stage the Gate Control Unit supplies six firing pulses at its output whose phase

position with respect to the synchronous voltage Usyn is in accordance with control

variable Ucontr An internal linearization ensures that the field voltage produced via the

firing pulses remains proportional to the control variable Ucontr throughout the entire

range As a result the circuit amplification of the control remains constant over the entire

range Whenever excitation is switched ON the set-point for Generator Voltage is set

automatically at the preset - ref Value This provision ensures that the generator voltage

always attains approximately its nominal value after the field flashing The Gate Control

Unit can be refunctioned ( by pre-selection with a switch ) for purposes of testing to act

as a purely firing pulse control In this case the control variable Ucontr is adjusted

directly using the RAISELOWER push buttons on the front of the module In this way

for example the relationship between the phase position of the firing pulses and the

control variable Ucontr can be checked easily

CHAPTER 3

PULSE SECTION

31 Pulse Generation and Amplification

The Gate Control Units of both AUTOMATIC channel and MANUAL channel

each supply six firing pulses for operating the 6-pulse thyristor bridges The low-power

pulse signals from these Gate Control Units are then amplified in the Intermediate Pulse

Stage galvanically isolated and then sent to the common pulse bus On the output end

the Intermediate Pulse Stage of the non-active channel) is always blocked The Gate

Control Units generate the pulses based on microprocessor control The reference voltage

used for the firing pulse phase location is the output voltage from the excitation

transformer (Usyn1 Usyn2) The commutation spikes of the synchronous voltage caused

by the converter are calculated prior to use of the voltage as a reference value and are

deliberately filtered out The lower limit for the firing pulses (double pulses) which are

offset from one another by 60deg is defined by the limit rectifier position (αmin) and the

upper limit by the limit inverter position (αmax) for the firing angle αmin and αmax can

be adjusted on the Gate Control Units using BCD (Binary Coded Decimal) switches

αmin ensures that the firing pulses will not be issued (premature firing) until there is

sufficient positive phase voltage on the thyristor involved αmax prevents a dangerous

ldquotippingrdquo of the thyristor bridge into the rectifier mode if the firing angle α is too large

(ldquolate firingrdquo) The critical factors determining αmax are the overlap time uumlmax (max

commutation time) and the ldquorecovery timerdquo of the thyristors (αmax lt 180deg - uumlmax - γ )

An external control signal can force the firing pulses into their inverter limit position

Other binary inputs can block or direct the firing pulses of the Gate Control Units so as to

produce freewheeling on the thyristor bridge During freewheeling the firing pulses for

the thyristor pair R and S are blocked and the pulse signals T+T- are engaged with

chains of pulses Both Gate Control Units (for the MANUAL amp AUTOMATIC channels)

contain a field current monitor that blocks the firing pulses immediately whenever the

current exceeds a preset threshold level In this case the field circuit-breaker is also

tripped via an output contact The purpose of these provisions is to prevent damage to

thyristors and thyristor fuses in case of a slip-ring short-circuit or to keep any damage

that does occur to a minimum The pulse signals are galvanically separated at the outputs

from the Intermediate Pulse Stage (with pulse transmitters) and are then directed to the

common pulse bus This transmission of the pulse signals to the pulse bus via passive

transmitters ensures a high degree of active channel autonomy Practically no possible

malfunctions on the inactive channel (including for example sustained pulses) affect the

active channel

32 Pulse Monitoring

The ldquoPulse Busrdquo and the pulse signals of the AUTOMATIC channel are

monitored This monitoring device consists of potential isolating stages and the common

monitor If the pulse monitoring of the ldquoPulse Busrdquo responds a switch-over is made to

MANUAL channel The function of the potential isolating stages is to couple the pulse

monitoring device to the pulse circuits without any feedback effect The pulse monitoring

checks the six pulse lines for the following malfunctions continuous or periodic failure

of one or more pulses Periodic occurrence of synchronous or asynchronous false pulses

Continuous pulses the pulse monitoring device can be tested while the machine is in

operation

CHAPTER 4

CONVERTER

Thyristor

The term thyristor usually refers to a family of four layer solid state device having

turn on characteristics that can be externally controlled by either current or voltage They

are also referred to as breakdown device because their working depends on avalanche

breakdown Thyristors have only two stages OFF and ON Thyristors have a similar

function to Uni-junctions they act as switches Thyristors use current flow as a switch

Thyristors have three states

1 Reverse blocking mode mdash Voltage is applied in the direction that would be

blocked by a diode

2 Forward blocking mode mdash Voltage is applied in the direction that would cause

a diode to conduct but the thyristor has not yet been triggered into conduction

3 Forward conducting mode mdash The thyristor has been triggered into conduction

and will remain conducting until the forward current drops below a threshold value

known as the holding current Converter is a semiconductor device which converts ac

input voltage into a constant dc output voltage In present excitation system three phase

fully controlled thyristor converter is used

Because of the following advantages thyristor converters are used

a) Thyristors are used for high power applications ie up to 10Kv3500A1KHz

b) Having high reliability and low losses

c) Uni-directional device like diode

d) Itrsquos operation as a rectifier which are low resistance in forward conduction

mode and high resistance in reverse conduction mode

PROTECTION OF THYRISTORS

For reliable operation of a thyristor demands that its specified ratings are not

exceeded When Subjected to or over voltages During the turn - on of SCR didt

prohibitively large False triggering of SCR by high value of dvdt andSpurious signals

between gate and cathode may leads to unwanted turn ndash on

DIDT AND PROTECTION

When thyristor starts conducting in forward conduction mode and is turned on by

gate pulse The anode current increases rapidly whole area of the gate to Cathode

junction then hot spots will be formed near the gate connection this locality of heating

destroys the thyristor Thyristor thermal time is constant The causes due to faults and

short circuits or surge currents Electronic crowbar protection is used against the over

voltages The rate rise of anode current must be kept at the time of turn on below the

rated or specified limiting value The didt value maintained below limited value by using

a inductor also called ldquodidt inductorrdquo in series with anode circuit The locality of heating

is avoided by applying gate current but not greater the maximum gate current

DVDT AND OVER VOLTAGE PROTECTION

With forward voltage across the anode and cathode of a thyristor the two outer

junctions are forward biased but the inner junction is reverse biased This reverse biased

junction J2 has the characteristics of a capacitor due to charges existing across the

junction In other words space-charges exist in the depletion region around junction J2

and therefore junction J2 behaves like a capacitance If the entire anode to cathode

forward voltage Va appears across J2 junction and the charge is denoted by Q then a

charging current i given by Eq (46) follows

i = dQdt =d(Cj Va )dt

= Cj (d Va dt) + Va(d Cj dt) helliphelliphelliphellip(46 a)

As Cj the capacitance of junction J2 is almost constant the current is given by

i = Cj (d Va dt) helliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip(46 b)

If the rate of rise of forward voltage dVadt is high the charging current i will be

more This charging current plays the role of gate current and turns on the SCR even

when gate signal is zero Such phenomena of turning-on a thyristor called dvdt turn-on

must be avoided as it leads to false operation of the thyristor circuit

For controllable operation of the thyristor the rate of rise of forward anode to

cathode voltage dVadt must be kept below the specified rated limit Typical values of

dvdt are 20 ndash 500 Vμsec False turn-on of a thyristor by large dvdt can be prevented by

using a snubber circuit in parallel with the device thyristor are very sensitive for over

voltage than the semiconductor devices

Over voltage transients are perhaps the main cause of thyristor failure

In thyristor there are mainly two types

1 Internal over voltages

Due to the commutation of the thyristors large voltages are generated internally

Because of the series inductance of the SCR circuit the large transient voltages L didt

produced This voltage several times the break over voltage of the device then thyristor

destroys permanently

2 External over voltages

External over voltages are caused due to the interruptions of current flow in an

inductive circuit and also due to the lightening strokes on the lines feeding the thyristor

system For the reliable operation of thyristor the over voltages must be suppressed by

adopting suitable techniques

Suppression of over voltages

The RC circuit called snubber circuit is connected across the device to protect In

order to keep the protective components to a minimum the thyristors are chosen with

their peak voltages ratings are 25 to 3 times of the normal peak working voltage

ldquoselenium thyrector diodes metal oxide varistors or avalanche diode suppressers are

commonly employed for protecting the thyristor circuit against the over voltages

Gate protection

Gate circuit should also be protected against the over voltages and surges Over

voltage at gate circuit can cause false triggering of the SCR may rises the junction

temperature behind specified limit leading to its damage Protection against over

voltage can be achieved by connecting a ZD across the gate circuit and a resister is

connected in series with gate circuit to protect against the s A capacitor and resister are

connected across gate to cathode to by pass the noise

41 Final Pulse Stages

The Final Pulse Stages adapt the output pulses from the Intermediate Pulse Stage

(pulses on the pulse bus) to the gate currents needed for the thyristors Each thyristor

bridge is equipped with its own Final Pulse Stage Each Final Pulse Stages is provided

with a power supply module The amplified output pulses from the Final Pulse Stages

start as a short strong steep pulse with an amplitude approx 2frac12 times that of the main

pulse This initial pulse edge assures proper firing of the thyristors being triggered

Subsequently the weaker part of main pulse keeps firing conditions steady As already

mentioned the Final Pulse Stages and their associated thyristor bridges form single units

All six pulse outputs from a Final Pulse Stage can be blocked by an external control

signal so that all thyristors in the associated thyristor bridge will block the current A

blocking of the pulses is initiated whenever there is a malfunction in the associated

thyristor bridge

42 Converter Power Section

The thyristor converter consists of three independent parallel rectifier blocks TY1

to TY3 which are all in service Even if one block fails the remaining blocks take over

automatically the full design current of the excitation circuit During normal operation

(with ideal current share) and all three bridges in operation each of these blocks has to

carry only (n-2)n (ie33)of its design current If 2 thyristor bridges fail the excitation

is limited Only when all three bridges fail the excitation is switched off Each thyristor

bridge arm is equipped with current flow monitoring CTrsquos Failure of conduction in any

arm is identified by a Current flow monitoring module

43 Converter Cooling

A cooling system is needed to dissipate heat losses in the converter blocks and

electronics Each converter block has therefore been equipped with a fan supplied with

power from the converterrsquos primary voltage (via transformer ndashT8 in field flashing

cubicle) The fans are protected with motor protection circuit breakers An air flow

monitoring unit is provided for monitoring the air flow through the thyristor bridge If a

circuit breaker failure is detected or if the air flow monitor drops off at one of the

thyristor bridges the bridge involved is immediately set out of operation by blocking its

firing pulses

44 Thyristor Converter Monitoring

A thyristor bridge in which defects occur that could threaten the safety of

operation or cause secondary damage is switched off automatically ie its firing pulses

are blocked This happens whenever A thyristor fuse is blown The fuses are monitored

individually with micro switches The Final Pulse Stage fails which is detected by

internal monitors (supply voltage sustained pulse short-circuit on the output end) The

power supply to the fan fails fan air flow as monitored by the Air flow monitor fails or

is insufficient Isolator on ACDC side is open

CHAPTER 5

Field Current Circuit Excitation Transformer

51 Field Circuit Breaker

The circuit-breaker in the field circuit is used to isolate the field circuit from the

converter It is capable of switching off the synchronous machine from full load under the

maximum conditions of a 3-phase short-circuit In addition to its main contacts the field

circuit-breaker also has a de-excitation contact with which the field energy stored in the

field can be dissipated across the de-excitation resistor The de-excitation contact closes

shortly before the main contacts open so as to ensure proper commutation of the field

current from the main contacts to the de-excitation contact when the breaker is switched

off The field circuit-breaker is switched on by electromagnetic force and is kept switched

on by a mechanical latch When the latch is released by a trip coil the circuit-breaker

opens The circuit-breaker also has auxiliary contacts that report its status

52 Field Flashing

In shunt supplied excitation circuits (excitation transformer connected to the

generator terminals) the generator does not have enough remnant voltage for a generator

voltage build-up via the converter In this case a field flashing circuit is provided It

consists of the field flashing contactor the diode bridge and a transformer used to adapt

the auxiliary input voltage to the voltage needed for field flashing when power is

supplied from the auxiliaries network

Fig Field Flashing

Because the field flashing contactor is not able to switch off the energy stored in

the field the control ensures that the contactor can only reopen if the field circuit breaker

has already been opened (generating the TRIP order) or in a normal field flashing

sequence when the converter has taken over the field current Field flashing occurs in the

following stages

1048729The excitation is switched on closing the field flashing contactor ( Field

Circuit Breaker is already closed )

1048729The start-up excitation current flows through the rotor driving the generator

voltage up to approx 15 U

1048729After about 10 U the firing pulses to the converter are released and it begins

to excite the generator to its rated voltage

1048729After about 30 U the field flashing contactor opens (with no current since

the converter is now supplying the current)

The diode bridge at the input to the field flashing contactor prevents a feed-back

from the converter to the source of field flashing while the contactor is still closed

53 De-excitation

When malfunctions occur the stored field energy must be dissipated as quickly

and safely as possible to protect the generator This is done by the converter the field

circuit-breaker and the de-excitation (discharge) resistor

De-excitation (with opening of the field circuit-breaker) takes place in the following

stages

1048729The converter drives to its inverter limit position (negative ceiling voltage)

recovers a portion of the field energy into the network A trip command is given to the

field circuit breaker

1048729The de-excitation contact closes diverting the field voltage to the de-excitation

resistor

1048729Then immediately the main contacts open building voltage The field voltage

commutates to the de-excitation resistor

1048729The current diminishes at a given time constant TE

(With linear resistance TE = Lf (Rf + Re))

Due to the reversal of the field voltage by the converter the field current

commutates from the main contacts of the field circuit-breaker to the de-excitation

resistor in a very early phase This reversal of the field voltage prevents burn-off on the

main contacts and provides effective protection for the field circuit-breaker Depending

on the operating policy an operational shut-down of the excitation can also be effected

with the field circuit-breaker closed This method is useful mainly when the excitation is

switched on and off frequently In this case the converter is merely driven into the

inverter limit position so that the field energy is recovered into the network The

converter then blocks since it is supplying positive current only

54 Excitation Transformer

The excitation transformer matches the generator voltage to the field voltage

(required ceiling voltage) It also serves as a commutation reactance for the thyristor

converter and as a potential isolator between the network and the excitation circuit In

addition the transformer functions as a current limiter in that it makes it possible to keep

any short circuits in the excitation circuit under better control The excitation transformer

is equipped with temperature monitoring probes which set off an alarm when the

temperature exceeds a first max limit and then trips the excitation if the temperature

continues rising to a second (higher)limit

CHAPTER 6

Monitoring and Protection

61 Excitation Monitoring

611 General Information

The main goal of Excitation Monitoring is to make optimum use of the

redundancies provided in the excitation system and to give alarm whenever a malfunction

makes these redundancies unavailable The field current is monitored to see that it does

not exceed a maximum level and if necessary a switch-over to the MANUAL channel is

initiated In addition the criterion for switching off the field flashing is generated The

excitation Monitoring consists of an autonomous processor system

612 over current Alarms

In the Excitation Monitoring the limits for are set at higher levels than the

settings on the Field Current Maximum Limiter Whenever the current exceeds 110 of

the nominal field current contact R1 and the binary output associated with it are

activated immediately If field current remains gt 110 then after a preset inverse-time

has lapsed relay R2 and - after a further delay - relay R0 and the binary outputs

associated with them are activated Parameters match the measurements for If1 and If2 to

the nominal value for field current so that the internal values can be processed and read as

pu values It can be used to falsify the actual value of the field current If (to raise it) so

as to cause a response from the alarm limits for purposes of testing The processed If

signal is always taken from on the active channel (CHANNEL I OR CHANNEL II) As

long as the field current If is above the threshold value 11 Ifn its peak value is

measured This is stored (until RESET) and can be read at any time on the Micro-

Terminal Once the value of If exceeds 11 Ifn integration of this value starts Whenever

the integrated time-current value (intisup2dt) exceeds the preselected reference value the

alarm OVER CURRENT INVERSE-TIME is set off and a command is simultaneously

issued to switch over to the stand by AUTO channel Software switch F758 enables the

three over current alarm functions (R0 R1 R2) and selects one of three possible inverse-

time curves T1 T2 or T3 Within the characteristic curve (T1 T2 T3) selected the

desired limit curve for response is set using the factor F216

613 Switch-Off Criterion for Field Flashing

The Excitation Monitoring supplies the criterion for switching off the field

flashing Whether this criterion is activated based on the actual value for generator voltage

Ug or for field current If or both depends on the settings of the two threshold values

F200F202 (0 setting means that the output is always ldquological 1rdquo) The measurements

Ug12 and If12 are switched over depending on the present status of the channels

(Channel 1 or Channel 2 ON) Whenever Flag F750 is not activated the binary output is

fixed at ldquological 1rdquo

614 Storage of Alarm Status

The outputs of the over current alarms (R1 R2 R0) and the messages NO

FAILURE MONITORING PARAMETERS CHANGED are stored messages can be

erased by activating the input ldquoGENERAL RESETrdquo or by using the RESET button on the

front of the module Erasure with the input ldquoGENERAL RESETrdquo is effective only if the

situation causing the alarm or the malfunction is no longer present Whenever the self-

diagnosis equipment in the processor detects a malfunction the output NO FAILURE of

MONITORING is set at ldquological 0rdquo (= alarm) The alarm ldquoPARAMETERS CHANGEDrdquo

is activated whenever parameters or settings of software switches have been changed via

the Micro-Terminal

615 Actual Value Monitoring

The actual values for generator voltage Ug synchronous voltage Usyn and field

current If are monitored for malfunctions This monitoring is active regardless of whether

or not the generator is in operation Essentially when the generator is in operation the

measurements are monitored by comparing the signals (the smaller signal reading is

detected as incorrect) When the generator is not in operation the measured data are

monitored for extreme values The percentage of deviation permissible in the

measurement signals being compared

(Ug1ampUsyn1 Ug1ampUg2 Ug2ampUsyn2 If1ampIf2) is defined by parameters F208 and

F20AIf the excitation transformer is being supplied from an auxiliary power source (no

shunt operation) the values of Ug and Usyn will be different in some operational

conditions

In that case Flag F75A can be used to deactivate comparative measurements Ug1 Usyn1

and Ug2 Usyn2 The ldquoprimary monitoringrdquo compares Ug1 with Ug2 and generates the

messages that Channel 1 or Channel 2 has suffered a malfunction Whenever Ug1 lt Ug2

and the binary message from CH1 reports no malfunction a malfunction on

Measurement Channel 1 is reported (Ug1Usyn1 FAILURE) A similar malfunction is

also present whenever the binary message CH1 DISTURBANCE is reported and a

discrepancy is detected between Ug1ampUg2 The generation of the alarm ldquoUg2 Usyn2

FAILURErdquo is analogous to that for Channel 1 The ldquosecondary monitoringrdquo compares

Ug1 with Usyn1 amp Ug2 with Usyn2 This is enabled whenever the binary message of the

comparison channel reports a malfunction or whenever both binary messages report no

malfunction - but both secondary monitors report a malfunction As long as the secondary

monitoring is blocked the differences Ug1neUsyn1 or Ug2neUsyn2 trigger malfunction

signals for the measurement channel involved (suspicion that there is a corresponding

error in Usyn) The measurement channel malfunctions are enabled operationally

whenever after excitation has been switched on generator voltage Ug exceeds the value

set on F204 The voltages Ug1 and Ug2 are checked 16 seconds after the excitation is

switched off to see that they do not exceed the limit value F210 that applies to both of

them At the same time g1Ug2Usyn1Usyn2 are checked for extreme values (gt or lt

the operating range) Monitoring for extreme values is likewise enabled during normal

operation ( Excitation ON and Ug gt F204 ) Flag F754 is used to enable or block the

malfunction signals to the binary outputs Basically the monitoring of the actual values

for If1If2 functions like that of the Ug1Ug2 monitoring

62 Excitation Protection

621 General Information

The Excitation Protection switches off the excitation (and de-excites the machine

rapidly) whenever a danger arises that threatens the excitation transformer the converter

or the generator Generally limiter or monitoring functions precede the emergency trips

and these normally respond before the Excitation Protection must initiate a trip

Protective trip commands are issued directly to the field circuit-breaker from potential

free contacts of the board via the trip relays They are directed redundantly to the

operative field circuit-breaker ldquoOFFrdquo command

622 Protection against Excitation Transformer Overheating

This equipment monitors the excitation transformer for overheating in the

windings that could result from over current short-circuits or inadequate cooling The

monitoring uses temperature monitoring modules in conjunction with temperature

sensors built into the windings Normally the temperature is monitored in two stages the

first stage sets off an alarm the second causes a trip of the excitation

623 Rotor Over voltage Protection

Malfunctions in the generator circuit (eg terminal short-circuit failed

synchronization asynchronous operation) cause induced negative field currents that

produce high voltages in the field circuit These must be restricted to a level with a

sufficient safety margin below the insulation capacity of the field winding (test voltage)

and also below the peak blocking voltage of the converter thyristors The crow bar

employs spark gap elements to detect over voltages in the field circuit Whenever they

respond the associated thyristors are fired immediately switching the de-excitation

resistor parallel to the field The de-excitation current generated thereby initiates an

excitation trip via a supervision circuit causing an immediate opening of the field circuit-

breaker The malfunction isets off an alarm and an internal malfunction is indicated at the

cubicle

TEST VALUES OBTAINED WHEN EXCITATION IS RAISED

TEST

SNO PARTICULARS ACTUAL VALUE OBTAINED VALUE

1 VREF 996 100

2 VACT 997 1003

3 IFACT 735 765

4 IGACT 703 707

5 ACTIVE

POWER

703 705

6 REACTIVE

POWER

101 142

7 POWER

FACTOR

099 IND 098 IND

8 ACTIVE

CURRENT(IR)

705 703

9 REACTIVE

CURRENT(IX)

102 138

10 POWER

ANGLE

566 546

11 FIRING

ANGLE

640 632

VALUES OBTAINED WHEN EXCITATION IS LOWERED

SNO PARTICULARS ACTUAL

VALUE

OBTAINED VALUE

1 VREF 100 997

2 VACT 100 997

3 IFACT 787 761

4 IGACT 839 837

5 ACTIVE

POWER

839 830

6 REACTIVE

POWER

155 110

7 POWER

FACTOR

098 IND 099 IND

8 ACTIVE

CURRENT(IR)

835 830

9 REACTIVE

CURRENT(IX)

140 94

10 POWER

ANGLE

613 DEG 631 DEG

11 FIRING

ANGLE

601 DEG 599 DEG

CONCLUSION

For generating the EMF in stator winding excitation is required to the rotor of a

generator There are two types of excitation

1 Static excitation system

2 Brushless excitation system

A certain disadvantage in brushless excitation system is the slow response time of

the field in case of fast load changes specified No slip-rings and brushes direct

measurements of the field parameters not possible

To avoid all loses static excitation is used Since it does not have any rotating

parts mechanical loses and windage loses This system has fast response and speed

control While preferring this excitation system there are no limitations for the

redundancy of Thyristor bridge circuits

Static excitation has fast field discharge by resistor and inverter operation direct

measurement of field quantity is possible The meaning of excitation is nothing but

continuous supply of DC current (ie field current) to the rotor to buildup required

output voltage in the stator

Field current is changed with respect to the change of load so the digital

automatic voltage regulator (DAVR) is used to regulate the output voltage according to

the load variations

So we conclude that static excitation system with DAVR is preferred since it is

having excellent dynamic performance and better options for R amp M

the permissible level the excitation is switched off by Excitation Protection The most

important measuring inputs for the excitation system (If Ug Usyn) are redundant (2-

fold) The Excitation Monitoring checks these measuring inputs for discrepancy and

plausibility An alarm is always initiated in case of malfunction In certain cases a

switch-over to MANUAL channel is also initiated The excitation system contains an

Excitation Protection to protect the excitation transformer the converters and the

synchronous machine The protection system can detect short-circuits in the excitation

circuit and keep secondary damage within acceptable limits by a quick tripping of the

excitation and an opening of the generator breaker An overheating of the excitation

transformer first sets off an alarm (at a given preset limit) and then likewise initiates a

protective shut-down at an even higher limit The over voltage protection in the de-

excitation equipment provides an autonomous protective function for the rotor and the

rectifier This protection system monitors the field voltage in both polarities for over

voltage and if necessary de-energizes the field via the de-excitation resistor

12 Principle of primary power supply

In the shunt excitation system the excitation transformer also provides

the power supply for the electronic equipment and the converter fans So a failure of the

auxiliary supply to the converter fans does not cause a shutdown of the excitation When

the auxiliary supply fails the supply to converter fans is switched over to Excitation

transformer OP with a contactor A station battery supply is absolutely necessary for the

control of the field circuit breaker It is the power source for the electronic devices till the

generator is able to supply voltage Auxiliary power to the field flashing equipment must

be present in order to build up the generator excitation The power supply for standstill

heating and the cubicle lighting is also from Station Auxiliary Power Supply and is of

secondary importance for operation of the plant Power supply to rotor earth fault

detection circuit too is from Station Auxiliary Supply The two synchronous voltages

Usyn are each supplied to the AUTOMATIC channel and the MANUAL channel

separately across transformers The Gate Control Units need these voltages to enable

4

them to issue the pulses at a given firing angle relative to the input voltage of the

converter

5

CHAPTER 2

Digital Automatic Voltage Regulator (DAVR)

21 Principle of Operation of the Regulator (DAVR)

To regulate the voltage and the reactive power of a synchronous machine the

field voltage must be adjusted quickly to the changes in the operating conditions (with a

response time that does not exceed a few ms) To accomplish this analog control systems

include amplifiers which make continuous comparison of the actual values against the

reference values and vary the control variable to the converter with almost no delay Most

of the delay that occurs originates in the converter since the firing pulses for changing

the rectifier phase angle are only issued periodically (every 33 ms)

The DVR digital voltage regulator calculates the control variable from the

measured and reference data in very short time intervals This results outwardly in a

quasi-continuous behavior with a negligible delay time (as in an analog regulator) The

calculations are made in the binary number system Analog measurement signals such as

those for generator voltage and generator current are converted into binary signals in

analogdigital converters The set-points and limit values have already been defined in

digital (binary) form An understanding of the actual computation processes in the digital

voltage regulator is not necessary for operation preventive maintenance or

troubleshooting Like the operator of a pocket calculator or a personal computer all the

operator needs is to know how to operate the instrument and the programming for this

working tool For that reason we will explain below only the principle division of work

among the various modules and the flow of data processing The purpose is above all to

make clear how the processor system has been integrated into the rest of the power

electronics system

6

22 Basic Structure of the Processor Systems

7

The signal processors 25 analog inputoutput modules Each of these processor

systems has a common bus circuit and output and the control lines There is a specific

range of addresses assigned to each assignment Board including the power supply bus

the address lines the two data lines to the input calculates the reactive current (I sin φ)

and the active current ( Icos φ) With these two channel processor Synchronized with

these interrupts (ie with the phase positions of current Ig the field current If and the

synchronous voltage Usyn From the exchange data with the microprocessor card across

the two data lines generator voltage Ug) this processor measures the generator current

Ig and then hardwired connections or multi-conductor cables Binary and analog

inputoutput modules ie for galvanic isolation and adaptation to the electronics level

The most important input interrupts per period to trigger the cycles for processing actual

values in the AUTOMATIC module on the processor bus) for filtering and further

processing

Monitoring each consist of the central microprocessor module and binary and

parameters to the AUTOMATIC channel are the generator voltage Ug the generator

peripheral unit Ug Ig and Usyn are sent to the Interrupt Generator (plug-in peripheral

units (wall-mounted units) peripheral units are used for preprocessing signals from

external measurement circuits power supply units Signals are exchanged among these

processor systems via processed across separate peripheral units for each channel These

processor working on the bus (a house address that can be adjusted using a switch)

systems The AUTOMATIC channel the programmable controls and the Excitation The

actual values measured from AUTOMATIC channel and MANUAL channel are The

AUTOMATIC channel and the MANUAL channel each have their own The digital

voltage regulator is broken down into several autonomous microprocessor The inputs and

outputs of the processor systems are directed across voltage-isolating The Interrupt

Generator also uses the 3-phase Ug signal to generate the 12 themselves contain a limited

number of hardware inputs and outputs with fixed equipment Whenever addresses from

this range are called up the signal processing module can results the processor is then

able to derive further operating parameters such as the load angle the active power etc

The functions of all microprocessor systems other than the programmable controls

have been accomplished in firmware The non-varying standard function modules can be

configured to the design desired for plant-specific purposes using software switches

(KFlags) Thus for example the stored status of a K-Flag determines whether or not a

Limiter is active and whether the de-excitation or the excitation limiters take precedence

Because these K-flags determine the software Scope of Supply for the installation they

cannot be changed permanently via the Micro-Terminal In this way they differ from

such setting data as the values of the parameters for the PID filter of the voltage regulator

or the set-points for the limiters These values can be permanently changed using the

Micro-Terminal Communication is possible with each of the processor systems via the

Micro-Terminal by plugging on the connecting cable In this way signals within the

processor and setting parameters can be viewed analog signals can be issued and the set

parameters can be altered temporarily (F range) or permanently (C range) Unlike the

other processor systems the programmable controls do not include any firmware for

realization of the functions They have been designed so that the designer can adapt and

change their functions easily using the ldquoFunctional Block Programming Language P10

Digital and analog functions can be implemented in practically any degree of complexity

desired using the P10 functional blocks The control variable of the voltage regulator

(AUTOMATIC channel) and the control variable of the field current regulator

(MANUAL channel) are each processed in separate Gate Control Unit and formed into a

chain of pulses at the appropriate firing angle The pulses of the active channel are

directed to the pulse bus via the associated Intermediate Pulse Stage The pulses for each

converter block are amplified sufficiently in Final Pulse Stage to fire the Thyristor

231 General Information

The functions of the automatic voltage regulator AVR are

1 to regulate the generator voltage

2 to regulate the effect of the reactive andor active current on the voltage

3 to limit VoltHz

4 to limit max and min field current

5 to limit inductive stator current

6 to limit capacitive stator current

7 to limit the load angle

8 to stabilize the power system

Block Diagram shows the software structure of AUTOMATIC channel The

generator limiters not provided for the installation in question (optional equipment) are

identified in this overview as ldquoNot Suppliedrdquo The parameter values signal values and

software switches (flags) marked with addresses (hexadecimal numbers) can be viewed

and altered via the Micro-Terminal The values selected are displayed in sec pu Hz

etc and can where necessary be changed directly in these formats The plant-specific

settings of the variables and the flags can be obtained from the Test and Commissioning

Report This block diagram provides information about the important functions and

possible settings of the AUTOMATIC channel For the sake of clarity no detailed

presentation has been given of special functions such as tracking circuits initializations

etc The page heading cross-refers this overview to the various sheets of the schematic

diagram Binary signals are shown in broken lines analog signals in solid lines The

corresponding text designations in the schematic diagram can be used for identification of

the input signals (hardware inputs) The only analog output signal from the automatic

voltage regulator control variable Ucontr is sent via the data bus (CRU bus) to the Gate

Control Unit Most of the binary messages (outputs) from the AVR are of no interest

functionally and they have been omitted for the sake of clarity The basic structure of the

digital voltage regulator and the limiters is simple This is necessary in order that the

behavior of the regulatorslimiters will remain calculable and understandable in all

operating situations and that there will be no problem in adjusting and optimizing them

The central PID filter in the digital voltage regulator defines the dynamic response of the

closed-loop controls both in the voltage regulator mode and after limiters have

intervened The ldquocontrol deviationrdquo at the input to the PID filter is either the control

deviation for voltage the control deviation of a de-excitation limiter (the value

determined by minimum value selection) or the control deviation of an excitation limiter

(the value determined by maximum value selection) Flag F730 (ldquoPRIORrdquo) is used to

determine whether the exciting (Min value) or the de-exciting signal takes precedence on

the minmax value limiter (normally F730 = 1111 ie the de-exciting signal takes

precedence) With the exception of the Minimum Field Current Limiter all other limiters

have variable factoring multipliers of the signal outputs so that they can be adjusted

individually together with the common PID filter which has been optimized for voltage

regulation The setting parameters for this PID filter are as follows

Vo = KR Static amplification

1

Ta = ---- Integration time constant

Tc1

Vp Proportional amplification

1

Tb = ---- Differential time constant

Tc2

Vinfin Amplification of high frequencies

The BODE diagram below shows the assignment of settings in accordance with

DINIEC standards based on a typical example

The PID filter amplifications Vo Vp and Vinfin can be adjusted in pu values But

the ldquoceiling factorrdquo pl+ must be adjusted correctly with parameter F310 if the total

amplification (circuit amplification) of the control circuit is actually to conform to the

pu settings This factor must agree with the ldquoexternalrdquo amplification ie with the

ceiling value of the transformer- converter circuit

Ceiling factor(pl+) = Ufmax Ufo

in which Ufmax = ceiling field voltage

Ufo = no-load field voltage

To attain a suitable response of the AVR when starting excitation

(ldquoEXCITATION ONrdquo) it may be necessary to change the proportional amplification of

the regulator during this phase Vp2 (transiently activated) and Vp1 (permanently

activated) can be adjusted for this purpose For example the value of Vp2 takes effect

immediately once the excitation is switched on and remains effective for a period as set at

F30C Once the period F30C (eg 5 sec) has expired Vp shifts over to Vp1 (becomes

the steady-state Vp) at the rate of change set The standard operating mode for the PID

filter is voltage regulation for which the discrepancy between the voltage set-point and

the current value for generator voltage Ug (the control deviation) is supplied at the input

To compensate for the voltage drop in the block transformer or whenever several

generators are operating to the same distributing bus the generator voltage must be

varied in proportion to the measured generator current (droop influence) To accomplish

this the voltage set-point is varied as a function of the measured reactive current IX

andor active current IR Flag F712 enables the IX droop Flag F710 the IR droop The

desired compensation is set in F282 and F286 respectively Flags F284 and F288 are used

to select whether this droop influence is to increase the voltage or to reduce it

(compensation) Combined influence of the active and reactive currents is attained by

enabling both droops IX and IR Flag F716 activates a so-called ldquoSoft-Startrdquo at the

starting of excitation This ldquoSoft-Startrdquo ensures that the voltage set-point integrates from

0 to 100 within the time set on F290 when the excitation is switched on

(ldquoEXCITATION ONrdquo) A ldquosmoothrdquo excitation of the generator can be achieved in this

way whenever there is no demand for a quick excitation

232 Voltage Set-Point

Various signals and settings control and limit the voltage set-point F270 For

example the values of F254 and F252 define the normal operating range possible for set-

point adjustment (eg 90 110) using external control commands (control room local

operatorrsquos panel superposed control system) The effective set-point adjustment rate is

governed The set-point can be set at the values of F250 and F256 by activating

appropriate control commands for ldquoSETrdquo input Enabling Flag F71A and activating a

binary input prior to switching on the excitation (ldquoEXCITATION OFFrdquo) sets the Ug set-

point at the value of UAUX This makes it possible for example to ensure that the

generator voltage will agree exactly with the network voltage after the voltage build-up

An external value with variable amplification can be added to the Ug set point by

enabling F724 (for example for stability tests)

233 Regulator Tracking in MANUAL Operation

Whenever the AUTOMATIC channel is not in operation (the MANUAL channel

is ON) a follow-up equipment ensures a smooth switch-back to the AUTOMATIC

mode will always be possible To track the voltage set-point is shifted by means of

RAISELOWER pulses from the Gate Control Unit so that control variable Ucontr at the

output from the PID filter is held steady and identical to the control variable Ucontr from

the MANUAL channel Because this tracking must react slowly resultant transient

control deviations resulting from the amplification in the PID filter might cause severe

interference with control variable Ucontr

To prevent this the follow-up equipment intervenes on the regulators mixing

point with a corresponding compensation signal

234 Ugf Limiter

At under frequency the Ugf Limiter reduces the generator voltage so as to

prevent saturation effects in the supply and measuring transformers To adjust this

limiter the max permissible generator voltage at rated frequency is defined and set

When any under-frequency occurs the generator voltage is thus reduced in proportion to

that setting

235 Field Current Maximum Limiter

The Field Current Maximum Limiter is provided to protect the generator rotor

from s occurring in steady-state and transient operation High field currents are normally

the result of a sharp drop in network voltage or of an improper raising of the voltage set-

point by the operating staff The field current is held steady at the value TH1 ie at the

maximum thermal value permissible for the excitation circuit and the rotor In order that

the generator can support the power network with its transient overload capacity during

brief collapses in voltage a temporary switch-over is made to the transient limit MAX1

(a higher setting) When the generator or the converter is operating at a reduced capacity

These limits TH1MAX1 can be switched over to the lower settings TH2MAX2 by

activating the corresponding binary signals The switch-over from the thermal limit

TH12 to the transient limit MAX12 can be configured in one of three ways

a) Depending on the over current with -dUdt ENABLE

Flag programming F418 = any setting desired F41A = 0000

This variant enables the transient value MAX12 whenever a collapse of voltage

in the network is detected The ENABLE time is fixed and can be set The example

below shows the typical behavior of the limiter configured in this way

b) Dependent on the time integral with -dUdt ENABLE

Flag programming F418 = inactive F41A = 1111

This variant likewise enables the transient value only when a collapse of network

voltage has been detected However the switch-back to the thermal limit is not made

dependent upon the time itself but on the calculated time integral intisup2dt of the The setting

on Parameter F414 in spu takes into account the time the rotor needs to cool down ie

the rate of temperature change in the case of intermittent operation The example below

shows how the timing of the switch-back to the thermal limit depends on the present

value for intisup2dt

The time integral is based on the formula

Example The setting of =isup2dt equivalent to Version a (F416) at a constant 16 times the

nominal field current for 10 seconds (with TH12 = 105) is

c) Dependent on the time integral without any preconditions

Flag programming F418 = 1111 F41A = 1111

In this variant the transient becomes available without any prior conditions

(without a -dUdt ENABLE) with the time integral intisup2dt

237 Inductive Stator Current Limiter

The Inductive Stator Current Limiter holds the stator current Ig within permissible

limits while the generator is in the ldquoover-excitedrdquo operating range by reducing the field

current accordingly The setting TH (thermal limit) provides the limit against stationary s

that might occur To take advantage of the generatorrsquos transient overload capacity a

switch-over is made to the higher setting MAX The principle of operation of this switch-

over to the value MAX permissible only transiently is identical to that employed for the

field current limiter (refer to the description above) When the drive output from the

turbine is very high stator current may exceed permissible limits even while inductive

loading of the generator is low In this case if the stator current limiter is not kept from

influencing the field current the control circuit will oscillate back and forth between the

Inductive Stator Current Limiter (de-

exciting) and the Capacitive Stator Current Limiter (exciting)The output signal of that

function then dominates the control variable of the Ig-dependent limiter via a maximum

value selection

238 Capacitive Stator Current Limiter

239 Load Angle Limiter

The Load Angle Limiter prevents the synchronous machine from slipping out of

phase due to slippage of the rotor The load angle δ the difference in phase between the

rotor and the stator rotating field results mainly from the driving torque (active power P)

acting on the generator and the level of rotor current (field current) If the driving torque

remains constant a increase in the field current reduces the load angle δ The current load

angle δ at any moment is obtained from the generator current and generator voltage based

on a simplified model of the generator Whenever this calculated load angle δ exceeds the

preset limit angle the limiter increases the field current until the load angle has dropped

back to its permissible value The quadrature reactance Xq of the generator and the

network reactance Xe during normal operation must be adjusted on the regulator in order

to obtain the load angle δ The graph below shows the Power Chart for a salient-pole

machine with typical limiter characteristics

The purpose of a Power System Stabilizer is to use the generator excitation to

damp electromechanical oscillations between the network and the generator Depending

on the design of the generator and the requirements imposed for network stability its

main function will be either to damp the oscillations originating in the machine or those

from the network A synchronous generator working in a combined power network is in

principle an oscillating structure In order to produce a torque the magnetic field of the

rotor and the stator must form a given angle (referred to as the rotor displacement or load

angle δ) The electrical torque ME increases as the angle δ increases just as with a

torsion spring Because the ME of the generator and the mechanical driving torque MA

from the turbine are in equilibrium during steady-state operation the angle δ remains in a

given position Whenever this state of equilibrium between MA and ME is disturbed the

load angle slips of this rest position and change thereby the electrical torque ME The

torque attempts to restore the load angle to a stationary position Due to the mass inertia

of the turbinegenerator rotor however this can only take place aperiodically It does so

in the form of more or less effectively damped oscillations (again similar to the effect of

mass inertia on a torsion spring) In order to damp the oscillations there must be a

damping torque produced depending not on the electrical torque ME associated with the

angle but on the difference in frequency (Df) between the rotor and the stator rotating

field ie on the slippage This torque is produced mainly by the so-called damper

winding in the rotor but the dimensioning of this is subject to limits imposed by

considerations of design and economy Some further action is therefore needed to

increase the damping effect The following drastically simplified formula shows the

parameters upon which the amount of active power PE supplied by the generator

depends

PE = active power

It can be seen from the above relationship that the active power that the generator

transfers depends not only on the load angle δ but also on the field current If That means

that a transient change can be made in the active power PE and with that in the effective

electrical torque ME by varying the field current The principle of operation of the DVR

Power System Stabilizer becomes clear from a consideration of the oscillations in power

output and frequency (ΔPE Δf) and the vector diagram If it is assumed that oscillations

in the network frequency generates load oscillations with the mass inertia of the rotor

then the active load of the generator (eg MW-measured) is influenced with a sinusoidal

value -ΔPE (ME-MA = -ΔPE) By inversion of -ΔPE one obtains the fluctuation in

power provided by the rotor +ΔPE As is known the slip signal Δf follows +ΔPE with a

phase delayed by 90deg The +ME produced by the periodic changes in the load angle δ is

in phase with +ΔPE A good damping is attained if ME is varied in phase with the slip

Δf However this signal must also be advanced somewhat to compensate for the time

constants in the excitation circuit and the generator

As mentioned above the electrical torque ME can be influenced by varying the

field current To accomplish this a suitable control signal referred to as variable

disturbance compensation must be imposed upon the voltage set-point or the converter

control variable Ucontr As can be seen from the vector diagram by applying proper

weighting factors (K1 K2) and then adding together the signals -ΔPE and Δf an overall

stabilization signal can be produced that rotates in advance of the Df signal by any angle

desired between 0deg and 90deg Because the amplitude of -DPE remains proportional to the

amplitude of Δf a constant angle in advance of Δf results for the compensation of the

time constants referred to above The optimum weighting factors K1 and K2 for a

synchronous generator working to a power network depend on its operating point at any

moment and the external reactance of the network Normally the selection of a

compromise setting is good enough to attain stability in all operating points and for all

external reactance For special demands these settings must be parameterized as a

function of the external reactance (which means optional equipment Xe-Identification)

The Power System Stabilizer PSS is a section of the AVR computer program and is

processed once per network cycle The voltage at the generator terminals and the

generator current are measured in order to define the signals ΔPE and Δf The calculated

signals for _P_ (=PE) and Δf are then sent across DC filters ldquoDrdquo (real differentiators) that

transmit only the dynamic portion of the signals The ΔPE and Δf signals obtained in this

way are then weighted (multiplied by) with the factors K1 and K2 and sent to the

summing point of the voltage regulator

The PSS stabilization signal is imposed on the automatic voltage regulator only if

the following prerequisites are met

bull Generator on line

bull Generator power output gt the value F338

bull Generator voltage in a range between F33C and F33A

The stabilization signal is limited at the output from the PSS to the lower and

upper limits Flag defines whether the stabilization signal is introduced before or after the

PID filter (usually before the filter) Because the PID filter as noted above already takes

the ceiling factor Vp1 into account the PSS signal needs to be multiplied by Vp1 if it is

added to the voltage regulator following the PID filter (divider at the input to the

minmax limiter) This precaution prevents the DC filter ldquoDrdquo in the P-channel from

producing an unnecessary ldquostabilizationrdquo effect in the case of rapid changes in turbine

load As an alternative for the AVRrsquos Power System Stabilizer a stabilization signal from

an outside system can be imposed by activating the binary input ldquoPSS-SIGNEXTrdquo Flag

F340 can be used to select between an analog and a 12-bit signal and F33E to select the

polarity desired for that signal

24 The MANUAL Channel

241 Summary

The MANUAL channel (Channel 2) has been built as a simple field current

regulator

without additional limiters Its main function is to maintain the excitation of the generator

even if the AUTOMATIC channel becomes non-operational The MANUAL channel

also performs valuable service for purposes of testing commissioning and preventive

maintenance Its measurements regulator generation of firing pulses and power supply

are physically separate from those on the AUTOMATIC channel

242 Principle of Operation

All the functions of the MANUAL channel including the generation of firing

pulses have been implemented in a single electronic module the Gate Control Unit The

control variable Ucontr of voltage regulator is used as the reference value for generating

firing pulses on the principle known as ldquoramp controlrdquo (Comparison of Ucontr with

Usynsynchronous sawtooth signal) For further processing in the UN 0096 Intermediate

Pulse Stage the Gate Control Unit supplies six firing pulses at its output whose phase

position with respect to the synchronous voltage Usyn is in accordance with control

variable Ucontr An internal linearization ensures that the field voltage produced via the

firing pulses remains proportional to the control variable Ucontr throughout the entire

range As a result the circuit amplification of the control remains constant over the entire

range Whenever excitation is switched ON the set-point for Generator Voltage is set

automatically at the preset - ref Value This provision ensures that the generator voltage

always attains approximately its nominal value after the field flashing The Gate Control

Unit can be refunctioned ( by pre-selection with a switch ) for purposes of testing to act

as a purely firing pulse control In this case the control variable Ucontr is adjusted

directly using the RAISELOWER push buttons on the front of the module In this way

for example the relationship between the phase position of the firing pulses and the

control variable Ucontr can be checked easily

CHAPTER 3

PULSE SECTION

31 Pulse Generation and Amplification

The Gate Control Units of both AUTOMATIC channel and MANUAL channel

each supply six firing pulses for operating the 6-pulse thyristor bridges The low-power

pulse signals from these Gate Control Units are then amplified in the Intermediate Pulse

Stage galvanically isolated and then sent to the common pulse bus On the output end

the Intermediate Pulse Stage of the non-active channel) is always blocked The Gate

Control Units generate the pulses based on microprocessor control The reference voltage

used for the firing pulse phase location is the output voltage from the excitation

transformer (Usyn1 Usyn2) The commutation spikes of the synchronous voltage caused

by the converter are calculated prior to use of the voltage as a reference value and are

deliberately filtered out The lower limit for the firing pulses (double pulses) which are

offset from one another by 60deg is defined by the limit rectifier position (αmin) and the

upper limit by the limit inverter position (αmax) for the firing angle αmin and αmax can

be adjusted on the Gate Control Units using BCD (Binary Coded Decimal) switches

αmin ensures that the firing pulses will not be issued (premature firing) until there is

sufficient positive phase voltage on the thyristor involved αmax prevents a dangerous

ldquotippingrdquo of the thyristor bridge into the rectifier mode if the firing angle α is too large

(ldquolate firingrdquo) The critical factors determining αmax are the overlap time uumlmax (max

commutation time) and the ldquorecovery timerdquo of the thyristors (αmax lt 180deg - uumlmax - γ )

An external control signal can force the firing pulses into their inverter limit position

Other binary inputs can block or direct the firing pulses of the Gate Control Units so as to

produce freewheeling on the thyristor bridge During freewheeling the firing pulses for

the thyristor pair R and S are blocked and the pulse signals T+T- are engaged with

chains of pulses Both Gate Control Units (for the MANUAL amp AUTOMATIC channels)

contain a field current monitor that blocks the firing pulses immediately whenever the

current exceeds a preset threshold level In this case the field circuit-breaker is also

tripped via an output contact The purpose of these provisions is to prevent damage to

thyristors and thyristor fuses in case of a slip-ring short-circuit or to keep any damage

that does occur to a minimum The pulse signals are galvanically separated at the outputs

from the Intermediate Pulse Stage (with pulse transmitters) and are then directed to the

common pulse bus This transmission of the pulse signals to the pulse bus via passive

transmitters ensures a high degree of active channel autonomy Practically no possible

malfunctions on the inactive channel (including for example sustained pulses) affect the

active channel

32 Pulse Monitoring

The ldquoPulse Busrdquo and the pulse signals of the AUTOMATIC channel are

monitored This monitoring device consists of potential isolating stages and the common

monitor If the pulse monitoring of the ldquoPulse Busrdquo responds a switch-over is made to

MANUAL channel The function of the potential isolating stages is to couple the pulse

monitoring device to the pulse circuits without any feedback effect The pulse monitoring

checks the six pulse lines for the following malfunctions continuous or periodic failure

of one or more pulses Periodic occurrence of synchronous or asynchronous false pulses

Continuous pulses the pulse monitoring device can be tested while the machine is in

operation

CHAPTER 4

CONVERTER

Thyristor

The term thyristor usually refers to a family of four layer solid state device having

turn on characteristics that can be externally controlled by either current or voltage They

are also referred to as breakdown device because their working depends on avalanche

breakdown Thyristors have only two stages OFF and ON Thyristors have a similar

function to Uni-junctions they act as switches Thyristors use current flow as a switch

Thyristors have three states

1 Reverse blocking mode mdash Voltage is applied in the direction that would be

blocked by a diode

2 Forward blocking mode mdash Voltage is applied in the direction that would cause

a diode to conduct but the thyristor has not yet been triggered into conduction

3 Forward conducting mode mdash The thyristor has been triggered into conduction

and will remain conducting until the forward current drops below a threshold value

known as the holding current Converter is a semiconductor device which converts ac

input voltage into a constant dc output voltage In present excitation system three phase

fully controlled thyristor converter is used

Because of the following advantages thyristor converters are used

a) Thyristors are used for high power applications ie up to 10Kv3500A1KHz

b) Having high reliability and low losses

c) Uni-directional device like diode

d) Itrsquos operation as a rectifier which are low resistance in forward conduction

mode and high resistance in reverse conduction mode

PROTECTION OF THYRISTORS

For reliable operation of a thyristor demands that its specified ratings are not

exceeded When Subjected to or over voltages During the turn - on of SCR didt

prohibitively large False triggering of SCR by high value of dvdt andSpurious signals

between gate and cathode may leads to unwanted turn ndash on

DIDT AND PROTECTION

When thyristor starts conducting in forward conduction mode and is turned on by

gate pulse The anode current increases rapidly whole area of the gate to Cathode

junction then hot spots will be formed near the gate connection this locality of heating

destroys the thyristor Thyristor thermal time is constant The causes due to faults and

short circuits or surge currents Electronic crowbar protection is used against the over

voltages The rate rise of anode current must be kept at the time of turn on below the

rated or specified limiting value The didt value maintained below limited value by using

a inductor also called ldquodidt inductorrdquo in series with anode circuit The locality of heating

is avoided by applying gate current but not greater the maximum gate current

DVDT AND OVER VOLTAGE PROTECTION

With forward voltage across the anode and cathode of a thyristor the two outer

junctions are forward biased but the inner junction is reverse biased This reverse biased

junction J2 has the characteristics of a capacitor due to charges existing across the

junction In other words space-charges exist in the depletion region around junction J2

and therefore junction J2 behaves like a capacitance If the entire anode to cathode

forward voltage Va appears across J2 junction and the charge is denoted by Q then a

charging current i given by Eq (46) follows

i = dQdt =d(Cj Va )dt

= Cj (d Va dt) + Va(d Cj dt) helliphelliphelliphellip(46 a)

As Cj the capacitance of junction J2 is almost constant the current is given by

i = Cj (d Va dt) helliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip(46 b)

If the rate of rise of forward voltage dVadt is high the charging current i will be

more This charging current plays the role of gate current and turns on the SCR even

when gate signal is zero Such phenomena of turning-on a thyristor called dvdt turn-on

must be avoided as it leads to false operation of the thyristor circuit

For controllable operation of the thyristor the rate of rise of forward anode to

cathode voltage dVadt must be kept below the specified rated limit Typical values of

dvdt are 20 ndash 500 Vμsec False turn-on of a thyristor by large dvdt can be prevented by

using a snubber circuit in parallel with the device thyristor are very sensitive for over

voltage than the semiconductor devices

Over voltage transients are perhaps the main cause of thyristor failure

In thyristor there are mainly two types

1 Internal over voltages

Due to the commutation of the thyristors large voltages are generated internally

Because of the series inductance of the SCR circuit the large transient voltages L didt

produced This voltage several times the break over voltage of the device then thyristor

destroys permanently

2 External over voltages

External over voltages are caused due to the interruptions of current flow in an

inductive circuit and also due to the lightening strokes on the lines feeding the thyristor

system For the reliable operation of thyristor the over voltages must be suppressed by

adopting suitable techniques

Suppression of over voltages

The RC circuit called snubber circuit is connected across the device to protect In

order to keep the protective components to a minimum the thyristors are chosen with

their peak voltages ratings are 25 to 3 times of the normal peak working voltage

ldquoselenium thyrector diodes metal oxide varistors or avalanche diode suppressers are

commonly employed for protecting the thyristor circuit against the over voltages

Gate protection

Gate circuit should also be protected against the over voltages and surges Over

voltage at gate circuit can cause false triggering of the SCR may rises the junction

temperature behind specified limit leading to its damage Protection against over

voltage can be achieved by connecting a ZD across the gate circuit and a resister is

connected in series with gate circuit to protect against the s A capacitor and resister are

connected across gate to cathode to by pass the noise

41 Final Pulse Stages

The Final Pulse Stages adapt the output pulses from the Intermediate Pulse Stage

(pulses on the pulse bus) to the gate currents needed for the thyristors Each thyristor

bridge is equipped with its own Final Pulse Stage Each Final Pulse Stages is provided

with a power supply module The amplified output pulses from the Final Pulse Stages

start as a short strong steep pulse with an amplitude approx 2frac12 times that of the main

pulse This initial pulse edge assures proper firing of the thyristors being triggered

Subsequently the weaker part of main pulse keeps firing conditions steady As already

mentioned the Final Pulse Stages and their associated thyristor bridges form single units

All six pulse outputs from a Final Pulse Stage can be blocked by an external control

signal so that all thyristors in the associated thyristor bridge will block the current A

blocking of the pulses is initiated whenever there is a malfunction in the associated

thyristor bridge

42 Converter Power Section

The thyristor converter consists of three independent parallel rectifier blocks TY1

to TY3 which are all in service Even if one block fails the remaining blocks take over

automatically the full design current of the excitation circuit During normal operation

(with ideal current share) and all three bridges in operation each of these blocks has to

carry only (n-2)n (ie33)of its design current If 2 thyristor bridges fail the excitation

is limited Only when all three bridges fail the excitation is switched off Each thyristor

bridge arm is equipped with current flow monitoring CTrsquos Failure of conduction in any

arm is identified by a Current flow monitoring module

43 Converter Cooling

A cooling system is needed to dissipate heat losses in the converter blocks and

electronics Each converter block has therefore been equipped with a fan supplied with

power from the converterrsquos primary voltage (via transformer ndashT8 in field flashing

cubicle) The fans are protected with motor protection circuit breakers An air flow

monitoring unit is provided for monitoring the air flow through the thyristor bridge If a

circuit breaker failure is detected or if the air flow monitor drops off at one of the

thyristor bridges the bridge involved is immediately set out of operation by blocking its

firing pulses

44 Thyristor Converter Monitoring

A thyristor bridge in which defects occur that could threaten the safety of

operation or cause secondary damage is switched off automatically ie its firing pulses

are blocked This happens whenever A thyristor fuse is blown The fuses are monitored

individually with micro switches The Final Pulse Stage fails which is detected by

internal monitors (supply voltage sustained pulse short-circuit on the output end) The

power supply to the fan fails fan air flow as monitored by the Air flow monitor fails or

is insufficient Isolator on ACDC side is open

CHAPTER 5

Field Current Circuit Excitation Transformer

51 Field Circuit Breaker

The circuit-breaker in the field circuit is used to isolate the field circuit from the

converter It is capable of switching off the synchronous machine from full load under the

maximum conditions of a 3-phase short-circuit In addition to its main contacts the field

circuit-breaker also has a de-excitation contact with which the field energy stored in the

field can be dissipated across the de-excitation resistor The de-excitation contact closes

shortly before the main contacts open so as to ensure proper commutation of the field

current from the main contacts to the de-excitation contact when the breaker is switched

off The field circuit-breaker is switched on by electromagnetic force and is kept switched

on by a mechanical latch When the latch is released by a trip coil the circuit-breaker

opens The circuit-breaker also has auxiliary contacts that report its status

52 Field Flashing

In shunt supplied excitation circuits (excitation transformer connected to the

generator terminals) the generator does not have enough remnant voltage for a generator

voltage build-up via the converter In this case a field flashing circuit is provided It

consists of the field flashing contactor the diode bridge and a transformer used to adapt

the auxiliary input voltage to the voltage needed for field flashing when power is

supplied from the auxiliaries network

Fig Field Flashing

Because the field flashing contactor is not able to switch off the energy stored in

the field the control ensures that the contactor can only reopen if the field circuit breaker

has already been opened (generating the TRIP order) or in a normal field flashing

sequence when the converter has taken over the field current Field flashing occurs in the

following stages

1048729The excitation is switched on closing the field flashing contactor ( Field

Circuit Breaker is already closed )

1048729The start-up excitation current flows through the rotor driving the generator

voltage up to approx 15 U

1048729After about 10 U the firing pulses to the converter are released and it begins

to excite the generator to its rated voltage

1048729After about 30 U the field flashing contactor opens (with no current since

the converter is now supplying the current)

The diode bridge at the input to the field flashing contactor prevents a feed-back

from the converter to the source of field flashing while the contactor is still closed

53 De-excitation

When malfunctions occur the stored field energy must be dissipated as quickly

and safely as possible to protect the generator This is done by the converter the field

circuit-breaker and the de-excitation (discharge) resistor

De-excitation (with opening of the field circuit-breaker) takes place in the following

stages

1048729The converter drives to its inverter limit position (negative ceiling voltage)

recovers a portion of the field energy into the network A trip command is given to the

field circuit breaker

1048729The de-excitation contact closes diverting the field voltage to the de-excitation

resistor

1048729Then immediately the main contacts open building voltage The field voltage

commutates to the de-excitation resistor

1048729The current diminishes at a given time constant TE

(With linear resistance TE = Lf (Rf + Re))

Due to the reversal of the field voltage by the converter the field current

commutates from the main contacts of the field circuit-breaker to the de-excitation

resistor in a very early phase This reversal of the field voltage prevents burn-off on the

main contacts and provides effective protection for the field circuit-breaker Depending

on the operating policy an operational shut-down of the excitation can also be effected

with the field circuit-breaker closed This method is useful mainly when the excitation is

switched on and off frequently In this case the converter is merely driven into the

inverter limit position so that the field energy is recovered into the network The

converter then blocks since it is supplying positive current only

54 Excitation Transformer

The excitation transformer matches the generator voltage to the field voltage

(required ceiling voltage) It also serves as a commutation reactance for the thyristor

converter and as a potential isolator between the network and the excitation circuit In

addition the transformer functions as a current limiter in that it makes it possible to keep

any short circuits in the excitation circuit under better control The excitation transformer

is equipped with temperature monitoring probes which set off an alarm when the

temperature exceeds a first max limit and then trips the excitation if the temperature

continues rising to a second (higher)limit

CHAPTER 6

Monitoring and Protection

61 Excitation Monitoring

611 General Information

The main goal of Excitation Monitoring is to make optimum use of the

redundancies provided in the excitation system and to give alarm whenever a malfunction

makes these redundancies unavailable The field current is monitored to see that it does

not exceed a maximum level and if necessary a switch-over to the MANUAL channel is

initiated In addition the criterion for switching off the field flashing is generated The

excitation Monitoring consists of an autonomous processor system

612 over current Alarms

In the Excitation Monitoring the limits for are set at higher levels than the

settings on the Field Current Maximum Limiter Whenever the current exceeds 110 of

the nominal field current contact R1 and the binary output associated with it are

activated immediately If field current remains gt 110 then after a preset inverse-time

has lapsed relay R2 and - after a further delay - relay R0 and the binary outputs

associated with them are activated Parameters match the measurements for If1 and If2 to

the nominal value for field current so that the internal values can be processed and read as

pu values It can be used to falsify the actual value of the field current If (to raise it) so

as to cause a response from the alarm limits for purposes of testing The processed If

signal is always taken from on the active channel (CHANNEL I OR CHANNEL II) As

long as the field current If is above the threshold value 11 Ifn its peak value is

measured This is stored (until RESET) and can be read at any time on the Micro-

Terminal Once the value of If exceeds 11 Ifn integration of this value starts Whenever

the integrated time-current value (intisup2dt) exceeds the preselected reference value the

alarm OVER CURRENT INVERSE-TIME is set off and a command is simultaneously

issued to switch over to the stand by AUTO channel Software switch F758 enables the

three over current alarm functions (R0 R1 R2) and selects one of three possible inverse-

time curves T1 T2 or T3 Within the characteristic curve (T1 T2 T3) selected the

desired limit curve for response is set using the factor F216

613 Switch-Off Criterion for Field Flashing

The Excitation Monitoring supplies the criterion for switching off the field

flashing Whether this criterion is activated based on the actual value for generator voltage

Ug or for field current If or both depends on the settings of the two threshold values

F200F202 (0 setting means that the output is always ldquological 1rdquo) The measurements

Ug12 and If12 are switched over depending on the present status of the channels

(Channel 1 or Channel 2 ON) Whenever Flag F750 is not activated the binary output is

fixed at ldquological 1rdquo

614 Storage of Alarm Status

The outputs of the over current alarms (R1 R2 R0) and the messages NO

FAILURE MONITORING PARAMETERS CHANGED are stored messages can be

erased by activating the input ldquoGENERAL RESETrdquo or by using the RESET button on the

front of the module Erasure with the input ldquoGENERAL RESETrdquo is effective only if the

situation causing the alarm or the malfunction is no longer present Whenever the self-

diagnosis equipment in the processor detects a malfunction the output NO FAILURE of

MONITORING is set at ldquological 0rdquo (= alarm) The alarm ldquoPARAMETERS CHANGEDrdquo

is activated whenever parameters or settings of software switches have been changed via

the Micro-Terminal

615 Actual Value Monitoring

The actual values for generator voltage Ug synchronous voltage Usyn and field

current If are monitored for malfunctions This monitoring is active regardless of whether

or not the generator is in operation Essentially when the generator is in operation the

measurements are monitored by comparing the signals (the smaller signal reading is

detected as incorrect) When the generator is not in operation the measured data are

monitored for extreme values The percentage of deviation permissible in the

measurement signals being compared

(Ug1ampUsyn1 Ug1ampUg2 Ug2ampUsyn2 If1ampIf2) is defined by parameters F208 and

F20AIf the excitation transformer is being supplied from an auxiliary power source (no

shunt operation) the values of Ug and Usyn will be different in some operational

conditions

In that case Flag F75A can be used to deactivate comparative measurements Ug1 Usyn1

and Ug2 Usyn2 The ldquoprimary monitoringrdquo compares Ug1 with Ug2 and generates the

messages that Channel 1 or Channel 2 has suffered a malfunction Whenever Ug1 lt Ug2

and the binary message from CH1 reports no malfunction a malfunction on

Measurement Channel 1 is reported (Ug1Usyn1 FAILURE) A similar malfunction is

also present whenever the binary message CH1 DISTURBANCE is reported and a

discrepancy is detected between Ug1ampUg2 The generation of the alarm ldquoUg2 Usyn2

FAILURErdquo is analogous to that for Channel 1 The ldquosecondary monitoringrdquo compares

Ug1 with Usyn1 amp Ug2 with Usyn2 This is enabled whenever the binary message of the

comparison channel reports a malfunction or whenever both binary messages report no

malfunction - but both secondary monitors report a malfunction As long as the secondary

monitoring is blocked the differences Ug1neUsyn1 or Ug2neUsyn2 trigger malfunction

signals for the measurement channel involved (suspicion that there is a corresponding

error in Usyn) The measurement channel malfunctions are enabled operationally

whenever after excitation has been switched on generator voltage Ug exceeds the value

set on F204 The voltages Ug1 and Ug2 are checked 16 seconds after the excitation is

switched off to see that they do not exceed the limit value F210 that applies to both of

them At the same time g1Ug2Usyn1Usyn2 are checked for extreme values (gt or lt

the operating range) Monitoring for extreme values is likewise enabled during normal

operation ( Excitation ON and Ug gt F204 ) Flag F754 is used to enable or block the

malfunction signals to the binary outputs Basically the monitoring of the actual values

for If1If2 functions like that of the Ug1Ug2 monitoring

62 Excitation Protection

621 General Information

The Excitation Protection switches off the excitation (and de-excites the machine

rapidly) whenever a danger arises that threatens the excitation transformer the converter

or the generator Generally limiter or monitoring functions precede the emergency trips

and these normally respond before the Excitation Protection must initiate a trip

Protective trip commands are issued directly to the field circuit-breaker from potential

free contacts of the board via the trip relays They are directed redundantly to the

operative field circuit-breaker ldquoOFFrdquo command

622 Protection against Excitation Transformer Overheating

This equipment monitors the excitation transformer for overheating in the

windings that could result from over current short-circuits or inadequate cooling The

monitoring uses temperature monitoring modules in conjunction with temperature

sensors built into the windings Normally the temperature is monitored in two stages the

first stage sets off an alarm the second causes a trip of the excitation

623 Rotor Over voltage Protection

Malfunctions in the generator circuit (eg terminal short-circuit failed

synchronization asynchronous operation) cause induced negative field currents that

produce high voltages in the field circuit These must be restricted to a level with a

sufficient safety margin below the insulation capacity of the field winding (test voltage)

and also below the peak blocking voltage of the converter thyristors The crow bar

employs spark gap elements to detect over voltages in the field circuit Whenever they

respond the associated thyristors are fired immediately switching the de-excitation

resistor parallel to the field The de-excitation current generated thereby initiates an

excitation trip via a supervision circuit causing an immediate opening of the field circuit-

breaker The malfunction isets off an alarm and an internal malfunction is indicated at the

cubicle

TEST VALUES OBTAINED WHEN EXCITATION IS RAISED

TEST

SNO PARTICULARS ACTUAL VALUE OBTAINED VALUE

1 VREF 996 100

2 VACT 997 1003

3 IFACT 735 765

4 IGACT 703 707

5 ACTIVE

POWER

703 705

6 REACTIVE

POWER

101 142

7 POWER

FACTOR

099 IND 098 IND

8 ACTIVE

CURRENT(IR)

705 703

9 REACTIVE

CURRENT(IX)

102 138

10 POWER

ANGLE

566 546

11 FIRING

ANGLE

640 632

VALUES OBTAINED WHEN EXCITATION IS LOWERED

SNO PARTICULARS ACTUAL

VALUE

OBTAINED VALUE

1 VREF 100 997

2 VACT 100 997

3 IFACT 787 761

4 IGACT 839 837

5 ACTIVE

POWER

839 830

6 REACTIVE

POWER

155 110

7 POWER

FACTOR

098 IND 099 IND

8 ACTIVE

CURRENT(IR)

835 830

9 REACTIVE

CURRENT(IX)

140 94

10 POWER

ANGLE

613 DEG 631 DEG

11 FIRING

ANGLE

601 DEG 599 DEG

CONCLUSION

For generating the EMF in stator winding excitation is required to the rotor of a

generator There are two types of excitation

1 Static excitation system

2 Brushless excitation system

A certain disadvantage in brushless excitation system is the slow response time of

the field in case of fast load changes specified No slip-rings and brushes direct

measurements of the field parameters not possible

To avoid all loses static excitation is used Since it does not have any rotating

parts mechanical loses and windage loses This system has fast response and speed

control While preferring this excitation system there are no limitations for the

redundancy of Thyristor bridge circuits

Static excitation has fast field discharge by resistor and inverter operation direct

measurement of field quantity is possible The meaning of excitation is nothing but

continuous supply of DC current (ie field current) to the rotor to buildup required

output voltage in the stator

Field current is changed with respect to the change of load so the digital

automatic voltage regulator (DAVR) is used to regulate the output voltage according to

the load variations

So we conclude that static excitation system with DAVR is preferred since it is

having excellent dynamic performance and better options for R amp M

them to issue the pulses at a given firing angle relative to the input voltage of the

converter

5

CHAPTER 2

Digital Automatic Voltage Regulator (DAVR)

21 Principle of Operation of the Regulator (DAVR)

To regulate the voltage and the reactive power of a synchronous machine the

field voltage must be adjusted quickly to the changes in the operating conditions (with a

response time that does not exceed a few ms) To accomplish this analog control systems

include amplifiers which make continuous comparison of the actual values against the

reference values and vary the control variable to the converter with almost no delay Most

of the delay that occurs originates in the converter since the firing pulses for changing

the rectifier phase angle are only issued periodically (every 33 ms)

The DVR digital voltage regulator calculates the control variable from the

measured and reference data in very short time intervals This results outwardly in a

quasi-continuous behavior with a negligible delay time (as in an analog regulator) The

calculations are made in the binary number system Analog measurement signals such as

those for generator voltage and generator current are converted into binary signals in

analogdigital converters The set-points and limit values have already been defined in

digital (binary) form An understanding of the actual computation processes in the digital

voltage regulator is not necessary for operation preventive maintenance or

troubleshooting Like the operator of a pocket calculator or a personal computer all the

operator needs is to know how to operate the instrument and the programming for this

working tool For that reason we will explain below only the principle division of work

among the various modules and the flow of data processing The purpose is above all to

make clear how the processor system has been integrated into the rest of the power

electronics system

6

22 Basic Structure of the Processor Systems

7

The signal processors 25 analog inputoutput modules Each of these processor

systems has a common bus circuit and output and the control lines There is a specific

range of addresses assigned to each assignment Board including the power supply bus

the address lines the two data lines to the input calculates the reactive current (I sin φ)

and the active current ( Icos φ) With these two channel processor Synchronized with

these interrupts (ie with the phase positions of current Ig the field current If and the

synchronous voltage Usyn From the exchange data with the microprocessor card across

the two data lines generator voltage Ug) this processor measures the generator current

Ig and then hardwired connections or multi-conductor cables Binary and analog

inputoutput modules ie for galvanic isolation and adaptation to the electronics level

The most important input interrupts per period to trigger the cycles for processing actual

values in the AUTOMATIC module on the processor bus) for filtering and further

processing

Monitoring each consist of the central microprocessor module and binary and

parameters to the AUTOMATIC channel are the generator voltage Ug the generator

peripheral unit Ug Ig and Usyn are sent to the Interrupt Generator (plug-in peripheral

units (wall-mounted units) peripheral units are used for preprocessing signals from

external measurement circuits power supply units Signals are exchanged among these

processor systems via processed across separate peripheral units for each channel These

processor working on the bus (a house address that can be adjusted using a switch)

systems The AUTOMATIC channel the programmable controls and the Excitation The

actual values measured from AUTOMATIC channel and MANUAL channel are The

AUTOMATIC channel and the MANUAL channel each have their own The digital

voltage regulator is broken down into several autonomous microprocessor The inputs and

outputs of the processor systems are directed across voltage-isolating The Interrupt

Generator also uses the 3-phase Ug signal to generate the 12 themselves contain a limited

number of hardware inputs and outputs with fixed equipment Whenever addresses from

this range are called up the signal processing module can results the processor is then

able to derive further operating parameters such as the load angle the active power etc

The functions of all microprocessor systems other than the programmable controls

have been accomplished in firmware The non-varying standard function modules can be

configured to the design desired for plant-specific purposes using software switches

(KFlags) Thus for example the stored status of a K-Flag determines whether or not a

Limiter is active and whether the de-excitation or the excitation limiters take precedence

Because these K-flags determine the software Scope of Supply for the installation they

cannot be changed permanently via the Micro-Terminal In this way they differ from

such setting data as the values of the parameters for the PID filter of the voltage regulator

or the set-points for the limiters These values can be permanently changed using the

Micro-Terminal Communication is possible with each of the processor systems via the

Micro-Terminal by plugging on the connecting cable In this way signals within the

processor and setting parameters can be viewed analog signals can be issued and the set

parameters can be altered temporarily (F range) or permanently (C range) Unlike the

other processor systems the programmable controls do not include any firmware for

realization of the functions They have been designed so that the designer can adapt and

change their functions easily using the ldquoFunctional Block Programming Language P10

Digital and analog functions can be implemented in practically any degree of complexity

desired using the P10 functional blocks The control variable of the voltage regulator

(AUTOMATIC channel) and the control variable of the field current regulator

(MANUAL channel) are each processed in separate Gate Control Unit and formed into a

chain of pulses at the appropriate firing angle The pulses of the active channel are

directed to the pulse bus via the associated Intermediate Pulse Stage The pulses for each

converter block are amplified sufficiently in Final Pulse Stage to fire the Thyristor

231 General Information

The functions of the automatic voltage regulator AVR are

1 to regulate the generator voltage

2 to regulate the effect of the reactive andor active current on the voltage

3 to limit VoltHz

4 to limit max and min field current

5 to limit inductive stator current

6 to limit capacitive stator current

7 to limit the load angle

8 to stabilize the power system

Block Diagram shows the software structure of AUTOMATIC channel The

generator limiters not provided for the installation in question (optional equipment) are

identified in this overview as ldquoNot Suppliedrdquo The parameter values signal values and

software switches (flags) marked with addresses (hexadecimal numbers) can be viewed

and altered via the Micro-Terminal The values selected are displayed in sec pu Hz

etc and can where necessary be changed directly in these formats The plant-specific

settings of the variables and the flags can be obtained from the Test and Commissioning

Report This block diagram provides information about the important functions and

possible settings of the AUTOMATIC channel For the sake of clarity no detailed

presentation has been given of special functions such as tracking circuits initializations

etc The page heading cross-refers this overview to the various sheets of the schematic

diagram Binary signals are shown in broken lines analog signals in solid lines The

corresponding text designations in the schematic diagram can be used for identification of

the input signals (hardware inputs) The only analog output signal from the automatic

voltage regulator control variable Ucontr is sent via the data bus (CRU bus) to the Gate

Control Unit Most of the binary messages (outputs) from the AVR are of no interest

functionally and they have been omitted for the sake of clarity The basic structure of the

digital voltage regulator and the limiters is simple This is necessary in order that the

behavior of the regulatorslimiters will remain calculable and understandable in all

operating situations and that there will be no problem in adjusting and optimizing them

The central PID filter in the digital voltage regulator defines the dynamic response of the

closed-loop controls both in the voltage regulator mode and after limiters have

intervened The ldquocontrol deviationrdquo at the input to the PID filter is either the control

deviation for voltage the control deviation of a de-excitation limiter (the value

determined by minimum value selection) or the control deviation of an excitation limiter

(the value determined by maximum value selection) Flag F730 (ldquoPRIORrdquo) is used to

determine whether the exciting (Min value) or the de-exciting signal takes precedence on

the minmax value limiter (normally F730 = 1111 ie the de-exciting signal takes

precedence) With the exception of the Minimum Field Current Limiter all other limiters

have variable factoring multipliers of the signal outputs so that they can be adjusted

individually together with the common PID filter which has been optimized for voltage

regulation The setting parameters for this PID filter are as follows

Vo = KR Static amplification

1

Ta = ---- Integration time constant

Tc1

Vp Proportional amplification

1

Tb = ---- Differential time constant

Tc2

Vinfin Amplification of high frequencies

The BODE diagram below shows the assignment of settings in accordance with

DINIEC standards based on a typical example

The PID filter amplifications Vo Vp and Vinfin can be adjusted in pu values But

the ldquoceiling factorrdquo pl+ must be adjusted correctly with parameter F310 if the total

amplification (circuit amplification) of the control circuit is actually to conform to the

pu settings This factor must agree with the ldquoexternalrdquo amplification ie with the

ceiling value of the transformer- converter circuit

Ceiling factor(pl+) = Ufmax Ufo

in which Ufmax = ceiling field voltage

Ufo = no-load field voltage

To attain a suitable response of the AVR when starting excitation

(ldquoEXCITATION ONrdquo) it may be necessary to change the proportional amplification of

the regulator during this phase Vp2 (transiently activated) and Vp1 (permanently

activated) can be adjusted for this purpose For example the value of Vp2 takes effect

immediately once the excitation is switched on and remains effective for a period as set at

F30C Once the period F30C (eg 5 sec) has expired Vp shifts over to Vp1 (becomes

the steady-state Vp) at the rate of change set The standard operating mode for the PID

filter is voltage regulation for which the discrepancy between the voltage set-point and

the current value for generator voltage Ug (the control deviation) is supplied at the input

To compensate for the voltage drop in the block transformer or whenever several

generators are operating to the same distributing bus the generator voltage must be

varied in proportion to the measured generator current (droop influence) To accomplish

this the voltage set-point is varied as a function of the measured reactive current IX

andor active current IR Flag F712 enables the IX droop Flag F710 the IR droop The

desired compensation is set in F282 and F286 respectively Flags F284 and F288 are used

to select whether this droop influence is to increase the voltage or to reduce it

(compensation) Combined influence of the active and reactive currents is attained by

enabling both droops IX and IR Flag F716 activates a so-called ldquoSoft-Startrdquo at the

starting of excitation This ldquoSoft-Startrdquo ensures that the voltage set-point integrates from

0 to 100 within the time set on F290 when the excitation is switched on

(ldquoEXCITATION ONrdquo) A ldquosmoothrdquo excitation of the generator can be achieved in this

way whenever there is no demand for a quick excitation

232 Voltage Set-Point

Various signals and settings control and limit the voltage set-point F270 For

example the values of F254 and F252 define the normal operating range possible for set-

point adjustment (eg 90 110) using external control commands (control room local

operatorrsquos panel superposed control system) The effective set-point adjustment rate is

governed The set-point can be set at the values of F250 and F256 by activating

appropriate control commands for ldquoSETrdquo input Enabling Flag F71A and activating a

binary input prior to switching on the excitation (ldquoEXCITATION OFFrdquo) sets the Ug set-

point at the value of UAUX This makes it possible for example to ensure that the

generator voltage will agree exactly with the network voltage after the voltage build-up

An external value with variable amplification can be added to the Ug set point by

enabling F724 (for example for stability tests)

233 Regulator Tracking in MANUAL Operation

Whenever the AUTOMATIC channel is not in operation (the MANUAL channel

is ON) a follow-up equipment ensures a smooth switch-back to the AUTOMATIC

mode will always be possible To track the voltage set-point is shifted by means of

RAISELOWER pulses from the Gate Control Unit so that control variable Ucontr at the

output from the PID filter is held steady and identical to the control variable Ucontr from

the MANUAL channel Because this tracking must react slowly resultant transient

control deviations resulting from the amplification in the PID filter might cause severe

interference with control variable Ucontr

To prevent this the follow-up equipment intervenes on the regulators mixing

point with a corresponding compensation signal

234 Ugf Limiter

At under frequency the Ugf Limiter reduces the generator voltage so as to

prevent saturation effects in the supply and measuring transformers To adjust this

limiter the max permissible generator voltage at rated frequency is defined and set

When any under-frequency occurs the generator voltage is thus reduced in proportion to

that setting

235 Field Current Maximum Limiter

The Field Current Maximum Limiter is provided to protect the generator rotor

from s occurring in steady-state and transient operation High field currents are normally

the result of a sharp drop in network voltage or of an improper raising of the voltage set-

point by the operating staff The field current is held steady at the value TH1 ie at the

maximum thermal value permissible for the excitation circuit and the rotor In order that

the generator can support the power network with its transient overload capacity during

brief collapses in voltage a temporary switch-over is made to the transient limit MAX1

(a higher setting) When the generator or the converter is operating at a reduced capacity

These limits TH1MAX1 can be switched over to the lower settings TH2MAX2 by

activating the corresponding binary signals The switch-over from the thermal limit

TH12 to the transient limit MAX12 can be configured in one of three ways

a) Depending on the over current with -dUdt ENABLE

Flag programming F418 = any setting desired F41A = 0000

This variant enables the transient value MAX12 whenever a collapse of voltage

in the network is detected The ENABLE time is fixed and can be set The example

below shows the typical behavior of the limiter configured in this way

b) Dependent on the time integral with -dUdt ENABLE

Flag programming F418 = inactive F41A = 1111

This variant likewise enables the transient value only when a collapse of network

voltage has been detected However the switch-back to the thermal limit is not made

dependent upon the time itself but on the calculated time integral intisup2dt of the The setting

on Parameter F414 in spu takes into account the time the rotor needs to cool down ie

the rate of temperature change in the case of intermittent operation The example below

shows how the timing of the switch-back to the thermal limit depends on the present

value for intisup2dt

The time integral is based on the formula

Example The setting of =isup2dt equivalent to Version a (F416) at a constant 16 times the

nominal field current for 10 seconds (with TH12 = 105) is

c) Dependent on the time integral without any preconditions

Flag programming F418 = 1111 F41A = 1111

In this variant the transient becomes available without any prior conditions

(without a -dUdt ENABLE) with the time integral intisup2dt

237 Inductive Stator Current Limiter

The Inductive Stator Current Limiter holds the stator current Ig within permissible

limits while the generator is in the ldquoover-excitedrdquo operating range by reducing the field

current accordingly The setting TH (thermal limit) provides the limit against stationary s

that might occur To take advantage of the generatorrsquos transient overload capacity a

switch-over is made to the higher setting MAX The principle of operation of this switch-

over to the value MAX permissible only transiently is identical to that employed for the

field current limiter (refer to the description above) When the drive output from the

turbine is very high stator current may exceed permissible limits even while inductive

loading of the generator is low In this case if the stator current limiter is not kept from

influencing the field current the control circuit will oscillate back and forth between the

Inductive Stator Current Limiter (de-

exciting) and the Capacitive Stator Current Limiter (exciting)The output signal of that

function then dominates the control variable of the Ig-dependent limiter via a maximum

value selection

238 Capacitive Stator Current Limiter

239 Load Angle Limiter

The Load Angle Limiter prevents the synchronous machine from slipping out of

phase due to slippage of the rotor The load angle δ the difference in phase between the

rotor and the stator rotating field results mainly from the driving torque (active power P)

acting on the generator and the level of rotor current (field current) If the driving torque

remains constant a increase in the field current reduces the load angle δ The current load

angle δ at any moment is obtained from the generator current and generator voltage based

on a simplified model of the generator Whenever this calculated load angle δ exceeds the

preset limit angle the limiter increases the field current until the load angle has dropped

back to its permissible value The quadrature reactance Xq of the generator and the

network reactance Xe during normal operation must be adjusted on the regulator in order

to obtain the load angle δ The graph below shows the Power Chart for a salient-pole

machine with typical limiter characteristics

The purpose of a Power System Stabilizer is to use the generator excitation to

damp electromechanical oscillations between the network and the generator Depending

on the design of the generator and the requirements imposed for network stability its

main function will be either to damp the oscillations originating in the machine or those

from the network A synchronous generator working in a combined power network is in

principle an oscillating structure In order to produce a torque the magnetic field of the

rotor and the stator must form a given angle (referred to as the rotor displacement or load

angle δ) The electrical torque ME increases as the angle δ increases just as with a

torsion spring Because the ME of the generator and the mechanical driving torque MA

from the turbine are in equilibrium during steady-state operation the angle δ remains in a

given position Whenever this state of equilibrium between MA and ME is disturbed the

load angle slips of this rest position and change thereby the electrical torque ME The

torque attempts to restore the load angle to a stationary position Due to the mass inertia

of the turbinegenerator rotor however this can only take place aperiodically It does so

in the form of more or less effectively damped oscillations (again similar to the effect of

mass inertia on a torsion spring) In order to damp the oscillations there must be a

damping torque produced depending not on the electrical torque ME associated with the

angle but on the difference in frequency (Df) between the rotor and the stator rotating

field ie on the slippage This torque is produced mainly by the so-called damper

winding in the rotor but the dimensioning of this is subject to limits imposed by

considerations of design and economy Some further action is therefore needed to

increase the damping effect The following drastically simplified formula shows the

parameters upon which the amount of active power PE supplied by the generator

depends

PE = active power

It can be seen from the above relationship that the active power that the generator

transfers depends not only on the load angle δ but also on the field current If That means

that a transient change can be made in the active power PE and with that in the effective

electrical torque ME by varying the field current The principle of operation of the DVR

Power System Stabilizer becomes clear from a consideration of the oscillations in power

output and frequency (ΔPE Δf) and the vector diagram If it is assumed that oscillations

in the network frequency generates load oscillations with the mass inertia of the rotor

then the active load of the generator (eg MW-measured) is influenced with a sinusoidal

value -ΔPE (ME-MA = -ΔPE) By inversion of -ΔPE one obtains the fluctuation in

power provided by the rotor +ΔPE As is known the slip signal Δf follows +ΔPE with a

phase delayed by 90deg The +ME produced by the periodic changes in the load angle δ is

in phase with +ΔPE A good damping is attained if ME is varied in phase with the slip

Δf However this signal must also be advanced somewhat to compensate for the time

constants in the excitation circuit and the generator

As mentioned above the electrical torque ME can be influenced by varying the

field current To accomplish this a suitable control signal referred to as variable

disturbance compensation must be imposed upon the voltage set-point or the converter

control variable Ucontr As can be seen from the vector diagram by applying proper

weighting factors (K1 K2) and then adding together the signals -ΔPE and Δf an overall

stabilization signal can be produced that rotates in advance of the Df signal by any angle

desired between 0deg and 90deg Because the amplitude of -DPE remains proportional to the

amplitude of Δf a constant angle in advance of Δf results for the compensation of the

time constants referred to above The optimum weighting factors K1 and K2 for a

synchronous generator working to a power network depend on its operating point at any

moment and the external reactance of the network Normally the selection of a

compromise setting is good enough to attain stability in all operating points and for all

external reactance For special demands these settings must be parameterized as a

function of the external reactance (which means optional equipment Xe-Identification)

The Power System Stabilizer PSS is a section of the AVR computer program and is

processed once per network cycle The voltage at the generator terminals and the

generator current are measured in order to define the signals ΔPE and Δf The calculated

signals for _P_ (=PE) and Δf are then sent across DC filters ldquoDrdquo (real differentiators) that

transmit only the dynamic portion of the signals The ΔPE and Δf signals obtained in this

way are then weighted (multiplied by) with the factors K1 and K2 and sent to the

summing point of the voltage regulator

The PSS stabilization signal is imposed on the automatic voltage regulator only if

the following prerequisites are met

bull Generator on line

bull Generator power output gt the value F338

bull Generator voltage in a range between F33C and F33A

The stabilization signal is limited at the output from the PSS to the lower and

upper limits Flag defines whether the stabilization signal is introduced before or after the

PID filter (usually before the filter) Because the PID filter as noted above already takes

the ceiling factor Vp1 into account the PSS signal needs to be multiplied by Vp1 if it is

added to the voltage regulator following the PID filter (divider at the input to the

minmax limiter) This precaution prevents the DC filter ldquoDrdquo in the P-channel from

producing an unnecessary ldquostabilizationrdquo effect in the case of rapid changes in turbine

load As an alternative for the AVRrsquos Power System Stabilizer a stabilization signal from

an outside system can be imposed by activating the binary input ldquoPSS-SIGNEXTrdquo Flag

F340 can be used to select between an analog and a 12-bit signal and F33E to select the

polarity desired for that signal

24 The MANUAL Channel

241 Summary

The MANUAL channel (Channel 2) has been built as a simple field current

regulator

without additional limiters Its main function is to maintain the excitation of the generator

even if the AUTOMATIC channel becomes non-operational The MANUAL channel

also performs valuable service for purposes of testing commissioning and preventive

maintenance Its measurements regulator generation of firing pulses and power supply

are physically separate from those on the AUTOMATIC channel

242 Principle of Operation

All the functions of the MANUAL channel including the generation of firing

pulses have been implemented in a single electronic module the Gate Control Unit The

control variable Ucontr of voltage regulator is used as the reference value for generating

firing pulses on the principle known as ldquoramp controlrdquo (Comparison of Ucontr with

Usynsynchronous sawtooth signal) For further processing in the UN 0096 Intermediate

Pulse Stage the Gate Control Unit supplies six firing pulses at its output whose phase

position with respect to the synchronous voltage Usyn is in accordance with control

variable Ucontr An internal linearization ensures that the field voltage produced via the

firing pulses remains proportional to the control variable Ucontr throughout the entire

range As a result the circuit amplification of the control remains constant over the entire

range Whenever excitation is switched ON the set-point for Generator Voltage is set

automatically at the preset - ref Value This provision ensures that the generator voltage

always attains approximately its nominal value after the field flashing The Gate Control

Unit can be refunctioned ( by pre-selection with a switch ) for purposes of testing to act

as a purely firing pulse control In this case the control variable Ucontr is adjusted

directly using the RAISELOWER push buttons on the front of the module In this way

for example the relationship between the phase position of the firing pulses and the

control variable Ucontr can be checked easily

CHAPTER 3

PULSE SECTION

31 Pulse Generation and Amplification

The Gate Control Units of both AUTOMATIC channel and MANUAL channel

each supply six firing pulses for operating the 6-pulse thyristor bridges The low-power

pulse signals from these Gate Control Units are then amplified in the Intermediate Pulse

Stage galvanically isolated and then sent to the common pulse bus On the output end

the Intermediate Pulse Stage of the non-active channel) is always blocked The Gate

Control Units generate the pulses based on microprocessor control The reference voltage

used for the firing pulse phase location is the output voltage from the excitation

transformer (Usyn1 Usyn2) The commutation spikes of the synchronous voltage caused

by the converter are calculated prior to use of the voltage as a reference value and are

deliberately filtered out The lower limit for the firing pulses (double pulses) which are

offset from one another by 60deg is defined by the limit rectifier position (αmin) and the

upper limit by the limit inverter position (αmax) for the firing angle αmin and αmax can

be adjusted on the Gate Control Units using BCD (Binary Coded Decimal) switches

αmin ensures that the firing pulses will not be issued (premature firing) until there is

sufficient positive phase voltage on the thyristor involved αmax prevents a dangerous

ldquotippingrdquo of the thyristor bridge into the rectifier mode if the firing angle α is too large

(ldquolate firingrdquo) The critical factors determining αmax are the overlap time uumlmax (max

commutation time) and the ldquorecovery timerdquo of the thyristors (αmax lt 180deg - uumlmax - γ )

An external control signal can force the firing pulses into their inverter limit position

Other binary inputs can block or direct the firing pulses of the Gate Control Units so as to

produce freewheeling on the thyristor bridge During freewheeling the firing pulses for

the thyristor pair R and S are blocked and the pulse signals T+T- are engaged with

chains of pulses Both Gate Control Units (for the MANUAL amp AUTOMATIC channels)

contain a field current monitor that blocks the firing pulses immediately whenever the

current exceeds a preset threshold level In this case the field circuit-breaker is also

tripped via an output contact The purpose of these provisions is to prevent damage to

thyristors and thyristor fuses in case of a slip-ring short-circuit or to keep any damage

that does occur to a minimum The pulse signals are galvanically separated at the outputs

from the Intermediate Pulse Stage (with pulse transmitters) and are then directed to the

common pulse bus This transmission of the pulse signals to the pulse bus via passive

transmitters ensures a high degree of active channel autonomy Practically no possible

malfunctions on the inactive channel (including for example sustained pulses) affect the

active channel

32 Pulse Monitoring

The ldquoPulse Busrdquo and the pulse signals of the AUTOMATIC channel are

monitored This monitoring device consists of potential isolating stages and the common

monitor If the pulse monitoring of the ldquoPulse Busrdquo responds a switch-over is made to

MANUAL channel The function of the potential isolating stages is to couple the pulse

monitoring device to the pulse circuits without any feedback effect The pulse monitoring

checks the six pulse lines for the following malfunctions continuous or periodic failure

of one or more pulses Periodic occurrence of synchronous or asynchronous false pulses

Continuous pulses the pulse monitoring device can be tested while the machine is in

operation

CHAPTER 4

CONVERTER

Thyristor

The term thyristor usually refers to a family of four layer solid state device having

turn on characteristics that can be externally controlled by either current or voltage They

are also referred to as breakdown device because their working depends on avalanche

breakdown Thyristors have only two stages OFF and ON Thyristors have a similar

function to Uni-junctions they act as switches Thyristors use current flow as a switch

Thyristors have three states

1 Reverse blocking mode mdash Voltage is applied in the direction that would be

blocked by a diode

2 Forward blocking mode mdash Voltage is applied in the direction that would cause

a diode to conduct but the thyristor has not yet been triggered into conduction

3 Forward conducting mode mdash The thyristor has been triggered into conduction

and will remain conducting until the forward current drops below a threshold value

known as the holding current Converter is a semiconductor device which converts ac

input voltage into a constant dc output voltage In present excitation system three phase

fully controlled thyristor converter is used

Because of the following advantages thyristor converters are used

a) Thyristors are used for high power applications ie up to 10Kv3500A1KHz

b) Having high reliability and low losses

c) Uni-directional device like diode

d) Itrsquos operation as a rectifier which are low resistance in forward conduction

mode and high resistance in reverse conduction mode

PROTECTION OF THYRISTORS

For reliable operation of a thyristor demands that its specified ratings are not

exceeded When Subjected to or over voltages During the turn - on of SCR didt

prohibitively large False triggering of SCR by high value of dvdt andSpurious signals

between gate and cathode may leads to unwanted turn ndash on

DIDT AND PROTECTION

When thyristor starts conducting in forward conduction mode and is turned on by

gate pulse The anode current increases rapidly whole area of the gate to Cathode

junction then hot spots will be formed near the gate connection this locality of heating

destroys the thyristor Thyristor thermal time is constant The causes due to faults and

short circuits or surge currents Electronic crowbar protection is used against the over

voltages The rate rise of anode current must be kept at the time of turn on below the

rated or specified limiting value The didt value maintained below limited value by using

a inductor also called ldquodidt inductorrdquo in series with anode circuit The locality of heating

is avoided by applying gate current but not greater the maximum gate current

DVDT AND OVER VOLTAGE PROTECTION

With forward voltage across the anode and cathode of a thyristor the two outer

junctions are forward biased but the inner junction is reverse biased This reverse biased

junction J2 has the characteristics of a capacitor due to charges existing across the

junction In other words space-charges exist in the depletion region around junction J2

and therefore junction J2 behaves like a capacitance If the entire anode to cathode

forward voltage Va appears across J2 junction and the charge is denoted by Q then a

charging current i given by Eq (46) follows

i = dQdt =d(Cj Va )dt

= Cj (d Va dt) + Va(d Cj dt) helliphelliphelliphellip(46 a)

As Cj the capacitance of junction J2 is almost constant the current is given by

i = Cj (d Va dt) helliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip(46 b)

If the rate of rise of forward voltage dVadt is high the charging current i will be

more This charging current plays the role of gate current and turns on the SCR even

when gate signal is zero Such phenomena of turning-on a thyristor called dvdt turn-on

must be avoided as it leads to false operation of the thyristor circuit

For controllable operation of the thyristor the rate of rise of forward anode to

cathode voltage dVadt must be kept below the specified rated limit Typical values of

dvdt are 20 ndash 500 Vμsec False turn-on of a thyristor by large dvdt can be prevented by

using a snubber circuit in parallel with the device thyristor are very sensitive for over

voltage than the semiconductor devices

Over voltage transients are perhaps the main cause of thyristor failure

In thyristor there are mainly two types

1 Internal over voltages

Due to the commutation of the thyristors large voltages are generated internally

Because of the series inductance of the SCR circuit the large transient voltages L didt

produced This voltage several times the break over voltage of the device then thyristor

destroys permanently

2 External over voltages

External over voltages are caused due to the interruptions of current flow in an

inductive circuit and also due to the lightening strokes on the lines feeding the thyristor

system For the reliable operation of thyristor the over voltages must be suppressed by

adopting suitable techniques

Suppression of over voltages

The RC circuit called snubber circuit is connected across the device to protect In

order to keep the protective components to a minimum the thyristors are chosen with

their peak voltages ratings are 25 to 3 times of the normal peak working voltage

ldquoselenium thyrector diodes metal oxide varistors or avalanche diode suppressers are

commonly employed for protecting the thyristor circuit against the over voltages

Gate protection

Gate circuit should also be protected against the over voltages and surges Over

voltage at gate circuit can cause false triggering of the SCR may rises the junction

temperature behind specified limit leading to its damage Protection against over

voltage can be achieved by connecting a ZD across the gate circuit and a resister is

connected in series with gate circuit to protect against the s A capacitor and resister are

connected across gate to cathode to by pass the noise

41 Final Pulse Stages

The Final Pulse Stages adapt the output pulses from the Intermediate Pulse Stage

(pulses on the pulse bus) to the gate currents needed for the thyristors Each thyristor

bridge is equipped with its own Final Pulse Stage Each Final Pulse Stages is provided

with a power supply module The amplified output pulses from the Final Pulse Stages

start as a short strong steep pulse with an amplitude approx 2frac12 times that of the main

pulse This initial pulse edge assures proper firing of the thyristors being triggered

Subsequently the weaker part of main pulse keeps firing conditions steady As already

mentioned the Final Pulse Stages and their associated thyristor bridges form single units

All six pulse outputs from a Final Pulse Stage can be blocked by an external control

signal so that all thyristors in the associated thyristor bridge will block the current A

blocking of the pulses is initiated whenever there is a malfunction in the associated

thyristor bridge

42 Converter Power Section

The thyristor converter consists of three independent parallel rectifier blocks TY1

to TY3 which are all in service Even if one block fails the remaining blocks take over

automatically the full design current of the excitation circuit During normal operation

(with ideal current share) and all three bridges in operation each of these blocks has to

carry only (n-2)n (ie33)of its design current If 2 thyristor bridges fail the excitation

is limited Only when all three bridges fail the excitation is switched off Each thyristor

bridge arm is equipped with current flow monitoring CTrsquos Failure of conduction in any

arm is identified by a Current flow monitoring module

43 Converter Cooling

A cooling system is needed to dissipate heat losses in the converter blocks and

electronics Each converter block has therefore been equipped with a fan supplied with

power from the converterrsquos primary voltage (via transformer ndashT8 in field flashing

cubicle) The fans are protected with motor protection circuit breakers An air flow

monitoring unit is provided for monitoring the air flow through the thyristor bridge If a

circuit breaker failure is detected or if the air flow monitor drops off at one of the

thyristor bridges the bridge involved is immediately set out of operation by blocking its

firing pulses

44 Thyristor Converter Monitoring

A thyristor bridge in which defects occur that could threaten the safety of

operation or cause secondary damage is switched off automatically ie its firing pulses

are blocked This happens whenever A thyristor fuse is blown The fuses are monitored

individually with micro switches The Final Pulse Stage fails which is detected by

internal monitors (supply voltage sustained pulse short-circuit on the output end) The

power supply to the fan fails fan air flow as monitored by the Air flow monitor fails or

is insufficient Isolator on ACDC side is open

CHAPTER 5

Field Current Circuit Excitation Transformer

51 Field Circuit Breaker

The circuit-breaker in the field circuit is used to isolate the field circuit from the

converter It is capable of switching off the synchronous machine from full load under the

maximum conditions of a 3-phase short-circuit In addition to its main contacts the field

circuit-breaker also has a de-excitation contact with which the field energy stored in the

field can be dissipated across the de-excitation resistor The de-excitation contact closes

shortly before the main contacts open so as to ensure proper commutation of the field

current from the main contacts to the de-excitation contact when the breaker is switched

off The field circuit-breaker is switched on by electromagnetic force and is kept switched

on by a mechanical latch When the latch is released by a trip coil the circuit-breaker

opens The circuit-breaker also has auxiliary contacts that report its status

52 Field Flashing

In shunt supplied excitation circuits (excitation transformer connected to the

generator terminals) the generator does not have enough remnant voltage for a generator

voltage build-up via the converter In this case a field flashing circuit is provided It

consists of the field flashing contactor the diode bridge and a transformer used to adapt

the auxiliary input voltage to the voltage needed for field flashing when power is

supplied from the auxiliaries network

Fig Field Flashing

Because the field flashing contactor is not able to switch off the energy stored in

the field the control ensures that the contactor can only reopen if the field circuit breaker

has already been opened (generating the TRIP order) or in a normal field flashing

sequence when the converter has taken over the field current Field flashing occurs in the

following stages

1048729The excitation is switched on closing the field flashing contactor ( Field

Circuit Breaker is already closed )

1048729The start-up excitation current flows through the rotor driving the generator

voltage up to approx 15 U

1048729After about 10 U the firing pulses to the converter are released and it begins

to excite the generator to its rated voltage

1048729After about 30 U the field flashing contactor opens (with no current since

the converter is now supplying the current)

The diode bridge at the input to the field flashing contactor prevents a feed-back

from the converter to the source of field flashing while the contactor is still closed

53 De-excitation

When malfunctions occur the stored field energy must be dissipated as quickly

and safely as possible to protect the generator This is done by the converter the field

circuit-breaker and the de-excitation (discharge) resistor

De-excitation (with opening of the field circuit-breaker) takes place in the following

stages

1048729The converter drives to its inverter limit position (negative ceiling voltage)

recovers a portion of the field energy into the network A trip command is given to the

field circuit breaker

1048729The de-excitation contact closes diverting the field voltage to the de-excitation

resistor

1048729Then immediately the main contacts open building voltage The field voltage

commutates to the de-excitation resistor

1048729The current diminishes at a given time constant TE

(With linear resistance TE = Lf (Rf + Re))

Due to the reversal of the field voltage by the converter the field current

commutates from the main contacts of the field circuit-breaker to the de-excitation

resistor in a very early phase This reversal of the field voltage prevents burn-off on the

main contacts and provides effective protection for the field circuit-breaker Depending

on the operating policy an operational shut-down of the excitation can also be effected

with the field circuit-breaker closed This method is useful mainly when the excitation is

switched on and off frequently In this case the converter is merely driven into the

inverter limit position so that the field energy is recovered into the network The

converter then blocks since it is supplying positive current only

54 Excitation Transformer

The excitation transformer matches the generator voltage to the field voltage

(required ceiling voltage) It also serves as a commutation reactance for the thyristor

converter and as a potential isolator between the network and the excitation circuit In

addition the transformer functions as a current limiter in that it makes it possible to keep

any short circuits in the excitation circuit under better control The excitation transformer

is equipped with temperature monitoring probes which set off an alarm when the

temperature exceeds a first max limit and then trips the excitation if the temperature

continues rising to a second (higher)limit

CHAPTER 6

Monitoring and Protection

61 Excitation Monitoring

611 General Information

The main goal of Excitation Monitoring is to make optimum use of the

redundancies provided in the excitation system and to give alarm whenever a malfunction

makes these redundancies unavailable The field current is monitored to see that it does

not exceed a maximum level and if necessary a switch-over to the MANUAL channel is

initiated In addition the criterion for switching off the field flashing is generated The

excitation Monitoring consists of an autonomous processor system

612 over current Alarms

In the Excitation Monitoring the limits for are set at higher levels than the

settings on the Field Current Maximum Limiter Whenever the current exceeds 110 of

the nominal field current contact R1 and the binary output associated with it are

activated immediately If field current remains gt 110 then after a preset inverse-time

has lapsed relay R2 and - after a further delay - relay R0 and the binary outputs

associated with them are activated Parameters match the measurements for If1 and If2 to

the nominal value for field current so that the internal values can be processed and read as

pu values It can be used to falsify the actual value of the field current If (to raise it) so

as to cause a response from the alarm limits for purposes of testing The processed If

signal is always taken from on the active channel (CHANNEL I OR CHANNEL II) As

long as the field current If is above the threshold value 11 Ifn its peak value is

measured This is stored (until RESET) and can be read at any time on the Micro-

Terminal Once the value of If exceeds 11 Ifn integration of this value starts Whenever

the integrated time-current value (intisup2dt) exceeds the preselected reference value the

alarm OVER CURRENT INVERSE-TIME is set off and a command is simultaneously

issued to switch over to the stand by AUTO channel Software switch F758 enables the

three over current alarm functions (R0 R1 R2) and selects one of three possible inverse-

time curves T1 T2 or T3 Within the characteristic curve (T1 T2 T3) selected the

desired limit curve for response is set using the factor F216

613 Switch-Off Criterion for Field Flashing

The Excitation Monitoring supplies the criterion for switching off the field

flashing Whether this criterion is activated based on the actual value for generator voltage

Ug or for field current If or both depends on the settings of the two threshold values

F200F202 (0 setting means that the output is always ldquological 1rdquo) The measurements

Ug12 and If12 are switched over depending on the present status of the channels

(Channel 1 or Channel 2 ON) Whenever Flag F750 is not activated the binary output is

fixed at ldquological 1rdquo

614 Storage of Alarm Status

The outputs of the over current alarms (R1 R2 R0) and the messages NO

FAILURE MONITORING PARAMETERS CHANGED are stored messages can be

erased by activating the input ldquoGENERAL RESETrdquo or by using the RESET button on the

front of the module Erasure with the input ldquoGENERAL RESETrdquo is effective only if the

situation causing the alarm or the malfunction is no longer present Whenever the self-

diagnosis equipment in the processor detects a malfunction the output NO FAILURE of

MONITORING is set at ldquological 0rdquo (= alarm) The alarm ldquoPARAMETERS CHANGEDrdquo

is activated whenever parameters or settings of software switches have been changed via

the Micro-Terminal

615 Actual Value Monitoring

The actual values for generator voltage Ug synchronous voltage Usyn and field

current If are monitored for malfunctions This monitoring is active regardless of whether

or not the generator is in operation Essentially when the generator is in operation the

measurements are monitored by comparing the signals (the smaller signal reading is

detected as incorrect) When the generator is not in operation the measured data are

monitored for extreme values The percentage of deviation permissible in the

measurement signals being compared

(Ug1ampUsyn1 Ug1ampUg2 Ug2ampUsyn2 If1ampIf2) is defined by parameters F208 and

F20AIf the excitation transformer is being supplied from an auxiliary power source (no

shunt operation) the values of Ug and Usyn will be different in some operational

conditions

In that case Flag F75A can be used to deactivate comparative measurements Ug1 Usyn1

and Ug2 Usyn2 The ldquoprimary monitoringrdquo compares Ug1 with Ug2 and generates the

messages that Channel 1 or Channel 2 has suffered a malfunction Whenever Ug1 lt Ug2

and the binary message from CH1 reports no malfunction a malfunction on

Measurement Channel 1 is reported (Ug1Usyn1 FAILURE) A similar malfunction is

also present whenever the binary message CH1 DISTURBANCE is reported and a

discrepancy is detected between Ug1ampUg2 The generation of the alarm ldquoUg2 Usyn2

FAILURErdquo is analogous to that for Channel 1 The ldquosecondary monitoringrdquo compares

Ug1 with Usyn1 amp Ug2 with Usyn2 This is enabled whenever the binary message of the

comparison channel reports a malfunction or whenever both binary messages report no

malfunction - but both secondary monitors report a malfunction As long as the secondary

monitoring is blocked the differences Ug1neUsyn1 or Ug2neUsyn2 trigger malfunction

signals for the measurement channel involved (suspicion that there is a corresponding

error in Usyn) The measurement channel malfunctions are enabled operationally

whenever after excitation has been switched on generator voltage Ug exceeds the value

set on F204 The voltages Ug1 and Ug2 are checked 16 seconds after the excitation is

switched off to see that they do not exceed the limit value F210 that applies to both of

them At the same time g1Ug2Usyn1Usyn2 are checked for extreme values (gt or lt

the operating range) Monitoring for extreme values is likewise enabled during normal

operation ( Excitation ON and Ug gt F204 ) Flag F754 is used to enable or block the

malfunction signals to the binary outputs Basically the monitoring of the actual values

for If1If2 functions like that of the Ug1Ug2 monitoring

62 Excitation Protection

621 General Information

The Excitation Protection switches off the excitation (and de-excites the machine

rapidly) whenever a danger arises that threatens the excitation transformer the converter

or the generator Generally limiter or monitoring functions precede the emergency trips

and these normally respond before the Excitation Protection must initiate a trip

Protective trip commands are issued directly to the field circuit-breaker from potential

free contacts of the board via the trip relays They are directed redundantly to the

operative field circuit-breaker ldquoOFFrdquo command

622 Protection against Excitation Transformer Overheating

This equipment monitors the excitation transformer for overheating in the

windings that could result from over current short-circuits or inadequate cooling The

monitoring uses temperature monitoring modules in conjunction with temperature

sensors built into the windings Normally the temperature is monitored in two stages the

first stage sets off an alarm the second causes a trip of the excitation

623 Rotor Over voltage Protection

Malfunctions in the generator circuit (eg terminal short-circuit failed

synchronization asynchronous operation) cause induced negative field currents that

produce high voltages in the field circuit These must be restricted to a level with a

sufficient safety margin below the insulation capacity of the field winding (test voltage)

and also below the peak blocking voltage of the converter thyristors The crow bar

employs spark gap elements to detect over voltages in the field circuit Whenever they

respond the associated thyristors are fired immediately switching the de-excitation

resistor parallel to the field The de-excitation current generated thereby initiates an

excitation trip via a supervision circuit causing an immediate opening of the field circuit-

breaker The malfunction isets off an alarm and an internal malfunction is indicated at the

cubicle

TEST VALUES OBTAINED WHEN EXCITATION IS RAISED

TEST

SNO PARTICULARS ACTUAL VALUE OBTAINED VALUE

1 VREF 996 100

2 VACT 997 1003

3 IFACT 735 765

4 IGACT 703 707

5 ACTIVE

POWER

703 705

6 REACTIVE

POWER

101 142

7 POWER

FACTOR

099 IND 098 IND

8 ACTIVE

CURRENT(IR)

705 703

9 REACTIVE

CURRENT(IX)

102 138

10 POWER

ANGLE

566 546

11 FIRING

ANGLE

640 632

VALUES OBTAINED WHEN EXCITATION IS LOWERED

SNO PARTICULARS ACTUAL

VALUE

OBTAINED VALUE

1 VREF 100 997

2 VACT 100 997

3 IFACT 787 761

4 IGACT 839 837

5 ACTIVE

POWER

839 830

6 REACTIVE

POWER

155 110

7 POWER

FACTOR

098 IND 099 IND

8 ACTIVE

CURRENT(IR)

835 830

9 REACTIVE

CURRENT(IX)

140 94

10 POWER

ANGLE

613 DEG 631 DEG

11 FIRING

ANGLE

601 DEG 599 DEG

CONCLUSION

For generating the EMF in stator winding excitation is required to the rotor of a

generator There are two types of excitation

1 Static excitation system

2 Brushless excitation system

A certain disadvantage in brushless excitation system is the slow response time of

the field in case of fast load changes specified No slip-rings and brushes direct

measurements of the field parameters not possible

To avoid all loses static excitation is used Since it does not have any rotating

parts mechanical loses and windage loses This system has fast response and speed

control While preferring this excitation system there are no limitations for the

redundancy of Thyristor bridge circuits

Static excitation has fast field discharge by resistor and inverter operation direct

measurement of field quantity is possible The meaning of excitation is nothing but

continuous supply of DC current (ie field current) to the rotor to buildup required

output voltage in the stator

Field current is changed with respect to the change of load so the digital

automatic voltage regulator (DAVR) is used to regulate the output voltage according to

the load variations

So we conclude that static excitation system with DAVR is preferred since it is

having excellent dynamic performance and better options for R amp M

CHAPTER 2

Digital Automatic Voltage Regulator (DAVR)

21 Principle of Operation of the Regulator (DAVR)

To regulate the voltage and the reactive power of a synchronous machine the

field voltage must be adjusted quickly to the changes in the operating conditions (with a

response time that does not exceed a few ms) To accomplish this analog control systems

include amplifiers which make continuous comparison of the actual values against the

reference values and vary the control variable to the converter with almost no delay Most

of the delay that occurs originates in the converter since the firing pulses for changing

the rectifier phase angle are only issued periodically (every 33 ms)

The DVR digital voltage regulator calculates the control variable from the

measured and reference data in very short time intervals This results outwardly in a

quasi-continuous behavior with a negligible delay time (as in an analog regulator) The

calculations are made in the binary number system Analog measurement signals such as

those for generator voltage and generator current are converted into binary signals in

analogdigital converters The set-points and limit values have already been defined in

digital (binary) form An understanding of the actual computation processes in the digital

voltage regulator is not necessary for operation preventive maintenance or

troubleshooting Like the operator of a pocket calculator or a personal computer all the

operator needs is to know how to operate the instrument and the programming for this

working tool For that reason we will explain below only the principle division of work

among the various modules and the flow of data processing The purpose is above all to

make clear how the processor system has been integrated into the rest of the power

electronics system

6

22 Basic Structure of the Processor Systems

7

The signal processors 25 analog inputoutput modules Each of these processor

systems has a common bus circuit and output and the control lines There is a specific

range of addresses assigned to each assignment Board including the power supply bus

the address lines the two data lines to the input calculates the reactive current (I sin φ)

and the active current ( Icos φ) With these two channel processor Synchronized with

these interrupts (ie with the phase positions of current Ig the field current If and the

synchronous voltage Usyn From the exchange data with the microprocessor card across

the two data lines generator voltage Ug) this processor measures the generator current

Ig and then hardwired connections or multi-conductor cables Binary and analog

inputoutput modules ie for galvanic isolation and adaptation to the electronics level

The most important input interrupts per period to trigger the cycles for processing actual

values in the AUTOMATIC module on the processor bus) for filtering and further

processing

Monitoring each consist of the central microprocessor module and binary and

parameters to the AUTOMATIC channel are the generator voltage Ug the generator

peripheral unit Ug Ig and Usyn are sent to the Interrupt Generator (plug-in peripheral

units (wall-mounted units) peripheral units are used for preprocessing signals from

external measurement circuits power supply units Signals are exchanged among these

processor systems via processed across separate peripheral units for each channel These

processor working on the bus (a house address that can be adjusted using a switch)

systems The AUTOMATIC channel the programmable controls and the Excitation The

actual values measured from AUTOMATIC channel and MANUAL channel are The

AUTOMATIC channel and the MANUAL channel each have their own The digital

voltage regulator is broken down into several autonomous microprocessor The inputs and

outputs of the processor systems are directed across voltage-isolating The Interrupt

Generator also uses the 3-phase Ug signal to generate the 12 themselves contain a limited

number of hardware inputs and outputs with fixed equipment Whenever addresses from

this range are called up the signal processing module can results the processor is then

able to derive further operating parameters such as the load angle the active power etc

The functions of all microprocessor systems other than the programmable controls

have been accomplished in firmware The non-varying standard function modules can be

configured to the design desired for plant-specific purposes using software switches

(KFlags) Thus for example the stored status of a K-Flag determines whether or not a

Limiter is active and whether the de-excitation or the excitation limiters take precedence

Because these K-flags determine the software Scope of Supply for the installation they

cannot be changed permanently via the Micro-Terminal In this way they differ from

such setting data as the values of the parameters for the PID filter of the voltage regulator

or the set-points for the limiters These values can be permanently changed using the

Micro-Terminal Communication is possible with each of the processor systems via the

Micro-Terminal by plugging on the connecting cable In this way signals within the

processor and setting parameters can be viewed analog signals can be issued and the set

parameters can be altered temporarily (F range) or permanently (C range) Unlike the

other processor systems the programmable controls do not include any firmware for

realization of the functions They have been designed so that the designer can adapt and

change their functions easily using the ldquoFunctional Block Programming Language P10

Digital and analog functions can be implemented in practically any degree of complexity

desired using the P10 functional blocks The control variable of the voltage regulator

(AUTOMATIC channel) and the control variable of the field current regulator

(MANUAL channel) are each processed in separate Gate Control Unit and formed into a

chain of pulses at the appropriate firing angle The pulses of the active channel are

directed to the pulse bus via the associated Intermediate Pulse Stage The pulses for each

converter block are amplified sufficiently in Final Pulse Stage to fire the Thyristor

231 General Information

The functions of the automatic voltage regulator AVR are

1 to regulate the generator voltage

2 to regulate the effect of the reactive andor active current on the voltage

3 to limit VoltHz

4 to limit max and min field current

5 to limit inductive stator current

6 to limit capacitive stator current

7 to limit the load angle

8 to stabilize the power system

Block Diagram shows the software structure of AUTOMATIC channel The

generator limiters not provided for the installation in question (optional equipment) are

identified in this overview as ldquoNot Suppliedrdquo The parameter values signal values and

software switches (flags) marked with addresses (hexadecimal numbers) can be viewed

and altered via the Micro-Terminal The values selected are displayed in sec pu Hz

etc and can where necessary be changed directly in these formats The plant-specific

settings of the variables and the flags can be obtained from the Test and Commissioning

Report This block diagram provides information about the important functions and

possible settings of the AUTOMATIC channel For the sake of clarity no detailed

presentation has been given of special functions such as tracking circuits initializations

etc The page heading cross-refers this overview to the various sheets of the schematic

diagram Binary signals are shown in broken lines analog signals in solid lines The

corresponding text designations in the schematic diagram can be used for identification of

the input signals (hardware inputs) The only analog output signal from the automatic

voltage regulator control variable Ucontr is sent via the data bus (CRU bus) to the Gate

Control Unit Most of the binary messages (outputs) from the AVR are of no interest

functionally and they have been omitted for the sake of clarity The basic structure of the

digital voltage regulator and the limiters is simple This is necessary in order that the

behavior of the regulatorslimiters will remain calculable and understandable in all

operating situations and that there will be no problem in adjusting and optimizing them

The central PID filter in the digital voltage regulator defines the dynamic response of the

closed-loop controls both in the voltage regulator mode and after limiters have

intervened The ldquocontrol deviationrdquo at the input to the PID filter is either the control

deviation for voltage the control deviation of a de-excitation limiter (the value

determined by minimum value selection) or the control deviation of an excitation limiter

(the value determined by maximum value selection) Flag F730 (ldquoPRIORrdquo) is used to

determine whether the exciting (Min value) or the de-exciting signal takes precedence on

the minmax value limiter (normally F730 = 1111 ie the de-exciting signal takes

precedence) With the exception of the Minimum Field Current Limiter all other limiters

have variable factoring multipliers of the signal outputs so that they can be adjusted

individually together with the common PID filter which has been optimized for voltage

regulation The setting parameters for this PID filter are as follows

Vo = KR Static amplification

1

Ta = ---- Integration time constant

Tc1

Vp Proportional amplification

1

Tb = ---- Differential time constant

Tc2

Vinfin Amplification of high frequencies

The BODE diagram below shows the assignment of settings in accordance with

DINIEC standards based on a typical example

The PID filter amplifications Vo Vp and Vinfin can be adjusted in pu values But

the ldquoceiling factorrdquo pl+ must be adjusted correctly with parameter F310 if the total

amplification (circuit amplification) of the control circuit is actually to conform to the

pu settings This factor must agree with the ldquoexternalrdquo amplification ie with the

ceiling value of the transformer- converter circuit

Ceiling factor(pl+) = Ufmax Ufo

in which Ufmax = ceiling field voltage

Ufo = no-load field voltage

To attain a suitable response of the AVR when starting excitation

(ldquoEXCITATION ONrdquo) it may be necessary to change the proportional amplification of

the regulator during this phase Vp2 (transiently activated) and Vp1 (permanently

activated) can be adjusted for this purpose For example the value of Vp2 takes effect

immediately once the excitation is switched on and remains effective for a period as set at

F30C Once the period F30C (eg 5 sec) has expired Vp shifts over to Vp1 (becomes

the steady-state Vp) at the rate of change set The standard operating mode for the PID

filter is voltage regulation for which the discrepancy between the voltage set-point and

the current value for generator voltage Ug (the control deviation) is supplied at the input

To compensate for the voltage drop in the block transformer or whenever several

generators are operating to the same distributing bus the generator voltage must be

varied in proportion to the measured generator current (droop influence) To accomplish

this the voltage set-point is varied as a function of the measured reactive current IX

andor active current IR Flag F712 enables the IX droop Flag F710 the IR droop The

desired compensation is set in F282 and F286 respectively Flags F284 and F288 are used

to select whether this droop influence is to increase the voltage or to reduce it

(compensation) Combined influence of the active and reactive currents is attained by

enabling both droops IX and IR Flag F716 activates a so-called ldquoSoft-Startrdquo at the

starting of excitation This ldquoSoft-Startrdquo ensures that the voltage set-point integrates from

0 to 100 within the time set on F290 when the excitation is switched on

(ldquoEXCITATION ONrdquo) A ldquosmoothrdquo excitation of the generator can be achieved in this

way whenever there is no demand for a quick excitation

232 Voltage Set-Point

Various signals and settings control and limit the voltage set-point F270 For

example the values of F254 and F252 define the normal operating range possible for set-

point adjustment (eg 90 110) using external control commands (control room local

operatorrsquos panel superposed control system) The effective set-point adjustment rate is

governed The set-point can be set at the values of F250 and F256 by activating

appropriate control commands for ldquoSETrdquo input Enabling Flag F71A and activating a

binary input prior to switching on the excitation (ldquoEXCITATION OFFrdquo) sets the Ug set-

point at the value of UAUX This makes it possible for example to ensure that the

generator voltage will agree exactly with the network voltage after the voltage build-up

An external value with variable amplification can be added to the Ug set point by

enabling F724 (for example for stability tests)

233 Regulator Tracking in MANUAL Operation

Whenever the AUTOMATIC channel is not in operation (the MANUAL channel

is ON) a follow-up equipment ensures a smooth switch-back to the AUTOMATIC

mode will always be possible To track the voltage set-point is shifted by means of

RAISELOWER pulses from the Gate Control Unit so that control variable Ucontr at the

output from the PID filter is held steady and identical to the control variable Ucontr from

the MANUAL channel Because this tracking must react slowly resultant transient

control deviations resulting from the amplification in the PID filter might cause severe

interference with control variable Ucontr

To prevent this the follow-up equipment intervenes on the regulators mixing

point with a corresponding compensation signal

234 Ugf Limiter

At under frequency the Ugf Limiter reduces the generator voltage so as to

prevent saturation effects in the supply and measuring transformers To adjust this

limiter the max permissible generator voltage at rated frequency is defined and set

When any under-frequency occurs the generator voltage is thus reduced in proportion to

that setting

235 Field Current Maximum Limiter

The Field Current Maximum Limiter is provided to protect the generator rotor

from s occurring in steady-state and transient operation High field currents are normally

the result of a sharp drop in network voltage or of an improper raising of the voltage set-

point by the operating staff The field current is held steady at the value TH1 ie at the

maximum thermal value permissible for the excitation circuit and the rotor In order that

the generator can support the power network with its transient overload capacity during

brief collapses in voltage a temporary switch-over is made to the transient limit MAX1

(a higher setting) When the generator or the converter is operating at a reduced capacity

These limits TH1MAX1 can be switched over to the lower settings TH2MAX2 by

activating the corresponding binary signals The switch-over from the thermal limit

TH12 to the transient limit MAX12 can be configured in one of three ways

a) Depending on the over current with -dUdt ENABLE

Flag programming F418 = any setting desired F41A = 0000

This variant enables the transient value MAX12 whenever a collapse of voltage

in the network is detected The ENABLE time is fixed and can be set The example

below shows the typical behavior of the limiter configured in this way

b) Dependent on the time integral with -dUdt ENABLE

Flag programming F418 = inactive F41A = 1111

This variant likewise enables the transient value only when a collapse of network

voltage has been detected However the switch-back to the thermal limit is not made

dependent upon the time itself but on the calculated time integral intisup2dt of the The setting

on Parameter F414 in spu takes into account the time the rotor needs to cool down ie

the rate of temperature change in the case of intermittent operation The example below

shows how the timing of the switch-back to the thermal limit depends on the present

value for intisup2dt

The time integral is based on the formula

Example The setting of =isup2dt equivalent to Version a (F416) at a constant 16 times the

nominal field current for 10 seconds (with TH12 = 105) is

c) Dependent on the time integral without any preconditions

Flag programming F418 = 1111 F41A = 1111

In this variant the transient becomes available without any prior conditions

(without a -dUdt ENABLE) with the time integral intisup2dt

237 Inductive Stator Current Limiter

The Inductive Stator Current Limiter holds the stator current Ig within permissible

limits while the generator is in the ldquoover-excitedrdquo operating range by reducing the field

current accordingly The setting TH (thermal limit) provides the limit against stationary s

that might occur To take advantage of the generatorrsquos transient overload capacity a

switch-over is made to the higher setting MAX The principle of operation of this switch-

over to the value MAX permissible only transiently is identical to that employed for the

field current limiter (refer to the description above) When the drive output from the

turbine is very high stator current may exceed permissible limits even while inductive

loading of the generator is low In this case if the stator current limiter is not kept from

influencing the field current the control circuit will oscillate back and forth between the

Inductive Stator Current Limiter (de-

exciting) and the Capacitive Stator Current Limiter (exciting)The output signal of that

function then dominates the control variable of the Ig-dependent limiter via a maximum

value selection

238 Capacitive Stator Current Limiter

239 Load Angle Limiter

The Load Angle Limiter prevents the synchronous machine from slipping out of

phase due to slippage of the rotor The load angle δ the difference in phase between the

rotor and the stator rotating field results mainly from the driving torque (active power P)

acting on the generator and the level of rotor current (field current) If the driving torque

remains constant a increase in the field current reduces the load angle δ The current load

angle δ at any moment is obtained from the generator current and generator voltage based

on a simplified model of the generator Whenever this calculated load angle δ exceeds the

preset limit angle the limiter increases the field current until the load angle has dropped

back to its permissible value The quadrature reactance Xq of the generator and the

network reactance Xe during normal operation must be adjusted on the regulator in order

to obtain the load angle δ The graph below shows the Power Chart for a salient-pole

machine with typical limiter characteristics

The purpose of a Power System Stabilizer is to use the generator excitation to

damp electromechanical oscillations between the network and the generator Depending

on the design of the generator and the requirements imposed for network stability its

main function will be either to damp the oscillations originating in the machine or those

from the network A synchronous generator working in a combined power network is in

principle an oscillating structure In order to produce a torque the magnetic field of the

rotor and the stator must form a given angle (referred to as the rotor displacement or load

angle δ) The electrical torque ME increases as the angle δ increases just as with a

torsion spring Because the ME of the generator and the mechanical driving torque MA

from the turbine are in equilibrium during steady-state operation the angle δ remains in a

given position Whenever this state of equilibrium between MA and ME is disturbed the

load angle slips of this rest position and change thereby the electrical torque ME The

torque attempts to restore the load angle to a stationary position Due to the mass inertia

of the turbinegenerator rotor however this can only take place aperiodically It does so

in the form of more or less effectively damped oscillations (again similar to the effect of

mass inertia on a torsion spring) In order to damp the oscillations there must be a

damping torque produced depending not on the electrical torque ME associated with the

angle but on the difference in frequency (Df) between the rotor and the stator rotating

field ie on the slippage This torque is produced mainly by the so-called damper

winding in the rotor but the dimensioning of this is subject to limits imposed by

considerations of design and economy Some further action is therefore needed to

increase the damping effect The following drastically simplified formula shows the

parameters upon which the amount of active power PE supplied by the generator

depends

PE = active power

It can be seen from the above relationship that the active power that the generator

transfers depends not only on the load angle δ but also on the field current If That means

that a transient change can be made in the active power PE and with that in the effective

electrical torque ME by varying the field current The principle of operation of the DVR

Power System Stabilizer becomes clear from a consideration of the oscillations in power

output and frequency (ΔPE Δf) and the vector diagram If it is assumed that oscillations

in the network frequency generates load oscillations with the mass inertia of the rotor

then the active load of the generator (eg MW-measured) is influenced with a sinusoidal

value -ΔPE (ME-MA = -ΔPE) By inversion of -ΔPE one obtains the fluctuation in

power provided by the rotor +ΔPE As is known the slip signal Δf follows +ΔPE with a

phase delayed by 90deg The +ME produced by the periodic changes in the load angle δ is

in phase with +ΔPE A good damping is attained if ME is varied in phase with the slip

Δf However this signal must also be advanced somewhat to compensate for the time

constants in the excitation circuit and the generator

As mentioned above the electrical torque ME can be influenced by varying the

field current To accomplish this a suitable control signal referred to as variable

disturbance compensation must be imposed upon the voltage set-point or the converter

control variable Ucontr As can be seen from the vector diagram by applying proper

weighting factors (K1 K2) and then adding together the signals -ΔPE and Δf an overall

stabilization signal can be produced that rotates in advance of the Df signal by any angle

desired between 0deg and 90deg Because the amplitude of -DPE remains proportional to the

amplitude of Δf a constant angle in advance of Δf results for the compensation of the

time constants referred to above The optimum weighting factors K1 and K2 for a

synchronous generator working to a power network depend on its operating point at any

moment and the external reactance of the network Normally the selection of a

compromise setting is good enough to attain stability in all operating points and for all

external reactance For special demands these settings must be parameterized as a

function of the external reactance (which means optional equipment Xe-Identification)

The Power System Stabilizer PSS is a section of the AVR computer program and is

processed once per network cycle The voltage at the generator terminals and the

generator current are measured in order to define the signals ΔPE and Δf The calculated

signals for _P_ (=PE) and Δf are then sent across DC filters ldquoDrdquo (real differentiators) that

transmit only the dynamic portion of the signals The ΔPE and Δf signals obtained in this

way are then weighted (multiplied by) with the factors K1 and K2 and sent to the

summing point of the voltage regulator

The PSS stabilization signal is imposed on the automatic voltage regulator only if

the following prerequisites are met

bull Generator on line

bull Generator power output gt the value F338

bull Generator voltage in a range between F33C and F33A

The stabilization signal is limited at the output from the PSS to the lower and

upper limits Flag defines whether the stabilization signal is introduced before or after the

PID filter (usually before the filter) Because the PID filter as noted above already takes

the ceiling factor Vp1 into account the PSS signal needs to be multiplied by Vp1 if it is

added to the voltage regulator following the PID filter (divider at the input to the

minmax limiter) This precaution prevents the DC filter ldquoDrdquo in the P-channel from

producing an unnecessary ldquostabilizationrdquo effect in the case of rapid changes in turbine

load As an alternative for the AVRrsquos Power System Stabilizer a stabilization signal from

an outside system can be imposed by activating the binary input ldquoPSS-SIGNEXTrdquo Flag

F340 can be used to select between an analog and a 12-bit signal and F33E to select the

polarity desired for that signal

24 The MANUAL Channel

241 Summary

The MANUAL channel (Channel 2) has been built as a simple field current

regulator

without additional limiters Its main function is to maintain the excitation of the generator

even if the AUTOMATIC channel becomes non-operational The MANUAL channel

also performs valuable service for purposes of testing commissioning and preventive

maintenance Its measurements regulator generation of firing pulses and power supply

are physically separate from those on the AUTOMATIC channel

242 Principle of Operation

All the functions of the MANUAL channel including the generation of firing

pulses have been implemented in a single electronic module the Gate Control Unit The

control variable Ucontr of voltage regulator is used as the reference value for generating

firing pulses on the principle known as ldquoramp controlrdquo (Comparison of Ucontr with

Usynsynchronous sawtooth signal) For further processing in the UN 0096 Intermediate

Pulse Stage the Gate Control Unit supplies six firing pulses at its output whose phase

position with respect to the synchronous voltage Usyn is in accordance with control

variable Ucontr An internal linearization ensures that the field voltage produced via the

firing pulses remains proportional to the control variable Ucontr throughout the entire

range As a result the circuit amplification of the control remains constant over the entire

range Whenever excitation is switched ON the set-point for Generator Voltage is set

automatically at the preset - ref Value This provision ensures that the generator voltage

always attains approximately its nominal value after the field flashing The Gate Control

Unit can be refunctioned ( by pre-selection with a switch ) for purposes of testing to act

as a purely firing pulse control In this case the control variable Ucontr is adjusted

directly using the RAISELOWER push buttons on the front of the module In this way

for example the relationship between the phase position of the firing pulses and the

control variable Ucontr can be checked easily

CHAPTER 3

PULSE SECTION

31 Pulse Generation and Amplification

The Gate Control Units of both AUTOMATIC channel and MANUAL channel

each supply six firing pulses for operating the 6-pulse thyristor bridges The low-power

pulse signals from these Gate Control Units are then amplified in the Intermediate Pulse

Stage galvanically isolated and then sent to the common pulse bus On the output end

the Intermediate Pulse Stage of the non-active channel) is always blocked The Gate

Control Units generate the pulses based on microprocessor control The reference voltage

used for the firing pulse phase location is the output voltage from the excitation

transformer (Usyn1 Usyn2) The commutation spikes of the synchronous voltage caused

by the converter are calculated prior to use of the voltage as a reference value and are

deliberately filtered out The lower limit for the firing pulses (double pulses) which are

offset from one another by 60deg is defined by the limit rectifier position (αmin) and the

upper limit by the limit inverter position (αmax) for the firing angle αmin and αmax can

be adjusted on the Gate Control Units using BCD (Binary Coded Decimal) switches

αmin ensures that the firing pulses will not be issued (premature firing) until there is

sufficient positive phase voltage on the thyristor involved αmax prevents a dangerous

ldquotippingrdquo of the thyristor bridge into the rectifier mode if the firing angle α is too large

(ldquolate firingrdquo) The critical factors determining αmax are the overlap time uumlmax (max

commutation time) and the ldquorecovery timerdquo of the thyristors (αmax lt 180deg - uumlmax - γ )

An external control signal can force the firing pulses into their inverter limit position

Other binary inputs can block or direct the firing pulses of the Gate Control Units so as to

produce freewheeling on the thyristor bridge During freewheeling the firing pulses for

the thyristor pair R and S are blocked and the pulse signals T+T- are engaged with

chains of pulses Both Gate Control Units (for the MANUAL amp AUTOMATIC channels)

contain a field current monitor that blocks the firing pulses immediately whenever the

current exceeds a preset threshold level In this case the field circuit-breaker is also

tripped via an output contact The purpose of these provisions is to prevent damage to

thyristors and thyristor fuses in case of a slip-ring short-circuit or to keep any damage

that does occur to a minimum The pulse signals are galvanically separated at the outputs

from the Intermediate Pulse Stage (with pulse transmitters) and are then directed to the

common pulse bus This transmission of the pulse signals to the pulse bus via passive

transmitters ensures a high degree of active channel autonomy Practically no possible

malfunctions on the inactive channel (including for example sustained pulses) affect the

active channel

32 Pulse Monitoring

The ldquoPulse Busrdquo and the pulse signals of the AUTOMATIC channel are

monitored This monitoring device consists of potential isolating stages and the common

monitor If the pulse monitoring of the ldquoPulse Busrdquo responds a switch-over is made to

MANUAL channel The function of the potential isolating stages is to couple the pulse

monitoring device to the pulse circuits without any feedback effect The pulse monitoring

checks the six pulse lines for the following malfunctions continuous or periodic failure

of one or more pulses Periodic occurrence of synchronous or asynchronous false pulses

Continuous pulses the pulse monitoring device can be tested while the machine is in

operation

CHAPTER 4

CONVERTER

Thyristor

The term thyristor usually refers to a family of four layer solid state device having

turn on characteristics that can be externally controlled by either current or voltage They

are also referred to as breakdown device because their working depends on avalanche

breakdown Thyristors have only two stages OFF and ON Thyristors have a similar

function to Uni-junctions they act as switches Thyristors use current flow as a switch

Thyristors have three states

1 Reverse blocking mode mdash Voltage is applied in the direction that would be

blocked by a diode

2 Forward blocking mode mdash Voltage is applied in the direction that would cause

a diode to conduct but the thyristor has not yet been triggered into conduction

3 Forward conducting mode mdash The thyristor has been triggered into conduction

and will remain conducting until the forward current drops below a threshold value

known as the holding current Converter is a semiconductor device which converts ac

input voltage into a constant dc output voltage In present excitation system three phase

fully controlled thyristor converter is used

Because of the following advantages thyristor converters are used

a) Thyristors are used for high power applications ie up to 10Kv3500A1KHz

b) Having high reliability and low losses

c) Uni-directional device like diode

d) Itrsquos operation as a rectifier which are low resistance in forward conduction

mode and high resistance in reverse conduction mode

PROTECTION OF THYRISTORS

For reliable operation of a thyristor demands that its specified ratings are not

exceeded When Subjected to or over voltages During the turn - on of SCR didt

prohibitively large False triggering of SCR by high value of dvdt andSpurious signals

between gate and cathode may leads to unwanted turn ndash on

DIDT AND PROTECTION

When thyristor starts conducting in forward conduction mode and is turned on by

gate pulse The anode current increases rapidly whole area of the gate to Cathode

junction then hot spots will be formed near the gate connection this locality of heating

destroys the thyristor Thyristor thermal time is constant The causes due to faults and

short circuits or surge currents Electronic crowbar protection is used against the over

voltages The rate rise of anode current must be kept at the time of turn on below the

rated or specified limiting value The didt value maintained below limited value by using

a inductor also called ldquodidt inductorrdquo in series with anode circuit The locality of heating

is avoided by applying gate current but not greater the maximum gate current

DVDT AND OVER VOLTAGE PROTECTION

With forward voltage across the anode and cathode of a thyristor the two outer

junctions are forward biased but the inner junction is reverse biased This reverse biased

junction J2 has the characteristics of a capacitor due to charges existing across the

junction In other words space-charges exist in the depletion region around junction J2

and therefore junction J2 behaves like a capacitance If the entire anode to cathode

forward voltage Va appears across J2 junction and the charge is denoted by Q then a

charging current i given by Eq (46) follows

i = dQdt =d(Cj Va )dt

= Cj (d Va dt) + Va(d Cj dt) helliphelliphelliphellip(46 a)

As Cj the capacitance of junction J2 is almost constant the current is given by

i = Cj (d Va dt) helliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip(46 b)

If the rate of rise of forward voltage dVadt is high the charging current i will be

more This charging current plays the role of gate current and turns on the SCR even

when gate signal is zero Such phenomena of turning-on a thyristor called dvdt turn-on

must be avoided as it leads to false operation of the thyristor circuit

For controllable operation of the thyristor the rate of rise of forward anode to

cathode voltage dVadt must be kept below the specified rated limit Typical values of

dvdt are 20 ndash 500 Vμsec False turn-on of a thyristor by large dvdt can be prevented by

using a snubber circuit in parallel with the device thyristor are very sensitive for over

voltage than the semiconductor devices

Over voltage transients are perhaps the main cause of thyristor failure

In thyristor there are mainly two types

1 Internal over voltages

Due to the commutation of the thyristors large voltages are generated internally

Because of the series inductance of the SCR circuit the large transient voltages L didt

produced This voltage several times the break over voltage of the device then thyristor

destroys permanently

2 External over voltages

External over voltages are caused due to the interruptions of current flow in an

inductive circuit and also due to the lightening strokes on the lines feeding the thyristor

system For the reliable operation of thyristor the over voltages must be suppressed by

adopting suitable techniques

Suppression of over voltages

The RC circuit called snubber circuit is connected across the device to protect In

order to keep the protective components to a minimum the thyristors are chosen with

their peak voltages ratings are 25 to 3 times of the normal peak working voltage

ldquoselenium thyrector diodes metal oxide varistors or avalanche diode suppressers are

commonly employed for protecting the thyristor circuit against the over voltages

Gate protection

Gate circuit should also be protected against the over voltages and surges Over

voltage at gate circuit can cause false triggering of the SCR may rises the junction

temperature behind specified limit leading to its damage Protection against over

voltage can be achieved by connecting a ZD across the gate circuit and a resister is

connected in series with gate circuit to protect against the s A capacitor and resister are

connected across gate to cathode to by pass the noise

41 Final Pulse Stages

The Final Pulse Stages adapt the output pulses from the Intermediate Pulse Stage

(pulses on the pulse bus) to the gate currents needed for the thyristors Each thyristor

bridge is equipped with its own Final Pulse Stage Each Final Pulse Stages is provided

with a power supply module The amplified output pulses from the Final Pulse Stages

start as a short strong steep pulse with an amplitude approx 2frac12 times that of the main

pulse This initial pulse edge assures proper firing of the thyristors being triggered

Subsequently the weaker part of main pulse keeps firing conditions steady As already

mentioned the Final Pulse Stages and their associated thyristor bridges form single units

All six pulse outputs from a Final Pulse Stage can be blocked by an external control

signal so that all thyristors in the associated thyristor bridge will block the current A

blocking of the pulses is initiated whenever there is a malfunction in the associated

thyristor bridge

42 Converter Power Section

The thyristor converter consists of three independent parallel rectifier blocks TY1

to TY3 which are all in service Even if one block fails the remaining blocks take over

automatically the full design current of the excitation circuit During normal operation

(with ideal current share) and all three bridges in operation each of these blocks has to

carry only (n-2)n (ie33)of its design current If 2 thyristor bridges fail the excitation

is limited Only when all three bridges fail the excitation is switched off Each thyristor

bridge arm is equipped with current flow monitoring CTrsquos Failure of conduction in any

arm is identified by a Current flow monitoring module

43 Converter Cooling

A cooling system is needed to dissipate heat losses in the converter blocks and

electronics Each converter block has therefore been equipped with a fan supplied with

power from the converterrsquos primary voltage (via transformer ndashT8 in field flashing

cubicle) The fans are protected with motor protection circuit breakers An air flow

monitoring unit is provided for monitoring the air flow through the thyristor bridge If a

circuit breaker failure is detected or if the air flow monitor drops off at one of the

thyristor bridges the bridge involved is immediately set out of operation by blocking its

firing pulses

44 Thyristor Converter Monitoring

A thyristor bridge in which defects occur that could threaten the safety of

operation or cause secondary damage is switched off automatically ie its firing pulses

are blocked This happens whenever A thyristor fuse is blown The fuses are monitored

individually with micro switches The Final Pulse Stage fails which is detected by

internal monitors (supply voltage sustained pulse short-circuit on the output end) The

power supply to the fan fails fan air flow as monitored by the Air flow monitor fails or

is insufficient Isolator on ACDC side is open

CHAPTER 5

Field Current Circuit Excitation Transformer

51 Field Circuit Breaker

The circuit-breaker in the field circuit is used to isolate the field circuit from the

converter It is capable of switching off the synchronous machine from full load under the

maximum conditions of a 3-phase short-circuit In addition to its main contacts the field

circuit-breaker also has a de-excitation contact with which the field energy stored in the

field can be dissipated across the de-excitation resistor The de-excitation contact closes

shortly before the main contacts open so as to ensure proper commutation of the field

current from the main contacts to the de-excitation contact when the breaker is switched

off The field circuit-breaker is switched on by electromagnetic force and is kept switched

on by a mechanical latch When the latch is released by a trip coil the circuit-breaker

opens The circuit-breaker also has auxiliary contacts that report its status

52 Field Flashing

In shunt supplied excitation circuits (excitation transformer connected to the

generator terminals) the generator does not have enough remnant voltage for a generator

voltage build-up via the converter In this case a field flashing circuit is provided It

consists of the field flashing contactor the diode bridge and a transformer used to adapt

the auxiliary input voltage to the voltage needed for field flashing when power is

supplied from the auxiliaries network

Fig Field Flashing

Because the field flashing contactor is not able to switch off the energy stored in

the field the control ensures that the contactor can only reopen if the field circuit breaker

has already been opened (generating the TRIP order) or in a normal field flashing

sequence when the converter has taken over the field current Field flashing occurs in the

following stages

1048729The excitation is switched on closing the field flashing contactor ( Field

Circuit Breaker is already closed )

1048729The start-up excitation current flows through the rotor driving the generator

voltage up to approx 15 U

1048729After about 10 U the firing pulses to the converter are released and it begins

to excite the generator to its rated voltage

1048729After about 30 U the field flashing contactor opens (with no current since

the converter is now supplying the current)

The diode bridge at the input to the field flashing contactor prevents a feed-back

from the converter to the source of field flashing while the contactor is still closed

53 De-excitation

When malfunctions occur the stored field energy must be dissipated as quickly

and safely as possible to protect the generator This is done by the converter the field

circuit-breaker and the de-excitation (discharge) resistor

De-excitation (with opening of the field circuit-breaker) takes place in the following

stages

1048729The converter drives to its inverter limit position (negative ceiling voltage)

recovers a portion of the field energy into the network A trip command is given to the

field circuit breaker

1048729The de-excitation contact closes diverting the field voltage to the de-excitation

resistor

1048729Then immediately the main contacts open building voltage The field voltage

commutates to the de-excitation resistor

1048729The current diminishes at a given time constant TE

(With linear resistance TE = Lf (Rf + Re))

Due to the reversal of the field voltage by the converter the field current

commutates from the main contacts of the field circuit-breaker to the de-excitation

resistor in a very early phase This reversal of the field voltage prevents burn-off on the

main contacts and provides effective protection for the field circuit-breaker Depending

on the operating policy an operational shut-down of the excitation can also be effected

with the field circuit-breaker closed This method is useful mainly when the excitation is

switched on and off frequently In this case the converter is merely driven into the

inverter limit position so that the field energy is recovered into the network The

converter then blocks since it is supplying positive current only

54 Excitation Transformer

The excitation transformer matches the generator voltage to the field voltage

(required ceiling voltage) It also serves as a commutation reactance for the thyristor

converter and as a potential isolator between the network and the excitation circuit In

addition the transformer functions as a current limiter in that it makes it possible to keep

any short circuits in the excitation circuit under better control The excitation transformer

is equipped with temperature monitoring probes which set off an alarm when the

temperature exceeds a first max limit and then trips the excitation if the temperature

continues rising to a second (higher)limit

CHAPTER 6

Monitoring and Protection

61 Excitation Monitoring

611 General Information

The main goal of Excitation Monitoring is to make optimum use of the

redundancies provided in the excitation system and to give alarm whenever a malfunction

makes these redundancies unavailable The field current is monitored to see that it does

not exceed a maximum level and if necessary a switch-over to the MANUAL channel is

initiated In addition the criterion for switching off the field flashing is generated The

excitation Monitoring consists of an autonomous processor system

612 over current Alarms

In the Excitation Monitoring the limits for are set at higher levels than the

settings on the Field Current Maximum Limiter Whenever the current exceeds 110 of

the nominal field current contact R1 and the binary output associated with it are

activated immediately If field current remains gt 110 then after a preset inverse-time

has lapsed relay R2 and - after a further delay - relay R0 and the binary outputs

associated with them are activated Parameters match the measurements for If1 and If2 to

the nominal value for field current so that the internal values can be processed and read as

pu values It can be used to falsify the actual value of the field current If (to raise it) so

as to cause a response from the alarm limits for purposes of testing The processed If

signal is always taken from on the active channel (CHANNEL I OR CHANNEL II) As

long as the field current If is above the threshold value 11 Ifn its peak value is

measured This is stored (until RESET) and can be read at any time on the Micro-

Terminal Once the value of If exceeds 11 Ifn integration of this value starts Whenever

the integrated time-current value (intisup2dt) exceeds the preselected reference value the

alarm OVER CURRENT INVERSE-TIME is set off and a command is simultaneously

issued to switch over to the stand by AUTO channel Software switch F758 enables the

three over current alarm functions (R0 R1 R2) and selects one of three possible inverse-

time curves T1 T2 or T3 Within the characteristic curve (T1 T2 T3) selected the

desired limit curve for response is set using the factor F216

613 Switch-Off Criterion for Field Flashing

The Excitation Monitoring supplies the criterion for switching off the field

flashing Whether this criterion is activated based on the actual value for generator voltage

Ug or for field current If or both depends on the settings of the two threshold values

F200F202 (0 setting means that the output is always ldquological 1rdquo) The measurements

Ug12 and If12 are switched over depending on the present status of the channels

(Channel 1 or Channel 2 ON) Whenever Flag F750 is not activated the binary output is

fixed at ldquological 1rdquo

614 Storage of Alarm Status

The outputs of the over current alarms (R1 R2 R0) and the messages NO

FAILURE MONITORING PARAMETERS CHANGED are stored messages can be

erased by activating the input ldquoGENERAL RESETrdquo or by using the RESET button on the

front of the module Erasure with the input ldquoGENERAL RESETrdquo is effective only if the

situation causing the alarm or the malfunction is no longer present Whenever the self-

diagnosis equipment in the processor detects a malfunction the output NO FAILURE of

MONITORING is set at ldquological 0rdquo (= alarm) The alarm ldquoPARAMETERS CHANGEDrdquo

is activated whenever parameters or settings of software switches have been changed via

the Micro-Terminal

615 Actual Value Monitoring

The actual values for generator voltage Ug synchronous voltage Usyn and field

current If are monitored for malfunctions This monitoring is active regardless of whether

or not the generator is in operation Essentially when the generator is in operation the

measurements are monitored by comparing the signals (the smaller signal reading is

detected as incorrect) When the generator is not in operation the measured data are

monitored for extreme values The percentage of deviation permissible in the

measurement signals being compared

(Ug1ampUsyn1 Ug1ampUg2 Ug2ampUsyn2 If1ampIf2) is defined by parameters F208 and

F20AIf the excitation transformer is being supplied from an auxiliary power source (no

shunt operation) the values of Ug and Usyn will be different in some operational

conditions

In that case Flag F75A can be used to deactivate comparative measurements Ug1 Usyn1

and Ug2 Usyn2 The ldquoprimary monitoringrdquo compares Ug1 with Ug2 and generates the

messages that Channel 1 or Channel 2 has suffered a malfunction Whenever Ug1 lt Ug2

and the binary message from CH1 reports no malfunction a malfunction on

Measurement Channel 1 is reported (Ug1Usyn1 FAILURE) A similar malfunction is

also present whenever the binary message CH1 DISTURBANCE is reported and a

discrepancy is detected between Ug1ampUg2 The generation of the alarm ldquoUg2 Usyn2

FAILURErdquo is analogous to that for Channel 1 The ldquosecondary monitoringrdquo compares

Ug1 with Usyn1 amp Ug2 with Usyn2 This is enabled whenever the binary message of the

comparison channel reports a malfunction or whenever both binary messages report no

malfunction - but both secondary monitors report a malfunction As long as the secondary

monitoring is blocked the differences Ug1neUsyn1 or Ug2neUsyn2 trigger malfunction

signals for the measurement channel involved (suspicion that there is a corresponding

error in Usyn) The measurement channel malfunctions are enabled operationally

whenever after excitation has been switched on generator voltage Ug exceeds the value

set on F204 The voltages Ug1 and Ug2 are checked 16 seconds after the excitation is

switched off to see that they do not exceed the limit value F210 that applies to both of

them At the same time g1Ug2Usyn1Usyn2 are checked for extreme values (gt or lt

the operating range) Monitoring for extreme values is likewise enabled during normal

operation ( Excitation ON and Ug gt F204 ) Flag F754 is used to enable or block the

malfunction signals to the binary outputs Basically the monitoring of the actual values

for If1If2 functions like that of the Ug1Ug2 monitoring

62 Excitation Protection

621 General Information

The Excitation Protection switches off the excitation (and de-excites the machine

rapidly) whenever a danger arises that threatens the excitation transformer the converter

or the generator Generally limiter or monitoring functions precede the emergency trips

and these normally respond before the Excitation Protection must initiate a trip

Protective trip commands are issued directly to the field circuit-breaker from potential

free contacts of the board via the trip relays They are directed redundantly to the

operative field circuit-breaker ldquoOFFrdquo command

622 Protection against Excitation Transformer Overheating

This equipment monitors the excitation transformer for overheating in the

windings that could result from over current short-circuits or inadequate cooling The

monitoring uses temperature monitoring modules in conjunction with temperature

sensors built into the windings Normally the temperature is monitored in two stages the

first stage sets off an alarm the second causes a trip of the excitation

623 Rotor Over voltage Protection

Malfunctions in the generator circuit (eg terminal short-circuit failed

synchronization asynchronous operation) cause induced negative field currents that

produce high voltages in the field circuit These must be restricted to a level with a

sufficient safety margin below the insulation capacity of the field winding (test voltage)

and also below the peak blocking voltage of the converter thyristors The crow bar

employs spark gap elements to detect over voltages in the field circuit Whenever they

respond the associated thyristors are fired immediately switching the de-excitation

resistor parallel to the field The de-excitation current generated thereby initiates an

excitation trip via a supervision circuit causing an immediate opening of the field circuit-

breaker The malfunction isets off an alarm and an internal malfunction is indicated at the

cubicle

TEST VALUES OBTAINED WHEN EXCITATION IS RAISED

TEST

SNO PARTICULARS ACTUAL VALUE OBTAINED VALUE

1 VREF 996 100

2 VACT 997 1003

3 IFACT 735 765

4 IGACT 703 707

5 ACTIVE

POWER

703 705

6 REACTIVE

POWER

101 142

7 POWER

FACTOR

099 IND 098 IND

8 ACTIVE

CURRENT(IR)

705 703

9 REACTIVE

CURRENT(IX)

102 138

10 POWER

ANGLE

566 546

11 FIRING

ANGLE

640 632

VALUES OBTAINED WHEN EXCITATION IS LOWERED

SNO PARTICULARS ACTUAL

VALUE

OBTAINED VALUE

1 VREF 100 997

2 VACT 100 997

3 IFACT 787 761

4 IGACT 839 837

5 ACTIVE

POWER

839 830

6 REACTIVE

POWER

155 110

7 POWER

FACTOR

098 IND 099 IND

8 ACTIVE

CURRENT(IR)

835 830

9 REACTIVE

CURRENT(IX)

140 94

10 POWER

ANGLE

613 DEG 631 DEG

11 FIRING

ANGLE

601 DEG 599 DEG

CONCLUSION

For generating the EMF in stator winding excitation is required to the rotor of a

generator There are two types of excitation

1 Static excitation system

2 Brushless excitation system

A certain disadvantage in brushless excitation system is the slow response time of

the field in case of fast load changes specified No slip-rings and brushes direct

measurements of the field parameters not possible

To avoid all loses static excitation is used Since it does not have any rotating

parts mechanical loses and windage loses This system has fast response and speed

control While preferring this excitation system there are no limitations for the

redundancy of Thyristor bridge circuits

Static excitation has fast field discharge by resistor and inverter operation direct

measurement of field quantity is possible The meaning of excitation is nothing but

continuous supply of DC current (ie field current) to the rotor to buildup required

output voltage in the stator

Field current is changed with respect to the change of load so the digital

automatic voltage regulator (DAVR) is used to regulate the output voltage according to

the load variations

So we conclude that static excitation system with DAVR is preferred since it is

having excellent dynamic performance and better options for R amp M

22 Basic Structure of the Processor Systems

7

The signal processors 25 analog inputoutput modules Each of these processor

systems has a common bus circuit and output and the control lines There is a specific

range of addresses assigned to each assignment Board including the power supply bus

the address lines the two data lines to the input calculates the reactive current (I sin φ)

and the active current ( Icos φ) With these two channel processor Synchronized with

these interrupts (ie with the phase positions of current Ig the field current If and the

synchronous voltage Usyn From the exchange data with the microprocessor card across

the two data lines generator voltage Ug) this processor measures the generator current

Ig and then hardwired connections or multi-conductor cables Binary and analog

inputoutput modules ie for galvanic isolation and adaptation to the electronics level

The most important input interrupts per period to trigger the cycles for processing actual

values in the AUTOMATIC module on the processor bus) for filtering and further

processing

Monitoring each consist of the central microprocessor module and binary and

parameters to the AUTOMATIC channel are the generator voltage Ug the generator

peripheral unit Ug Ig and Usyn are sent to the Interrupt Generator (plug-in peripheral

units (wall-mounted units) peripheral units are used for preprocessing signals from

external measurement circuits power supply units Signals are exchanged among these

processor systems via processed across separate peripheral units for each channel These

processor working on the bus (a house address that can be adjusted using a switch)

systems The AUTOMATIC channel the programmable controls and the Excitation The

actual values measured from AUTOMATIC channel and MANUAL channel are The

AUTOMATIC channel and the MANUAL channel each have their own The digital

voltage regulator is broken down into several autonomous microprocessor The inputs and

outputs of the processor systems are directed across voltage-isolating The Interrupt

Generator also uses the 3-phase Ug signal to generate the 12 themselves contain a limited

number of hardware inputs and outputs with fixed equipment Whenever addresses from

this range are called up the signal processing module can results the processor is then

able to derive further operating parameters such as the load angle the active power etc

The functions of all microprocessor systems other than the programmable controls

have been accomplished in firmware The non-varying standard function modules can be

configured to the design desired for plant-specific purposes using software switches

(KFlags) Thus for example the stored status of a K-Flag determines whether or not a

Limiter is active and whether the de-excitation or the excitation limiters take precedence

Because these K-flags determine the software Scope of Supply for the installation they

cannot be changed permanently via the Micro-Terminal In this way they differ from

such setting data as the values of the parameters for the PID filter of the voltage regulator

or the set-points for the limiters These values can be permanently changed using the

Micro-Terminal Communication is possible with each of the processor systems via the

Micro-Terminal by plugging on the connecting cable In this way signals within the

processor and setting parameters can be viewed analog signals can be issued and the set

parameters can be altered temporarily (F range) or permanently (C range) Unlike the

other processor systems the programmable controls do not include any firmware for

realization of the functions They have been designed so that the designer can adapt and

change their functions easily using the ldquoFunctional Block Programming Language P10

Digital and analog functions can be implemented in practically any degree of complexity

desired using the P10 functional blocks The control variable of the voltage regulator

(AUTOMATIC channel) and the control variable of the field current regulator

(MANUAL channel) are each processed in separate Gate Control Unit and formed into a

chain of pulses at the appropriate firing angle The pulses of the active channel are

directed to the pulse bus via the associated Intermediate Pulse Stage The pulses for each

converter block are amplified sufficiently in Final Pulse Stage to fire the Thyristor

231 General Information

The functions of the automatic voltage regulator AVR are

1 to regulate the generator voltage

2 to regulate the effect of the reactive andor active current on the voltage

3 to limit VoltHz

4 to limit max and min field current

5 to limit inductive stator current

6 to limit capacitive stator current

7 to limit the load angle

8 to stabilize the power system

Block Diagram shows the software structure of AUTOMATIC channel The

generator limiters not provided for the installation in question (optional equipment) are

identified in this overview as ldquoNot Suppliedrdquo The parameter values signal values and

software switches (flags) marked with addresses (hexadecimal numbers) can be viewed

and altered via the Micro-Terminal The values selected are displayed in sec pu Hz

etc and can where necessary be changed directly in these formats The plant-specific

settings of the variables and the flags can be obtained from the Test and Commissioning

Report This block diagram provides information about the important functions and

possible settings of the AUTOMATIC channel For the sake of clarity no detailed

presentation has been given of special functions such as tracking circuits initializations

etc The page heading cross-refers this overview to the various sheets of the schematic

diagram Binary signals are shown in broken lines analog signals in solid lines The

corresponding text designations in the schematic diagram can be used for identification of

the input signals (hardware inputs) The only analog output signal from the automatic

voltage regulator control variable Ucontr is sent via the data bus (CRU bus) to the Gate

Control Unit Most of the binary messages (outputs) from the AVR are of no interest

functionally and they have been omitted for the sake of clarity The basic structure of the

digital voltage regulator and the limiters is simple This is necessary in order that the

behavior of the regulatorslimiters will remain calculable and understandable in all

operating situations and that there will be no problem in adjusting and optimizing them

The central PID filter in the digital voltage regulator defines the dynamic response of the

closed-loop controls both in the voltage regulator mode and after limiters have

intervened The ldquocontrol deviationrdquo at the input to the PID filter is either the control

deviation for voltage the control deviation of a de-excitation limiter (the value

determined by minimum value selection) or the control deviation of an excitation limiter

(the value determined by maximum value selection) Flag F730 (ldquoPRIORrdquo) is used to

determine whether the exciting (Min value) or the de-exciting signal takes precedence on

the minmax value limiter (normally F730 = 1111 ie the de-exciting signal takes

precedence) With the exception of the Minimum Field Current Limiter all other limiters

have variable factoring multipliers of the signal outputs so that they can be adjusted

individually together with the common PID filter which has been optimized for voltage

regulation The setting parameters for this PID filter are as follows

Vo = KR Static amplification

1

Ta = ---- Integration time constant

Tc1

Vp Proportional amplification

1

Tb = ---- Differential time constant

Tc2

Vinfin Amplification of high frequencies

The BODE diagram below shows the assignment of settings in accordance with

DINIEC standards based on a typical example

The PID filter amplifications Vo Vp and Vinfin can be adjusted in pu values But

the ldquoceiling factorrdquo pl+ must be adjusted correctly with parameter F310 if the total

amplification (circuit amplification) of the control circuit is actually to conform to the

pu settings This factor must agree with the ldquoexternalrdquo amplification ie with the

ceiling value of the transformer- converter circuit

Ceiling factor(pl+) = Ufmax Ufo

in which Ufmax = ceiling field voltage

Ufo = no-load field voltage

To attain a suitable response of the AVR when starting excitation

(ldquoEXCITATION ONrdquo) it may be necessary to change the proportional amplification of

the regulator during this phase Vp2 (transiently activated) and Vp1 (permanently

activated) can be adjusted for this purpose For example the value of Vp2 takes effect

immediately once the excitation is switched on and remains effective for a period as set at

F30C Once the period F30C (eg 5 sec) has expired Vp shifts over to Vp1 (becomes

the steady-state Vp) at the rate of change set The standard operating mode for the PID

filter is voltage regulation for which the discrepancy between the voltage set-point and

the current value for generator voltage Ug (the control deviation) is supplied at the input

To compensate for the voltage drop in the block transformer or whenever several

generators are operating to the same distributing bus the generator voltage must be

varied in proportion to the measured generator current (droop influence) To accomplish

this the voltage set-point is varied as a function of the measured reactive current IX

andor active current IR Flag F712 enables the IX droop Flag F710 the IR droop The

desired compensation is set in F282 and F286 respectively Flags F284 and F288 are used

to select whether this droop influence is to increase the voltage or to reduce it

(compensation) Combined influence of the active and reactive currents is attained by

enabling both droops IX and IR Flag F716 activates a so-called ldquoSoft-Startrdquo at the

starting of excitation This ldquoSoft-Startrdquo ensures that the voltage set-point integrates from

0 to 100 within the time set on F290 when the excitation is switched on

(ldquoEXCITATION ONrdquo) A ldquosmoothrdquo excitation of the generator can be achieved in this

way whenever there is no demand for a quick excitation

232 Voltage Set-Point

Various signals and settings control and limit the voltage set-point F270 For

example the values of F254 and F252 define the normal operating range possible for set-

point adjustment (eg 90 110) using external control commands (control room local

operatorrsquos panel superposed control system) The effective set-point adjustment rate is

governed The set-point can be set at the values of F250 and F256 by activating

appropriate control commands for ldquoSETrdquo input Enabling Flag F71A and activating a

binary input prior to switching on the excitation (ldquoEXCITATION OFFrdquo) sets the Ug set-

point at the value of UAUX This makes it possible for example to ensure that the

generator voltage will agree exactly with the network voltage after the voltage build-up

An external value with variable amplification can be added to the Ug set point by

enabling F724 (for example for stability tests)

233 Regulator Tracking in MANUAL Operation

Whenever the AUTOMATIC channel is not in operation (the MANUAL channel

is ON) a follow-up equipment ensures a smooth switch-back to the AUTOMATIC

mode will always be possible To track the voltage set-point is shifted by means of

RAISELOWER pulses from the Gate Control Unit so that control variable Ucontr at the

output from the PID filter is held steady and identical to the control variable Ucontr from

the MANUAL channel Because this tracking must react slowly resultant transient

control deviations resulting from the amplification in the PID filter might cause severe

interference with control variable Ucontr

To prevent this the follow-up equipment intervenes on the regulators mixing

point with a corresponding compensation signal

234 Ugf Limiter

At under frequency the Ugf Limiter reduces the generator voltage so as to

prevent saturation effects in the supply and measuring transformers To adjust this

limiter the max permissible generator voltage at rated frequency is defined and set

When any under-frequency occurs the generator voltage is thus reduced in proportion to

that setting

235 Field Current Maximum Limiter

The Field Current Maximum Limiter is provided to protect the generator rotor

from s occurring in steady-state and transient operation High field currents are normally

the result of a sharp drop in network voltage or of an improper raising of the voltage set-

point by the operating staff The field current is held steady at the value TH1 ie at the

maximum thermal value permissible for the excitation circuit and the rotor In order that

the generator can support the power network with its transient overload capacity during

brief collapses in voltage a temporary switch-over is made to the transient limit MAX1

(a higher setting) When the generator or the converter is operating at a reduced capacity

These limits TH1MAX1 can be switched over to the lower settings TH2MAX2 by

activating the corresponding binary signals The switch-over from the thermal limit

TH12 to the transient limit MAX12 can be configured in one of three ways

a) Depending on the over current with -dUdt ENABLE

Flag programming F418 = any setting desired F41A = 0000

This variant enables the transient value MAX12 whenever a collapse of voltage

in the network is detected The ENABLE time is fixed and can be set The example

below shows the typical behavior of the limiter configured in this way

b) Dependent on the time integral with -dUdt ENABLE

Flag programming F418 = inactive F41A = 1111

This variant likewise enables the transient value only when a collapse of network

voltage has been detected However the switch-back to the thermal limit is not made

dependent upon the time itself but on the calculated time integral intisup2dt of the The setting

on Parameter F414 in spu takes into account the time the rotor needs to cool down ie

the rate of temperature change in the case of intermittent operation The example below

shows how the timing of the switch-back to the thermal limit depends on the present

value for intisup2dt

The time integral is based on the formula

Example The setting of =isup2dt equivalent to Version a (F416) at a constant 16 times the

nominal field current for 10 seconds (with TH12 = 105) is

c) Dependent on the time integral without any preconditions

Flag programming F418 = 1111 F41A = 1111

In this variant the transient becomes available without any prior conditions

(without a -dUdt ENABLE) with the time integral intisup2dt

237 Inductive Stator Current Limiter

The Inductive Stator Current Limiter holds the stator current Ig within permissible

limits while the generator is in the ldquoover-excitedrdquo operating range by reducing the field

current accordingly The setting TH (thermal limit) provides the limit against stationary s

that might occur To take advantage of the generatorrsquos transient overload capacity a

switch-over is made to the higher setting MAX The principle of operation of this switch-

over to the value MAX permissible only transiently is identical to that employed for the

field current limiter (refer to the description above) When the drive output from the

turbine is very high stator current may exceed permissible limits even while inductive

loading of the generator is low In this case if the stator current limiter is not kept from

influencing the field current the control circuit will oscillate back and forth between the

Inductive Stator Current Limiter (de-

exciting) and the Capacitive Stator Current Limiter (exciting)The output signal of that

function then dominates the control variable of the Ig-dependent limiter via a maximum

value selection

238 Capacitive Stator Current Limiter

239 Load Angle Limiter

The Load Angle Limiter prevents the synchronous machine from slipping out of

phase due to slippage of the rotor The load angle δ the difference in phase between the

rotor and the stator rotating field results mainly from the driving torque (active power P)

acting on the generator and the level of rotor current (field current) If the driving torque

remains constant a increase in the field current reduces the load angle δ The current load

angle δ at any moment is obtained from the generator current and generator voltage based

on a simplified model of the generator Whenever this calculated load angle δ exceeds the

preset limit angle the limiter increases the field current until the load angle has dropped

back to its permissible value The quadrature reactance Xq of the generator and the

network reactance Xe during normal operation must be adjusted on the regulator in order

to obtain the load angle δ The graph below shows the Power Chart for a salient-pole

machine with typical limiter characteristics

The purpose of a Power System Stabilizer is to use the generator excitation to

damp electromechanical oscillations between the network and the generator Depending

on the design of the generator and the requirements imposed for network stability its

main function will be either to damp the oscillations originating in the machine or those

from the network A synchronous generator working in a combined power network is in

principle an oscillating structure In order to produce a torque the magnetic field of the

rotor and the stator must form a given angle (referred to as the rotor displacement or load

angle δ) The electrical torque ME increases as the angle δ increases just as with a

torsion spring Because the ME of the generator and the mechanical driving torque MA

from the turbine are in equilibrium during steady-state operation the angle δ remains in a

given position Whenever this state of equilibrium between MA and ME is disturbed the

load angle slips of this rest position and change thereby the electrical torque ME The

torque attempts to restore the load angle to a stationary position Due to the mass inertia

of the turbinegenerator rotor however this can only take place aperiodically It does so

in the form of more or less effectively damped oscillations (again similar to the effect of

mass inertia on a torsion spring) In order to damp the oscillations there must be a

damping torque produced depending not on the electrical torque ME associated with the

angle but on the difference in frequency (Df) between the rotor and the stator rotating

field ie on the slippage This torque is produced mainly by the so-called damper

winding in the rotor but the dimensioning of this is subject to limits imposed by

considerations of design and economy Some further action is therefore needed to

increase the damping effect The following drastically simplified formula shows the

parameters upon which the amount of active power PE supplied by the generator

depends

PE = active power

It can be seen from the above relationship that the active power that the generator

transfers depends not only on the load angle δ but also on the field current If That means

that a transient change can be made in the active power PE and with that in the effective

electrical torque ME by varying the field current The principle of operation of the DVR

Power System Stabilizer becomes clear from a consideration of the oscillations in power

output and frequency (ΔPE Δf) and the vector diagram If it is assumed that oscillations

in the network frequency generates load oscillations with the mass inertia of the rotor

then the active load of the generator (eg MW-measured) is influenced with a sinusoidal

value -ΔPE (ME-MA = -ΔPE) By inversion of -ΔPE one obtains the fluctuation in

power provided by the rotor +ΔPE As is known the slip signal Δf follows +ΔPE with a

phase delayed by 90deg The +ME produced by the periodic changes in the load angle δ is

in phase with +ΔPE A good damping is attained if ME is varied in phase with the slip

Δf However this signal must also be advanced somewhat to compensate for the time

constants in the excitation circuit and the generator

As mentioned above the electrical torque ME can be influenced by varying the

field current To accomplish this a suitable control signal referred to as variable

disturbance compensation must be imposed upon the voltage set-point or the converter

control variable Ucontr As can be seen from the vector diagram by applying proper

weighting factors (K1 K2) and then adding together the signals -ΔPE and Δf an overall

stabilization signal can be produced that rotates in advance of the Df signal by any angle

desired between 0deg and 90deg Because the amplitude of -DPE remains proportional to the

amplitude of Δf a constant angle in advance of Δf results for the compensation of the

time constants referred to above The optimum weighting factors K1 and K2 for a

synchronous generator working to a power network depend on its operating point at any

moment and the external reactance of the network Normally the selection of a

compromise setting is good enough to attain stability in all operating points and for all

external reactance For special demands these settings must be parameterized as a

function of the external reactance (which means optional equipment Xe-Identification)

The Power System Stabilizer PSS is a section of the AVR computer program and is

processed once per network cycle The voltage at the generator terminals and the

generator current are measured in order to define the signals ΔPE and Δf The calculated

signals for _P_ (=PE) and Δf are then sent across DC filters ldquoDrdquo (real differentiators) that

transmit only the dynamic portion of the signals The ΔPE and Δf signals obtained in this

way are then weighted (multiplied by) with the factors K1 and K2 and sent to the

summing point of the voltage regulator

The PSS stabilization signal is imposed on the automatic voltage regulator only if

the following prerequisites are met

bull Generator on line

bull Generator power output gt the value F338

bull Generator voltage in a range between F33C and F33A

The stabilization signal is limited at the output from the PSS to the lower and

upper limits Flag defines whether the stabilization signal is introduced before or after the

PID filter (usually before the filter) Because the PID filter as noted above already takes

the ceiling factor Vp1 into account the PSS signal needs to be multiplied by Vp1 if it is

added to the voltage regulator following the PID filter (divider at the input to the

minmax limiter) This precaution prevents the DC filter ldquoDrdquo in the P-channel from

producing an unnecessary ldquostabilizationrdquo effect in the case of rapid changes in turbine

load As an alternative for the AVRrsquos Power System Stabilizer a stabilization signal from

an outside system can be imposed by activating the binary input ldquoPSS-SIGNEXTrdquo Flag

F340 can be used to select between an analog and a 12-bit signal and F33E to select the

polarity desired for that signal

24 The MANUAL Channel

241 Summary

The MANUAL channel (Channel 2) has been built as a simple field current

regulator

without additional limiters Its main function is to maintain the excitation of the generator

even if the AUTOMATIC channel becomes non-operational The MANUAL channel

also performs valuable service for purposes of testing commissioning and preventive

maintenance Its measurements regulator generation of firing pulses and power supply

are physically separate from those on the AUTOMATIC channel

242 Principle of Operation

All the functions of the MANUAL channel including the generation of firing

pulses have been implemented in a single electronic module the Gate Control Unit The

control variable Ucontr of voltage regulator is used as the reference value for generating

firing pulses on the principle known as ldquoramp controlrdquo (Comparison of Ucontr with

Usynsynchronous sawtooth signal) For further processing in the UN 0096 Intermediate

Pulse Stage the Gate Control Unit supplies six firing pulses at its output whose phase

position with respect to the synchronous voltage Usyn is in accordance with control

variable Ucontr An internal linearization ensures that the field voltage produced via the

firing pulses remains proportional to the control variable Ucontr throughout the entire

range As a result the circuit amplification of the control remains constant over the entire

range Whenever excitation is switched ON the set-point for Generator Voltage is set

automatically at the preset - ref Value This provision ensures that the generator voltage

always attains approximately its nominal value after the field flashing The Gate Control

Unit can be refunctioned ( by pre-selection with a switch ) for purposes of testing to act

as a purely firing pulse control In this case the control variable Ucontr is adjusted

directly using the RAISELOWER push buttons on the front of the module In this way

for example the relationship between the phase position of the firing pulses and the

control variable Ucontr can be checked easily

CHAPTER 3

PULSE SECTION

31 Pulse Generation and Amplification

The Gate Control Units of both AUTOMATIC channel and MANUAL channel

each supply six firing pulses for operating the 6-pulse thyristor bridges The low-power

pulse signals from these Gate Control Units are then amplified in the Intermediate Pulse

Stage galvanically isolated and then sent to the common pulse bus On the output end

the Intermediate Pulse Stage of the non-active channel) is always blocked The Gate

Control Units generate the pulses based on microprocessor control The reference voltage

used for the firing pulse phase location is the output voltage from the excitation

transformer (Usyn1 Usyn2) The commutation spikes of the synchronous voltage caused

by the converter are calculated prior to use of the voltage as a reference value and are

deliberately filtered out The lower limit for the firing pulses (double pulses) which are

offset from one another by 60deg is defined by the limit rectifier position (αmin) and the

upper limit by the limit inverter position (αmax) for the firing angle αmin and αmax can

be adjusted on the Gate Control Units using BCD (Binary Coded Decimal) switches

αmin ensures that the firing pulses will not be issued (premature firing) until there is

sufficient positive phase voltage on the thyristor involved αmax prevents a dangerous

ldquotippingrdquo of the thyristor bridge into the rectifier mode if the firing angle α is too large

(ldquolate firingrdquo) The critical factors determining αmax are the overlap time uumlmax (max

commutation time) and the ldquorecovery timerdquo of the thyristors (αmax lt 180deg - uumlmax - γ )

An external control signal can force the firing pulses into their inverter limit position

Other binary inputs can block or direct the firing pulses of the Gate Control Units so as to

produce freewheeling on the thyristor bridge During freewheeling the firing pulses for

the thyristor pair R and S are blocked and the pulse signals T+T- are engaged with

chains of pulses Both Gate Control Units (for the MANUAL amp AUTOMATIC channels)

contain a field current monitor that blocks the firing pulses immediately whenever the

current exceeds a preset threshold level In this case the field circuit-breaker is also

tripped via an output contact The purpose of these provisions is to prevent damage to

thyristors and thyristor fuses in case of a slip-ring short-circuit or to keep any damage

that does occur to a minimum The pulse signals are galvanically separated at the outputs

from the Intermediate Pulse Stage (with pulse transmitters) and are then directed to the

common pulse bus This transmission of the pulse signals to the pulse bus via passive

transmitters ensures a high degree of active channel autonomy Practically no possible

malfunctions on the inactive channel (including for example sustained pulses) affect the

active channel

32 Pulse Monitoring

The ldquoPulse Busrdquo and the pulse signals of the AUTOMATIC channel are

monitored This monitoring device consists of potential isolating stages and the common

monitor If the pulse monitoring of the ldquoPulse Busrdquo responds a switch-over is made to

MANUAL channel The function of the potential isolating stages is to couple the pulse

monitoring device to the pulse circuits without any feedback effect The pulse monitoring

checks the six pulse lines for the following malfunctions continuous or periodic failure

of one or more pulses Periodic occurrence of synchronous or asynchronous false pulses

Continuous pulses the pulse monitoring device can be tested while the machine is in

operation

CHAPTER 4

CONVERTER

Thyristor

The term thyristor usually refers to a family of four layer solid state device having

turn on characteristics that can be externally controlled by either current or voltage They

are also referred to as breakdown device because their working depends on avalanche

breakdown Thyristors have only two stages OFF and ON Thyristors have a similar

function to Uni-junctions they act as switches Thyristors use current flow as a switch

Thyristors have three states

1 Reverse blocking mode mdash Voltage is applied in the direction that would be

blocked by a diode

2 Forward blocking mode mdash Voltage is applied in the direction that would cause

a diode to conduct but the thyristor has not yet been triggered into conduction

3 Forward conducting mode mdash The thyristor has been triggered into conduction

and will remain conducting until the forward current drops below a threshold value

known as the holding current Converter is a semiconductor device which converts ac

input voltage into a constant dc output voltage In present excitation system three phase

fully controlled thyristor converter is used

Because of the following advantages thyristor converters are used

a) Thyristors are used for high power applications ie up to 10Kv3500A1KHz

b) Having high reliability and low losses

c) Uni-directional device like diode

d) Itrsquos operation as a rectifier which are low resistance in forward conduction

mode and high resistance in reverse conduction mode

PROTECTION OF THYRISTORS

For reliable operation of a thyristor demands that its specified ratings are not

exceeded When Subjected to or over voltages During the turn - on of SCR didt

prohibitively large False triggering of SCR by high value of dvdt andSpurious signals

between gate and cathode may leads to unwanted turn ndash on

DIDT AND PROTECTION

When thyristor starts conducting in forward conduction mode and is turned on by

gate pulse The anode current increases rapidly whole area of the gate to Cathode

junction then hot spots will be formed near the gate connection this locality of heating

destroys the thyristor Thyristor thermal time is constant The causes due to faults and

short circuits or surge currents Electronic crowbar protection is used against the over

voltages The rate rise of anode current must be kept at the time of turn on below the

rated or specified limiting value The didt value maintained below limited value by using

a inductor also called ldquodidt inductorrdquo in series with anode circuit The locality of heating

is avoided by applying gate current but not greater the maximum gate current

DVDT AND OVER VOLTAGE PROTECTION

With forward voltage across the anode and cathode of a thyristor the two outer

junctions are forward biased but the inner junction is reverse biased This reverse biased

junction J2 has the characteristics of a capacitor due to charges existing across the

junction In other words space-charges exist in the depletion region around junction J2

and therefore junction J2 behaves like a capacitance If the entire anode to cathode

forward voltage Va appears across J2 junction and the charge is denoted by Q then a

charging current i given by Eq (46) follows

i = dQdt =d(Cj Va )dt

= Cj (d Va dt) + Va(d Cj dt) helliphelliphelliphellip(46 a)

As Cj the capacitance of junction J2 is almost constant the current is given by

i = Cj (d Va dt) helliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip(46 b)

If the rate of rise of forward voltage dVadt is high the charging current i will be

more This charging current plays the role of gate current and turns on the SCR even

when gate signal is zero Such phenomena of turning-on a thyristor called dvdt turn-on

must be avoided as it leads to false operation of the thyristor circuit

For controllable operation of the thyristor the rate of rise of forward anode to

cathode voltage dVadt must be kept below the specified rated limit Typical values of

dvdt are 20 ndash 500 Vμsec False turn-on of a thyristor by large dvdt can be prevented by

using a snubber circuit in parallel with the device thyristor are very sensitive for over

voltage than the semiconductor devices

Over voltage transients are perhaps the main cause of thyristor failure

In thyristor there are mainly two types

1 Internal over voltages

Due to the commutation of the thyristors large voltages are generated internally

Because of the series inductance of the SCR circuit the large transient voltages L didt

produced This voltage several times the break over voltage of the device then thyristor

destroys permanently

2 External over voltages

External over voltages are caused due to the interruptions of current flow in an

inductive circuit and also due to the lightening strokes on the lines feeding the thyristor

system For the reliable operation of thyristor the over voltages must be suppressed by

adopting suitable techniques

Suppression of over voltages

The RC circuit called snubber circuit is connected across the device to protect In

order to keep the protective components to a minimum the thyristors are chosen with

their peak voltages ratings are 25 to 3 times of the normal peak working voltage

ldquoselenium thyrector diodes metal oxide varistors or avalanche diode suppressers are

commonly employed for protecting the thyristor circuit against the over voltages

Gate protection

Gate circuit should also be protected against the over voltages and surges Over

voltage at gate circuit can cause false triggering of the SCR may rises the junction

temperature behind specified limit leading to its damage Protection against over

voltage can be achieved by connecting a ZD across the gate circuit and a resister is

connected in series with gate circuit to protect against the s A capacitor and resister are

connected across gate to cathode to by pass the noise

41 Final Pulse Stages

The Final Pulse Stages adapt the output pulses from the Intermediate Pulse Stage

(pulses on the pulse bus) to the gate currents needed for the thyristors Each thyristor

bridge is equipped with its own Final Pulse Stage Each Final Pulse Stages is provided

with a power supply module The amplified output pulses from the Final Pulse Stages

start as a short strong steep pulse with an amplitude approx 2frac12 times that of the main

pulse This initial pulse edge assures proper firing of the thyristors being triggered

Subsequently the weaker part of main pulse keeps firing conditions steady As already

mentioned the Final Pulse Stages and their associated thyristor bridges form single units

All six pulse outputs from a Final Pulse Stage can be blocked by an external control

signal so that all thyristors in the associated thyristor bridge will block the current A

blocking of the pulses is initiated whenever there is a malfunction in the associated

thyristor bridge

42 Converter Power Section

The thyristor converter consists of three independent parallel rectifier blocks TY1

to TY3 which are all in service Even if one block fails the remaining blocks take over

automatically the full design current of the excitation circuit During normal operation

(with ideal current share) and all three bridges in operation each of these blocks has to

carry only (n-2)n (ie33)of its design current If 2 thyristor bridges fail the excitation

is limited Only when all three bridges fail the excitation is switched off Each thyristor

bridge arm is equipped with current flow monitoring CTrsquos Failure of conduction in any

arm is identified by a Current flow monitoring module

43 Converter Cooling

A cooling system is needed to dissipate heat losses in the converter blocks and

electronics Each converter block has therefore been equipped with a fan supplied with

power from the converterrsquos primary voltage (via transformer ndashT8 in field flashing

cubicle) The fans are protected with motor protection circuit breakers An air flow

monitoring unit is provided for monitoring the air flow through the thyristor bridge If a

circuit breaker failure is detected or if the air flow monitor drops off at one of the

thyristor bridges the bridge involved is immediately set out of operation by blocking its

firing pulses

44 Thyristor Converter Monitoring

A thyristor bridge in which defects occur that could threaten the safety of

operation or cause secondary damage is switched off automatically ie its firing pulses

are blocked This happens whenever A thyristor fuse is blown The fuses are monitored

individually with micro switches The Final Pulse Stage fails which is detected by

internal monitors (supply voltage sustained pulse short-circuit on the output end) The

power supply to the fan fails fan air flow as monitored by the Air flow monitor fails or

is insufficient Isolator on ACDC side is open

CHAPTER 5

Field Current Circuit Excitation Transformer

51 Field Circuit Breaker

The circuit-breaker in the field circuit is used to isolate the field circuit from the

converter It is capable of switching off the synchronous machine from full load under the

maximum conditions of a 3-phase short-circuit In addition to its main contacts the field

circuit-breaker also has a de-excitation contact with which the field energy stored in the

field can be dissipated across the de-excitation resistor The de-excitation contact closes

shortly before the main contacts open so as to ensure proper commutation of the field

current from the main contacts to the de-excitation contact when the breaker is switched

off The field circuit-breaker is switched on by electromagnetic force and is kept switched

on by a mechanical latch When the latch is released by a trip coil the circuit-breaker

opens The circuit-breaker also has auxiliary contacts that report its status

52 Field Flashing

In shunt supplied excitation circuits (excitation transformer connected to the

generator terminals) the generator does not have enough remnant voltage for a generator

voltage build-up via the converter In this case a field flashing circuit is provided It

consists of the field flashing contactor the diode bridge and a transformer used to adapt

the auxiliary input voltage to the voltage needed for field flashing when power is

supplied from the auxiliaries network

Fig Field Flashing

Because the field flashing contactor is not able to switch off the energy stored in

the field the control ensures that the contactor can only reopen if the field circuit breaker

has already been opened (generating the TRIP order) or in a normal field flashing

sequence when the converter has taken over the field current Field flashing occurs in the

following stages

1048729The excitation is switched on closing the field flashing contactor ( Field

Circuit Breaker is already closed )

1048729The start-up excitation current flows through the rotor driving the generator

voltage up to approx 15 U

1048729After about 10 U the firing pulses to the converter are released and it begins

to excite the generator to its rated voltage

1048729After about 30 U the field flashing contactor opens (with no current since

the converter is now supplying the current)

The diode bridge at the input to the field flashing contactor prevents a feed-back

from the converter to the source of field flashing while the contactor is still closed

53 De-excitation

When malfunctions occur the stored field energy must be dissipated as quickly

and safely as possible to protect the generator This is done by the converter the field

circuit-breaker and the de-excitation (discharge) resistor

De-excitation (with opening of the field circuit-breaker) takes place in the following

stages

1048729The converter drives to its inverter limit position (negative ceiling voltage)

recovers a portion of the field energy into the network A trip command is given to the

field circuit breaker

1048729The de-excitation contact closes diverting the field voltage to the de-excitation

resistor

1048729Then immediately the main contacts open building voltage The field voltage

commutates to the de-excitation resistor

1048729The current diminishes at a given time constant TE

(With linear resistance TE = Lf (Rf + Re))

Due to the reversal of the field voltage by the converter the field current

commutates from the main contacts of the field circuit-breaker to the de-excitation

resistor in a very early phase This reversal of the field voltage prevents burn-off on the

main contacts and provides effective protection for the field circuit-breaker Depending

on the operating policy an operational shut-down of the excitation can also be effected

with the field circuit-breaker closed This method is useful mainly when the excitation is

switched on and off frequently In this case the converter is merely driven into the

inverter limit position so that the field energy is recovered into the network The

converter then blocks since it is supplying positive current only

54 Excitation Transformer

The excitation transformer matches the generator voltage to the field voltage

(required ceiling voltage) It also serves as a commutation reactance for the thyristor

converter and as a potential isolator between the network and the excitation circuit In

addition the transformer functions as a current limiter in that it makes it possible to keep

any short circuits in the excitation circuit under better control The excitation transformer

is equipped with temperature monitoring probes which set off an alarm when the

temperature exceeds a first max limit and then trips the excitation if the temperature

continues rising to a second (higher)limit

CHAPTER 6

Monitoring and Protection

61 Excitation Monitoring

611 General Information

The main goal of Excitation Monitoring is to make optimum use of the

redundancies provided in the excitation system and to give alarm whenever a malfunction

makes these redundancies unavailable The field current is monitored to see that it does

not exceed a maximum level and if necessary a switch-over to the MANUAL channel is

initiated In addition the criterion for switching off the field flashing is generated The

excitation Monitoring consists of an autonomous processor system

612 over current Alarms

In the Excitation Monitoring the limits for are set at higher levels than the

settings on the Field Current Maximum Limiter Whenever the current exceeds 110 of

the nominal field current contact R1 and the binary output associated with it are

activated immediately If field current remains gt 110 then after a preset inverse-time

has lapsed relay R2 and - after a further delay - relay R0 and the binary outputs

associated with them are activated Parameters match the measurements for If1 and If2 to

the nominal value for field current so that the internal values can be processed and read as

pu values It can be used to falsify the actual value of the field current If (to raise it) so

as to cause a response from the alarm limits for purposes of testing The processed If

signal is always taken from on the active channel (CHANNEL I OR CHANNEL II) As

long as the field current If is above the threshold value 11 Ifn its peak value is

measured This is stored (until RESET) and can be read at any time on the Micro-

Terminal Once the value of If exceeds 11 Ifn integration of this value starts Whenever

the integrated time-current value (intisup2dt) exceeds the preselected reference value the

alarm OVER CURRENT INVERSE-TIME is set off and a command is simultaneously

issued to switch over to the stand by AUTO channel Software switch F758 enables the

three over current alarm functions (R0 R1 R2) and selects one of three possible inverse-

time curves T1 T2 or T3 Within the characteristic curve (T1 T2 T3) selected the

desired limit curve for response is set using the factor F216

613 Switch-Off Criterion for Field Flashing

The Excitation Monitoring supplies the criterion for switching off the field

flashing Whether this criterion is activated based on the actual value for generator voltage

Ug or for field current If or both depends on the settings of the two threshold values

F200F202 (0 setting means that the output is always ldquological 1rdquo) The measurements

Ug12 and If12 are switched over depending on the present status of the channels

(Channel 1 or Channel 2 ON) Whenever Flag F750 is not activated the binary output is

fixed at ldquological 1rdquo

614 Storage of Alarm Status

The outputs of the over current alarms (R1 R2 R0) and the messages NO

FAILURE MONITORING PARAMETERS CHANGED are stored messages can be

erased by activating the input ldquoGENERAL RESETrdquo or by using the RESET button on the

front of the module Erasure with the input ldquoGENERAL RESETrdquo is effective only if the

situation causing the alarm or the malfunction is no longer present Whenever the self-

diagnosis equipment in the processor detects a malfunction the output NO FAILURE of

MONITORING is set at ldquological 0rdquo (= alarm) The alarm ldquoPARAMETERS CHANGEDrdquo

is activated whenever parameters or settings of software switches have been changed via

the Micro-Terminal

615 Actual Value Monitoring

The actual values for generator voltage Ug synchronous voltage Usyn and field

current If are monitored for malfunctions This monitoring is active regardless of whether

or not the generator is in operation Essentially when the generator is in operation the

measurements are monitored by comparing the signals (the smaller signal reading is

detected as incorrect) When the generator is not in operation the measured data are

monitored for extreme values The percentage of deviation permissible in the

measurement signals being compared

(Ug1ampUsyn1 Ug1ampUg2 Ug2ampUsyn2 If1ampIf2) is defined by parameters F208 and

F20AIf the excitation transformer is being supplied from an auxiliary power source (no

shunt operation) the values of Ug and Usyn will be different in some operational

conditions

In that case Flag F75A can be used to deactivate comparative measurements Ug1 Usyn1

and Ug2 Usyn2 The ldquoprimary monitoringrdquo compares Ug1 with Ug2 and generates the

messages that Channel 1 or Channel 2 has suffered a malfunction Whenever Ug1 lt Ug2

and the binary message from CH1 reports no malfunction a malfunction on

Measurement Channel 1 is reported (Ug1Usyn1 FAILURE) A similar malfunction is

also present whenever the binary message CH1 DISTURBANCE is reported and a

discrepancy is detected between Ug1ampUg2 The generation of the alarm ldquoUg2 Usyn2

FAILURErdquo is analogous to that for Channel 1 The ldquosecondary monitoringrdquo compares

Ug1 with Usyn1 amp Ug2 with Usyn2 This is enabled whenever the binary message of the

comparison channel reports a malfunction or whenever both binary messages report no

malfunction - but both secondary monitors report a malfunction As long as the secondary

monitoring is blocked the differences Ug1neUsyn1 or Ug2neUsyn2 trigger malfunction

signals for the measurement channel involved (suspicion that there is a corresponding

error in Usyn) The measurement channel malfunctions are enabled operationally

whenever after excitation has been switched on generator voltage Ug exceeds the value

set on F204 The voltages Ug1 and Ug2 are checked 16 seconds after the excitation is

switched off to see that they do not exceed the limit value F210 that applies to both of

them At the same time g1Ug2Usyn1Usyn2 are checked for extreme values (gt or lt

the operating range) Monitoring for extreme values is likewise enabled during normal

operation ( Excitation ON and Ug gt F204 ) Flag F754 is used to enable or block the

malfunction signals to the binary outputs Basically the monitoring of the actual values

for If1If2 functions like that of the Ug1Ug2 monitoring

62 Excitation Protection

621 General Information

The Excitation Protection switches off the excitation (and de-excites the machine

rapidly) whenever a danger arises that threatens the excitation transformer the converter

or the generator Generally limiter or monitoring functions precede the emergency trips

and these normally respond before the Excitation Protection must initiate a trip

Protective trip commands are issued directly to the field circuit-breaker from potential

free contacts of the board via the trip relays They are directed redundantly to the

operative field circuit-breaker ldquoOFFrdquo command

622 Protection against Excitation Transformer Overheating

This equipment monitors the excitation transformer for overheating in the

windings that could result from over current short-circuits or inadequate cooling The

monitoring uses temperature monitoring modules in conjunction with temperature

sensors built into the windings Normally the temperature is monitored in two stages the

first stage sets off an alarm the second causes a trip of the excitation

623 Rotor Over voltage Protection

Malfunctions in the generator circuit (eg terminal short-circuit failed

synchronization asynchronous operation) cause induced negative field currents that

produce high voltages in the field circuit These must be restricted to a level with a

sufficient safety margin below the insulation capacity of the field winding (test voltage)

and also below the peak blocking voltage of the converter thyristors The crow bar

employs spark gap elements to detect over voltages in the field circuit Whenever they

respond the associated thyristors are fired immediately switching the de-excitation

resistor parallel to the field The de-excitation current generated thereby initiates an

excitation trip via a supervision circuit causing an immediate opening of the field circuit-

breaker The malfunction isets off an alarm and an internal malfunction is indicated at the

cubicle

TEST VALUES OBTAINED WHEN EXCITATION IS RAISED

TEST

SNO PARTICULARS ACTUAL VALUE OBTAINED VALUE

1 VREF 996 100

2 VACT 997 1003

3 IFACT 735 765

4 IGACT 703 707

5 ACTIVE

POWER

703 705

6 REACTIVE

POWER

101 142

7 POWER

FACTOR

099 IND 098 IND

8 ACTIVE

CURRENT(IR)

705 703

9 REACTIVE

CURRENT(IX)

102 138

10 POWER

ANGLE

566 546

11 FIRING

ANGLE

640 632

VALUES OBTAINED WHEN EXCITATION IS LOWERED

SNO PARTICULARS ACTUAL

VALUE

OBTAINED VALUE

1 VREF 100 997

2 VACT 100 997

3 IFACT 787 761

4 IGACT 839 837

5 ACTIVE

POWER

839 830

6 REACTIVE

POWER

155 110

7 POWER

FACTOR

098 IND 099 IND

8 ACTIVE

CURRENT(IR)

835 830

9 REACTIVE

CURRENT(IX)

140 94

10 POWER

ANGLE

613 DEG 631 DEG

11 FIRING

ANGLE

601 DEG 599 DEG

CONCLUSION

For generating the EMF in stator winding excitation is required to the rotor of a

generator There are two types of excitation

1 Static excitation system

2 Brushless excitation system

A certain disadvantage in brushless excitation system is the slow response time of

the field in case of fast load changes specified No slip-rings and brushes direct

measurements of the field parameters not possible

To avoid all loses static excitation is used Since it does not have any rotating

parts mechanical loses and windage loses This system has fast response and speed

control While preferring this excitation system there are no limitations for the

redundancy of Thyristor bridge circuits

Static excitation has fast field discharge by resistor and inverter operation direct

measurement of field quantity is possible The meaning of excitation is nothing but

continuous supply of DC current (ie field current) to the rotor to buildup required

output voltage in the stator

Field current is changed with respect to the change of load so the digital

automatic voltage regulator (DAVR) is used to regulate the output voltage according to

the load variations

So we conclude that static excitation system with DAVR is preferred since it is

having excellent dynamic performance and better options for R amp M

The signal processors 25 analog inputoutput modules Each of these processor

systems has a common bus circuit and output and the control lines There is a specific

range of addresses assigned to each assignment Board including the power supply bus

the address lines the two data lines to the input calculates the reactive current (I sin φ)

and the active current ( Icos φ) With these two channel processor Synchronized with

these interrupts (ie with the phase positions of current Ig the field current If and the

synchronous voltage Usyn From the exchange data with the microprocessor card across

the two data lines generator voltage Ug) this processor measures the generator current

Ig and then hardwired connections or multi-conductor cables Binary and analog

inputoutput modules ie for galvanic isolation and adaptation to the electronics level

The most important input interrupts per period to trigger the cycles for processing actual

values in the AUTOMATIC module on the processor bus) for filtering and further

processing

Monitoring each consist of the central microprocessor module and binary and

parameters to the AUTOMATIC channel are the generator voltage Ug the generator

peripheral unit Ug Ig and Usyn are sent to the Interrupt Generator (plug-in peripheral

units (wall-mounted units) peripheral units are used for preprocessing signals from

external measurement circuits power supply units Signals are exchanged among these

processor systems via processed across separate peripheral units for each channel These

processor working on the bus (a house address that can be adjusted using a switch)

systems The AUTOMATIC channel the programmable controls and the Excitation The

actual values measured from AUTOMATIC channel and MANUAL channel are The

AUTOMATIC channel and the MANUAL channel each have their own The digital

voltage regulator is broken down into several autonomous microprocessor The inputs and

outputs of the processor systems are directed across voltage-isolating The Interrupt

Generator also uses the 3-phase Ug signal to generate the 12 themselves contain a limited

number of hardware inputs and outputs with fixed equipment Whenever addresses from

this range are called up the signal processing module can results the processor is then

able to derive further operating parameters such as the load angle the active power etc

The functions of all microprocessor systems other than the programmable controls

have been accomplished in firmware The non-varying standard function modules can be

configured to the design desired for plant-specific purposes using software switches

(KFlags) Thus for example the stored status of a K-Flag determines whether or not a

Limiter is active and whether the de-excitation or the excitation limiters take precedence

Because these K-flags determine the software Scope of Supply for the installation they

cannot be changed permanently via the Micro-Terminal In this way they differ from

such setting data as the values of the parameters for the PID filter of the voltage regulator

or the set-points for the limiters These values can be permanently changed using the

Micro-Terminal Communication is possible with each of the processor systems via the

Micro-Terminal by plugging on the connecting cable In this way signals within the

processor and setting parameters can be viewed analog signals can be issued and the set

parameters can be altered temporarily (F range) or permanently (C range) Unlike the

other processor systems the programmable controls do not include any firmware for

realization of the functions They have been designed so that the designer can adapt and

change their functions easily using the ldquoFunctional Block Programming Language P10

Digital and analog functions can be implemented in practically any degree of complexity

desired using the P10 functional blocks The control variable of the voltage regulator

(AUTOMATIC channel) and the control variable of the field current regulator

(MANUAL channel) are each processed in separate Gate Control Unit and formed into a

chain of pulses at the appropriate firing angle The pulses of the active channel are

directed to the pulse bus via the associated Intermediate Pulse Stage The pulses for each

converter block are amplified sufficiently in Final Pulse Stage to fire the Thyristor

231 General Information

The functions of the automatic voltage regulator AVR are

1 to regulate the generator voltage

2 to regulate the effect of the reactive andor active current on the voltage

3 to limit VoltHz

4 to limit max and min field current

5 to limit inductive stator current

6 to limit capacitive stator current

7 to limit the load angle

8 to stabilize the power system

Block Diagram shows the software structure of AUTOMATIC channel The

generator limiters not provided for the installation in question (optional equipment) are

identified in this overview as ldquoNot Suppliedrdquo The parameter values signal values and

software switches (flags) marked with addresses (hexadecimal numbers) can be viewed

and altered via the Micro-Terminal The values selected are displayed in sec pu Hz

etc and can where necessary be changed directly in these formats The plant-specific

settings of the variables and the flags can be obtained from the Test and Commissioning

Report This block diagram provides information about the important functions and

possible settings of the AUTOMATIC channel For the sake of clarity no detailed

presentation has been given of special functions such as tracking circuits initializations

etc The page heading cross-refers this overview to the various sheets of the schematic

diagram Binary signals are shown in broken lines analog signals in solid lines The

corresponding text designations in the schematic diagram can be used for identification of

the input signals (hardware inputs) The only analog output signal from the automatic

voltage regulator control variable Ucontr is sent via the data bus (CRU bus) to the Gate

Control Unit Most of the binary messages (outputs) from the AVR are of no interest

functionally and they have been omitted for the sake of clarity The basic structure of the

digital voltage regulator and the limiters is simple This is necessary in order that the

behavior of the regulatorslimiters will remain calculable and understandable in all

operating situations and that there will be no problem in adjusting and optimizing them

The central PID filter in the digital voltage regulator defines the dynamic response of the

closed-loop controls both in the voltage regulator mode and after limiters have

intervened The ldquocontrol deviationrdquo at the input to the PID filter is either the control

deviation for voltage the control deviation of a de-excitation limiter (the value

determined by minimum value selection) or the control deviation of an excitation limiter

(the value determined by maximum value selection) Flag F730 (ldquoPRIORrdquo) is used to

determine whether the exciting (Min value) or the de-exciting signal takes precedence on

the minmax value limiter (normally F730 = 1111 ie the de-exciting signal takes

precedence) With the exception of the Minimum Field Current Limiter all other limiters

have variable factoring multipliers of the signal outputs so that they can be adjusted

individually together with the common PID filter which has been optimized for voltage

regulation The setting parameters for this PID filter are as follows

Vo = KR Static amplification

1

Ta = ---- Integration time constant

Tc1

Vp Proportional amplification

1

Tb = ---- Differential time constant

Tc2

Vinfin Amplification of high frequencies

The BODE diagram below shows the assignment of settings in accordance with

DINIEC standards based on a typical example

The PID filter amplifications Vo Vp and Vinfin can be adjusted in pu values But

the ldquoceiling factorrdquo pl+ must be adjusted correctly with parameter F310 if the total

amplification (circuit amplification) of the control circuit is actually to conform to the

pu settings This factor must agree with the ldquoexternalrdquo amplification ie with the

ceiling value of the transformer- converter circuit

Ceiling factor(pl+) = Ufmax Ufo

in which Ufmax = ceiling field voltage

Ufo = no-load field voltage

To attain a suitable response of the AVR when starting excitation

(ldquoEXCITATION ONrdquo) it may be necessary to change the proportional amplification of

the regulator during this phase Vp2 (transiently activated) and Vp1 (permanently

activated) can be adjusted for this purpose For example the value of Vp2 takes effect

immediately once the excitation is switched on and remains effective for a period as set at

F30C Once the period F30C (eg 5 sec) has expired Vp shifts over to Vp1 (becomes

the steady-state Vp) at the rate of change set The standard operating mode for the PID

filter is voltage regulation for which the discrepancy between the voltage set-point and

the current value for generator voltage Ug (the control deviation) is supplied at the input

To compensate for the voltage drop in the block transformer or whenever several

generators are operating to the same distributing bus the generator voltage must be

varied in proportion to the measured generator current (droop influence) To accomplish

this the voltage set-point is varied as a function of the measured reactive current IX

andor active current IR Flag F712 enables the IX droop Flag F710 the IR droop The

desired compensation is set in F282 and F286 respectively Flags F284 and F288 are used

to select whether this droop influence is to increase the voltage or to reduce it

(compensation) Combined influence of the active and reactive currents is attained by

enabling both droops IX and IR Flag F716 activates a so-called ldquoSoft-Startrdquo at the

starting of excitation This ldquoSoft-Startrdquo ensures that the voltage set-point integrates from

0 to 100 within the time set on F290 when the excitation is switched on

(ldquoEXCITATION ONrdquo) A ldquosmoothrdquo excitation of the generator can be achieved in this

way whenever there is no demand for a quick excitation

232 Voltage Set-Point

Various signals and settings control and limit the voltage set-point F270 For

example the values of F254 and F252 define the normal operating range possible for set-

point adjustment (eg 90 110) using external control commands (control room local

operatorrsquos panel superposed control system) The effective set-point adjustment rate is

governed The set-point can be set at the values of F250 and F256 by activating

appropriate control commands for ldquoSETrdquo input Enabling Flag F71A and activating a

binary input prior to switching on the excitation (ldquoEXCITATION OFFrdquo) sets the Ug set-

point at the value of UAUX This makes it possible for example to ensure that the

generator voltage will agree exactly with the network voltage after the voltage build-up

An external value with variable amplification can be added to the Ug set point by

enabling F724 (for example for stability tests)

233 Regulator Tracking in MANUAL Operation

Whenever the AUTOMATIC channel is not in operation (the MANUAL channel

is ON) a follow-up equipment ensures a smooth switch-back to the AUTOMATIC

mode will always be possible To track the voltage set-point is shifted by means of

RAISELOWER pulses from the Gate Control Unit so that control variable Ucontr at the

output from the PID filter is held steady and identical to the control variable Ucontr from

the MANUAL channel Because this tracking must react slowly resultant transient

control deviations resulting from the amplification in the PID filter might cause severe

interference with control variable Ucontr

To prevent this the follow-up equipment intervenes on the regulators mixing

point with a corresponding compensation signal

234 Ugf Limiter

At under frequency the Ugf Limiter reduces the generator voltage so as to

prevent saturation effects in the supply and measuring transformers To adjust this

limiter the max permissible generator voltage at rated frequency is defined and set

When any under-frequency occurs the generator voltage is thus reduced in proportion to

that setting

235 Field Current Maximum Limiter

The Field Current Maximum Limiter is provided to protect the generator rotor

from s occurring in steady-state and transient operation High field currents are normally

the result of a sharp drop in network voltage or of an improper raising of the voltage set-

point by the operating staff The field current is held steady at the value TH1 ie at the

maximum thermal value permissible for the excitation circuit and the rotor In order that

the generator can support the power network with its transient overload capacity during

brief collapses in voltage a temporary switch-over is made to the transient limit MAX1

(a higher setting) When the generator or the converter is operating at a reduced capacity

These limits TH1MAX1 can be switched over to the lower settings TH2MAX2 by

activating the corresponding binary signals The switch-over from the thermal limit

TH12 to the transient limit MAX12 can be configured in one of three ways

a) Depending on the over current with -dUdt ENABLE

Flag programming F418 = any setting desired F41A = 0000

This variant enables the transient value MAX12 whenever a collapse of voltage

in the network is detected The ENABLE time is fixed and can be set The example

below shows the typical behavior of the limiter configured in this way

b) Dependent on the time integral with -dUdt ENABLE

Flag programming F418 = inactive F41A = 1111

This variant likewise enables the transient value only when a collapse of network

voltage has been detected However the switch-back to the thermal limit is not made

dependent upon the time itself but on the calculated time integral intisup2dt of the The setting

on Parameter F414 in spu takes into account the time the rotor needs to cool down ie

the rate of temperature change in the case of intermittent operation The example below

shows how the timing of the switch-back to the thermal limit depends on the present

value for intisup2dt

The time integral is based on the formula

Example The setting of =isup2dt equivalent to Version a (F416) at a constant 16 times the

nominal field current for 10 seconds (with TH12 = 105) is

c) Dependent on the time integral without any preconditions

Flag programming F418 = 1111 F41A = 1111

In this variant the transient becomes available without any prior conditions

(without a -dUdt ENABLE) with the time integral intisup2dt

237 Inductive Stator Current Limiter

The Inductive Stator Current Limiter holds the stator current Ig within permissible

limits while the generator is in the ldquoover-excitedrdquo operating range by reducing the field

current accordingly The setting TH (thermal limit) provides the limit against stationary s

that might occur To take advantage of the generatorrsquos transient overload capacity a

switch-over is made to the higher setting MAX The principle of operation of this switch-

over to the value MAX permissible only transiently is identical to that employed for the

field current limiter (refer to the description above) When the drive output from the

turbine is very high stator current may exceed permissible limits even while inductive

loading of the generator is low In this case if the stator current limiter is not kept from

influencing the field current the control circuit will oscillate back and forth between the

Inductive Stator Current Limiter (de-

exciting) and the Capacitive Stator Current Limiter (exciting)The output signal of that

function then dominates the control variable of the Ig-dependent limiter via a maximum

value selection

238 Capacitive Stator Current Limiter

239 Load Angle Limiter

The Load Angle Limiter prevents the synchronous machine from slipping out of

phase due to slippage of the rotor The load angle δ the difference in phase between the

rotor and the stator rotating field results mainly from the driving torque (active power P)

acting on the generator and the level of rotor current (field current) If the driving torque

remains constant a increase in the field current reduces the load angle δ The current load

angle δ at any moment is obtained from the generator current and generator voltage based

on a simplified model of the generator Whenever this calculated load angle δ exceeds the

preset limit angle the limiter increases the field current until the load angle has dropped

back to its permissible value The quadrature reactance Xq of the generator and the

network reactance Xe during normal operation must be adjusted on the regulator in order

to obtain the load angle δ The graph below shows the Power Chart for a salient-pole

machine with typical limiter characteristics

The purpose of a Power System Stabilizer is to use the generator excitation to

damp electromechanical oscillations between the network and the generator Depending

on the design of the generator and the requirements imposed for network stability its

main function will be either to damp the oscillations originating in the machine or those

from the network A synchronous generator working in a combined power network is in

principle an oscillating structure In order to produce a torque the magnetic field of the

rotor and the stator must form a given angle (referred to as the rotor displacement or load

angle δ) The electrical torque ME increases as the angle δ increases just as with a

torsion spring Because the ME of the generator and the mechanical driving torque MA

from the turbine are in equilibrium during steady-state operation the angle δ remains in a

given position Whenever this state of equilibrium between MA and ME is disturbed the

load angle slips of this rest position and change thereby the electrical torque ME The

torque attempts to restore the load angle to a stationary position Due to the mass inertia

of the turbinegenerator rotor however this can only take place aperiodically It does so

in the form of more or less effectively damped oscillations (again similar to the effect of

mass inertia on a torsion spring) In order to damp the oscillations there must be a

damping torque produced depending not on the electrical torque ME associated with the

angle but on the difference in frequency (Df) between the rotor and the stator rotating

field ie on the slippage This torque is produced mainly by the so-called damper

winding in the rotor but the dimensioning of this is subject to limits imposed by

considerations of design and economy Some further action is therefore needed to

increase the damping effect The following drastically simplified formula shows the

parameters upon which the amount of active power PE supplied by the generator

depends

PE = active power

It can be seen from the above relationship that the active power that the generator

transfers depends not only on the load angle δ but also on the field current If That means

that a transient change can be made in the active power PE and with that in the effective

electrical torque ME by varying the field current The principle of operation of the DVR

Power System Stabilizer becomes clear from a consideration of the oscillations in power

output and frequency (ΔPE Δf) and the vector diagram If it is assumed that oscillations

in the network frequency generates load oscillations with the mass inertia of the rotor

then the active load of the generator (eg MW-measured) is influenced with a sinusoidal

value -ΔPE (ME-MA = -ΔPE) By inversion of -ΔPE one obtains the fluctuation in

power provided by the rotor +ΔPE As is known the slip signal Δf follows +ΔPE with a

phase delayed by 90deg The +ME produced by the periodic changes in the load angle δ is

in phase with +ΔPE A good damping is attained if ME is varied in phase with the slip

Δf However this signal must also be advanced somewhat to compensate for the time

constants in the excitation circuit and the generator

As mentioned above the electrical torque ME can be influenced by varying the

field current To accomplish this a suitable control signal referred to as variable

disturbance compensation must be imposed upon the voltage set-point or the converter

control variable Ucontr As can be seen from the vector diagram by applying proper

weighting factors (K1 K2) and then adding together the signals -ΔPE and Δf an overall

stabilization signal can be produced that rotates in advance of the Df signal by any angle

desired between 0deg and 90deg Because the amplitude of -DPE remains proportional to the

amplitude of Δf a constant angle in advance of Δf results for the compensation of the

time constants referred to above The optimum weighting factors K1 and K2 for a

synchronous generator working to a power network depend on its operating point at any

moment and the external reactance of the network Normally the selection of a

compromise setting is good enough to attain stability in all operating points and for all

external reactance For special demands these settings must be parameterized as a

function of the external reactance (which means optional equipment Xe-Identification)

The Power System Stabilizer PSS is a section of the AVR computer program and is

processed once per network cycle The voltage at the generator terminals and the

generator current are measured in order to define the signals ΔPE and Δf The calculated

signals for _P_ (=PE) and Δf are then sent across DC filters ldquoDrdquo (real differentiators) that

transmit only the dynamic portion of the signals The ΔPE and Δf signals obtained in this

way are then weighted (multiplied by) with the factors K1 and K2 and sent to the

summing point of the voltage regulator

The PSS stabilization signal is imposed on the automatic voltage regulator only if

the following prerequisites are met

bull Generator on line

bull Generator power output gt the value F338

bull Generator voltage in a range between F33C and F33A

The stabilization signal is limited at the output from the PSS to the lower and

upper limits Flag defines whether the stabilization signal is introduced before or after the

PID filter (usually before the filter) Because the PID filter as noted above already takes

the ceiling factor Vp1 into account the PSS signal needs to be multiplied by Vp1 if it is

added to the voltage regulator following the PID filter (divider at the input to the

minmax limiter) This precaution prevents the DC filter ldquoDrdquo in the P-channel from

producing an unnecessary ldquostabilizationrdquo effect in the case of rapid changes in turbine

load As an alternative for the AVRrsquos Power System Stabilizer a stabilization signal from

an outside system can be imposed by activating the binary input ldquoPSS-SIGNEXTrdquo Flag

F340 can be used to select between an analog and a 12-bit signal and F33E to select the

polarity desired for that signal

24 The MANUAL Channel

241 Summary

The MANUAL channel (Channel 2) has been built as a simple field current

regulator

without additional limiters Its main function is to maintain the excitation of the generator

even if the AUTOMATIC channel becomes non-operational The MANUAL channel

also performs valuable service for purposes of testing commissioning and preventive

maintenance Its measurements regulator generation of firing pulses and power supply

are physically separate from those on the AUTOMATIC channel

242 Principle of Operation

All the functions of the MANUAL channel including the generation of firing

pulses have been implemented in a single electronic module the Gate Control Unit The

control variable Ucontr of voltage regulator is used as the reference value for generating

firing pulses on the principle known as ldquoramp controlrdquo (Comparison of Ucontr with

Usynsynchronous sawtooth signal) For further processing in the UN 0096 Intermediate

Pulse Stage the Gate Control Unit supplies six firing pulses at its output whose phase

position with respect to the synchronous voltage Usyn is in accordance with control

variable Ucontr An internal linearization ensures that the field voltage produced via the

firing pulses remains proportional to the control variable Ucontr throughout the entire

range As a result the circuit amplification of the control remains constant over the entire

range Whenever excitation is switched ON the set-point for Generator Voltage is set

automatically at the preset - ref Value This provision ensures that the generator voltage

always attains approximately its nominal value after the field flashing The Gate Control

Unit can be refunctioned ( by pre-selection with a switch ) for purposes of testing to act

as a purely firing pulse control In this case the control variable Ucontr is adjusted

directly using the RAISELOWER push buttons on the front of the module In this way

for example the relationship between the phase position of the firing pulses and the

control variable Ucontr can be checked easily

CHAPTER 3

PULSE SECTION

31 Pulse Generation and Amplification

The Gate Control Units of both AUTOMATIC channel and MANUAL channel

each supply six firing pulses for operating the 6-pulse thyristor bridges The low-power

pulse signals from these Gate Control Units are then amplified in the Intermediate Pulse

Stage galvanically isolated and then sent to the common pulse bus On the output end

the Intermediate Pulse Stage of the non-active channel) is always blocked The Gate

Control Units generate the pulses based on microprocessor control The reference voltage

used for the firing pulse phase location is the output voltage from the excitation

transformer (Usyn1 Usyn2) The commutation spikes of the synchronous voltage caused

by the converter are calculated prior to use of the voltage as a reference value and are

deliberately filtered out The lower limit for the firing pulses (double pulses) which are

offset from one another by 60deg is defined by the limit rectifier position (αmin) and the

upper limit by the limit inverter position (αmax) for the firing angle αmin and αmax can

be adjusted on the Gate Control Units using BCD (Binary Coded Decimal) switches

αmin ensures that the firing pulses will not be issued (premature firing) until there is

sufficient positive phase voltage on the thyristor involved αmax prevents a dangerous

ldquotippingrdquo of the thyristor bridge into the rectifier mode if the firing angle α is too large

(ldquolate firingrdquo) The critical factors determining αmax are the overlap time uumlmax (max

commutation time) and the ldquorecovery timerdquo of the thyristors (αmax lt 180deg - uumlmax - γ )

An external control signal can force the firing pulses into their inverter limit position

Other binary inputs can block or direct the firing pulses of the Gate Control Units so as to

produce freewheeling on the thyristor bridge During freewheeling the firing pulses for

the thyristor pair R and S are blocked and the pulse signals T+T- are engaged with

chains of pulses Both Gate Control Units (for the MANUAL amp AUTOMATIC channels)

contain a field current monitor that blocks the firing pulses immediately whenever the

current exceeds a preset threshold level In this case the field circuit-breaker is also

tripped via an output contact The purpose of these provisions is to prevent damage to

thyristors and thyristor fuses in case of a slip-ring short-circuit or to keep any damage

that does occur to a minimum The pulse signals are galvanically separated at the outputs

from the Intermediate Pulse Stage (with pulse transmitters) and are then directed to the

common pulse bus This transmission of the pulse signals to the pulse bus via passive

transmitters ensures a high degree of active channel autonomy Practically no possible

malfunctions on the inactive channel (including for example sustained pulses) affect the

active channel

32 Pulse Monitoring

The ldquoPulse Busrdquo and the pulse signals of the AUTOMATIC channel are

monitored This monitoring device consists of potential isolating stages and the common

monitor If the pulse monitoring of the ldquoPulse Busrdquo responds a switch-over is made to

MANUAL channel The function of the potential isolating stages is to couple the pulse

monitoring device to the pulse circuits without any feedback effect The pulse monitoring

checks the six pulse lines for the following malfunctions continuous or periodic failure

of one or more pulses Periodic occurrence of synchronous or asynchronous false pulses

Continuous pulses the pulse monitoring device can be tested while the machine is in

operation

CHAPTER 4

CONVERTER

Thyristor

The term thyristor usually refers to a family of four layer solid state device having

turn on characteristics that can be externally controlled by either current or voltage They

are also referred to as breakdown device because their working depends on avalanche

breakdown Thyristors have only two stages OFF and ON Thyristors have a similar

function to Uni-junctions they act as switches Thyristors use current flow as a switch

Thyristors have three states

1 Reverse blocking mode mdash Voltage is applied in the direction that would be

blocked by a diode

2 Forward blocking mode mdash Voltage is applied in the direction that would cause

a diode to conduct but the thyristor has not yet been triggered into conduction

3 Forward conducting mode mdash The thyristor has been triggered into conduction

and will remain conducting until the forward current drops below a threshold value

known as the holding current Converter is a semiconductor device which converts ac

input voltage into a constant dc output voltage In present excitation system three phase

fully controlled thyristor converter is used

Because of the following advantages thyristor converters are used

a) Thyristors are used for high power applications ie up to 10Kv3500A1KHz

b) Having high reliability and low losses

c) Uni-directional device like diode

d) Itrsquos operation as a rectifier which are low resistance in forward conduction

mode and high resistance in reverse conduction mode

PROTECTION OF THYRISTORS

For reliable operation of a thyristor demands that its specified ratings are not

exceeded When Subjected to or over voltages During the turn - on of SCR didt

prohibitively large False triggering of SCR by high value of dvdt andSpurious signals

between gate and cathode may leads to unwanted turn ndash on

DIDT AND PROTECTION

When thyristor starts conducting in forward conduction mode and is turned on by

gate pulse The anode current increases rapidly whole area of the gate to Cathode

junction then hot spots will be formed near the gate connection this locality of heating

destroys the thyristor Thyristor thermal time is constant The causes due to faults and

short circuits or surge currents Electronic crowbar protection is used against the over

voltages The rate rise of anode current must be kept at the time of turn on below the

rated or specified limiting value The didt value maintained below limited value by using

a inductor also called ldquodidt inductorrdquo in series with anode circuit The locality of heating

is avoided by applying gate current but not greater the maximum gate current

DVDT AND OVER VOLTAGE PROTECTION

With forward voltage across the anode and cathode of a thyristor the two outer

junctions are forward biased but the inner junction is reverse biased This reverse biased

junction J2 has the characteristics of a capacitor due to charges existing across the

junction In other words space-charges exist in the depletion region around junction J2

and therefore junction J2 behaves like a capacitance If the entire anode to cathode

forward voltage Va appears across J2 junction and the charge is denoted by Q then a

charging current i given by Eq (46) follows

i = dQdt =d(Cj Va )dt

= Cj (d Va dt) + Va(d Cj dt) helliphelliphelliphellip(46 a)

As Cj the capacitance of junction J2 is almost constant the current is given by

i = Cj (d Va dt) helliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip(46 b)

If the rate of rise of forward voltage dVadt is high the charging current i will be

more This charging current plays the role of gate current and turns on the SCR even

when gate signal is zero Such phenomena of turning-on a thyristor called dvdt turn-on

must be avoided as it leads to false operation of the thyristor circuit

For controllable operation of the thyristor the rate of rise of forward anode to

cathode voltage dVadt must be kept below the specified rated limit Typical values of

dvdt are 20 ndash 500 Vμsec False turn-on of a thyristor by large dvdt can be prevented by

using a snubber circuit in parallel with the device thyristor are very sensitive for over

voltage than the semiconductor devices

Over voltage transients are perhaps the main cause of thyristor failure

In thyristor there are mainly two types

1 Internal over voltages

Due to the commutation of the thyristors large voltages are generated internally

Because of the series inductance of the SCR circuit the large transient voltages L didt

produced This voltage several times the break over voltage of the device then thyristor

destroys permanently

2 External over voltages

External over voltages are caused due to the interruptions of current flow in an

inductive circuit and also due to the lightening strokes on the lines feeding the thyristor

system For the reliable operation of thyristor the over voltages must be suppressed by

adopting suitable techniques

Suppression of over voltages

The RC circuit called snubber circuit is connected across the device to protect In

order to keep the protective components to a minimum the thyristors are chosen with

their peak voltages ratings are 25 to 3 times of the normal peak working voltage

ldquoselenium thyrector diodes metal oxide varistors or avalanche diode suppressers are

commonly employed for protecting the thyristor circuit against the over voltages

Gate protection

Gate circuit should also be protected against the over voltages and surges Over

voltage at gate circuit can cause false triggering of the SCR may rises the junction

temperature behind specified limit leading to its damage Protection against over

voltage can be achieved by connecting a ZD across the gate circuit and a resister is

connected in series with gate circuit to protect against the s A capacitor and resister are

connected across gate to cathode to by pass the noise

41 Final Pulse Stages

The Final Pulse Stages adapt the output pulses from the Intermediate Pulse Stage

(pulses on the pulse bus) to the gate currents needed for the thyristors Each thyristor

bridge is equipped with its own Final Pulse Stage Each Final Pulse Stages is provided

with a power supply module The amplified output pulses from the Final Pulse Stages

start as a short strong steep pulse with an amplitude approx 2frac12 times that of the main

pulse This initial pulse edge assures proper firing of the thyristors being triggered

Subsequently the weaker part of main pulse keeps firing conditions steady As already

mentioned the Final Pulse Stages and their associated thyristor bridges form single units

All six pulse outputs from a Final Pulse Stage can be blocked by an external control

signal so that all thyristors in the associated thyristor bridge will block the current A

blocking of the pulses is initiated whenever there is a malfunction in the associated

thyristor bridge

42 Converter Power Section

The thyristor converter consists of three independent parallel rectifier blocks TY1

to TY3 which are all in service Even if one block fails the remaining blocks take over

automatically the full design current of the excitation circuit During normal operation

(with ideal current share) and all three bridges in operation each of these blocks has to

carry only (n-2)n (ie33)of its design current If 2 thyristor bridges fail the excitation

is limited Only when all three bridges fail the excitation is switched off Each thyristor

bridge arm is equipped with current flow monitoring CTrsquos Failure of conduction in any

arm is identified by a Current flow monitoring module

43 Converter Cooling

A cooling system is needed to dissipate heat losses in the converter blocks and

electronics Each converter block has therefore been equipped with a fan supplied with

power from the converterrsquos primary voltage (via transformer ndashT8 in field flashing

cubicle) The fans are protected with motor protection circuit breakers An air flow

monitoring unit is provided for monitoring the air flow through the thyristor bridge If a

circuit breaker failure is detected or if the air flow monitor drops off at one of the

thyristor bridges the bridge involved is immediately set out of operation by blocking its

firing pulses

44 Thyristor Converter Monitoring

A thyristor bridge in which defects occur that could threaten the safety of

operation or cause secondary damage is switched off automatically ie its firing pulses

are blocked This happens whenever A thyristor fuse is blown The fuses are monitored

individually with micro switches The Final Pulse Stage fails which is detected by

internal monitors (supply voltage sustained pulse short-circuit on the output end) The

power supply to the fan fails fan air flow as monitored by the Air flow monitor fails or

is insufficient Isolator on ACDC side is open

CHAPTER 5

Field Current Circuit Excitation Transformer

51 Field Circuit Breaker

The circuit-breaker in the field circuit is used to isolate the field circuit from the

converter It is capable of switching off the synchronous machine from full load under the

maximum conditions of a 3-phase short-circuit In addition to its main contacts the field

circuit-breaker also has a de-excitation contact with which the field energy stored in the

field can be dissipated across the de-excitation resistor The de-excitation contact closes

shortly before the main contacts open so as to ensure proper commutation of the field

current from the main contacts to the de-excitation contact when the breaker is switched

off The field circuit-breaker is switched on by electromagnetic force and is kept switched

on by a mechanical latch When the latch is released by a trip coil the circuit-breaker

opens The circuit-breaker also has auxiliary contacts that report its status

52 Field Flashing

In shunt supplied excitation circuits (excitation transformer connected to the

generator terminals) the generator does not have enough remnant voltage for a generator

voltage build-up via the converter In this case a field flashing circuit is provided It

consists of the field flashing contactor the diode bridge and a transformer used to adapt

the auxiliary input voltage to the voltage needed for field flashing when power is

supplied from the auxiliaries network

Fig Field Flashing

Because the field flashing contactor is not able to switch off the energy stored in

the field the control ensures that the contactor can only reopen if the field circuit breaker

has already been opened (generating the TRIP order) or in a normal field flashing

sequence when the converter has taken over the field current Field flashing occurs in the

following stages

1048729The excitation is switched on closing the field flashing contactor ( Field

Circuit Breaker is already closed )

1048729The start-up excitation current flows through the rotor driving the generator

voltage up to approx 15 U

1048729After about 10 U the firing pulses to the converter are released and it begins

to excite the generator to its rated voltage

1048729After about 30 U the field flashing contactor opens (with no current since

the converter is now supplying the current)

The diode bridge at the input to the field flashing contactor prevents a feed-back

from the converter to the source of field flashing while the contactor is still closed

53 De-excitation

When malfunctions occur the stored field energy must be dissipated as quickly

and safely as possible to protect the generator This is done by the converter the field

circuit-breaker and the de-excitation (discharge) resistor

De-excitation (with opening of the field circuit-breaker) takes place in the following

stages

1048729The converter drives to its inverter limit position (negative ceiling voltage)

recovers a portion of the field energy into the network A trip command is given to the

field circuit breaker

1048729The de-excitation contact closes diverting the field voltage to the de-excitation

resistor

1048729Then immediately the main contacts open building voltage The field voltage

commutates to the de-excitation resistor

1048729The current diminishes at a given time constant TE

(With linear resistance TE = Lf (Rf + Re))

Due to the reversal of the field voltage by the converter the field current

commutates from the main contacts of the field circuit-breaker to the de-excitation

resistor in a very early phase This reversal of the field voltage prevents burn-off on the

main contacts and provides effective protection for the field circuit-breaker Depending

on the operating policy an operational shut-down of the excitation can also be effected

with the field circuit-breaker closed This method is useful mainly when the excitation is

switched on and off frequently In this case the converter is merely driven into the

inverter limit position so that the field energy is recovered into the network The

converter then blocks since it is supplying positive current only

54 Excitation Transformer

The excitation transformer matches the generator voltage to the field voltage

(required ceiling voltage) It also serves as a commutation reactance for the thyristor

converter and as a potential isolator between the network and the excitation circuit In

addition the transformer functions as a current limiter in that it makes it possible to keep

any short circuits in the excitation circuit under better control The excitation transformer

is equipped with temperature monitoring probes which set off an alarm when the

temperature exceeds a first max limit and then trips the excitation if the temperature

continues rising to a second (higher)limit

CHAPTER 6

Monitoring and Protection

61 Excitation Monitoring

611 General Information

The main goal of Excitation Monitoring is to make optimum use of the

redundancies provided in the excitation system and to give alarm whenever a malfunction

makes these redundancies unavailable The field current is monitored to see that it does

not exceed a maximum level and if necessary a switch-over to the MANUAL channel is

initiated In addition the criterion for switching off the field flashing is generated The

excitation Monitoring consists of an autonomous processor system

612 over current Alarms

In the Excitation Monitoring the limits for are set at higher levels than the

settings on the Field Current Maximum Limiter Whenever the current exceeds 110 of

the nominal field current contact R1 and the binary output associated with it are

activated immediately If field current remains gt 110 then after a preset inverse-time

has lapsed relay R2 and - after a further delay - relay R0 and the binary outputs

associated with them are activated Parameters match the measurements for If1 and If2 to

the nominal value for field current so that the internal values can be processed and read as

pu values It can be used to falsify the actual value of the field current If (to raise it) so

as to cause a response from the alarm limits for purposes of testing The processed If

signal is always taken from on the active channel (CHANNEL I OR CHANNEL II) As

long as the field current If is above the threshold value 11 Ifn its peak value is

measured This is stored (until RESET) and can be read at any time on the Micro-

Terminal Once the value of If exceeds 11 Ifn integration of this value starts Whenever

the integrated time-current value (intisup2dt) exceeds the preselected reference value the

alarm OVER CURRENT INVERSE-TIME is set off and a command is simultaneously

issued to switch over to the stand by AUTO channel Software switch F758 enables the

three over current alarm functions (R0 R1 R2) and selects one of three possible inverse-

time curves T1 T2 or T3 Within the characteristic curve (T1 T2 T3) selected the

desired limit curve for response is set using the factor F216

613 Switch-Off Criterion for Field Flashing

The Excitation Monitoring supplies the criterion for switching off the field

flashing Whether this criterion is activated based on the actual value for generator voltage

Ug or for field current If or both depends on the settings of the two threshold values

F200F202 (0 setting means that the output is always ldquological 1rdquo) The measurements

Ug12 and If12 are switched over depending on the present status of the channels

(Channel 1 or Channel 2 ON) Whenever Flag F750 is not activated the binary output is

fixed at ldquological 1rdquo

614 Storage of Alarm Status

The outputs of the over current alarms (R1 R2 R0) and the messages NO

FAILURE MONITORING PARAMETERS CHANGED are stored messages can be

erased by activating the input ldquoGENERAL RESETrdquo or by using the RESET button on the

front of the module Erasure with the input ldquoGENERAL RESETrdquo is effective only if the

situation causing the alarm or the malfunction is no longer present Whenever the self-

diagnosis equipment in the processor detects a malfunction the output NO FAILURE of

MONITORING is set at ldquological 0rdquo (= alarm) The alarm ldquoPARAMETERS CHANGEDrdquo

is activated whenever parameters or settings of software switches have been changed via

the Micro-Terminal

615 Actual Value Monitoring

The actual values for generator voltage Ug synchronous voltage Usyn and field

current If are monitored for malfunctions This monitoring is active regardless of whether

or not the generator is in operation Essentially when the generator is in operation the

measurements are monitored by comparing the signals (the smaller signal reading is

detected as incorrect) When the generator is not in operation the measured data are

monitored for extreme values The percentage of deviation permissible in the

measurement signals being compared

(Ug1ampUsyn1 Ug1ampUg2 Ug2ampUsyn2 If1ampIf2) is defined by parameters F208 and

F20AIf the excitation transformer is being supplied from an auxiliary power source (no

shunt operation) the values of Ug and Usyn will be different in some operational

conditions

In that case Flag F75A can be used to deactivate comparative measurements Ug1 Usyn1

and Ug2 Usyn2 The ldquoprimary monitoringrdquo compares Ug1 with Ug2 and generates the

messages that Channel 1 or Channel 2 has suffered a malfunction Whenever Ug1 lt Ug2

and the binary message from CH1 reports no malfunction a malfunction on

Measurement Channel 1 is reported (Ug1Usyn1 FAILURE) A similar malfunction is

also present whenever the binary message CH1 DISTURBANCE is reported and a

discrepancy is detected between Ug1ampUg2 The generation of the alarm ldquoUg2 Usyn2

FAILURErdquo is analogous to that for Channel 1 The ldquosecondary monitoringrdquo compares

Ug1 with Usyn1 amp Ug2 with Usyn2 This is enabled whenever the binary message of the

comparison channel reports a malfunction or whenever both binary messages report no

malfunction - but both secondary monitors report a malfunction As long as the secondary

monitoring is blocked the differences Ug1neUsyn1 or Ug2neUsyn2 trigger malfunction

signals for the measurement channel involved (suspicion that there is a corresponding

error in Usyn) The measurement channel malfunctions are enabled operationally

whenever after excitation has been switched on generator voltage Ug exceeds the value

set on F204 The voltages Ug1 and Ug2 are checked 16 seconds after the excitation is

switched off to see that they do not exceed the limit value F210 that applies to both of

them At the same time g1Ug2Usyn1Usyn2 are checked for extreme values (gt or lt

the operating range) Monitoring for extreme values is likewise enabled during normal

operation ( Excitation ON and Ug gt F204 ) Flag F754 is used to enable or block the

malfunction signals to the binary outputs Basically the monitoring of the actual values

for If1If2 functions like that of the Ug1Ug2 monitoring

62 Excitation Protection

621 General Information

The Excitation Protection switches off the excitation (and de-excites the machine

rapidly) whenever a danger arises that threatens the excitation transformer the converter

or the generator Generally limiter or monitoring functions precede the emergency trips

and these normally respond before the Excitation Protection must initiate a trip

Protective trip commands are issued directly to the field circuit-breaker from potential

free contacts of the board via the trip relays They are directed redundantly to the

operative field circuit-breaker ldquoOFFrdquo command

622 Protection against Excitation Transformer Overheating

This equipment monitors the excitation transformer for overheating in the

windings that could result from over current short-circuits or inadequate cooling The

monitoring uses temperature monitoring modules in conjunction with temperature

sensors built into the windings Normally the temperature is monitored in two stages the

first stage sets off an alarm the second causes a trip of the excitation

623 Rotor Over voltage Protection

Malfunctions in the generator circuit (eg terminal short-circuit failed

synchronization asynchronous operation) cause induced negative field currents that

produce high voltages in the field circuit These must be restricted to a level with a

sufficient safety margin below the insulation capacity of the field winding (test voltage)

and also below the peak blocking voltage of the converter thyristors The crow bar

employs spark gap elements to detect over voltages in the field circuit Whenever they

respond the associated thyristors are fired immediately switching the de-excitation

resistor parallel to the field The de-excitation current generated thereby initiates an

excitation trip via a supervision circuit causing an immediate opening of the field circuit-

breaker The malfunction isets off an alarm and an internal malfunction is indicated at the

cubicle

TEST VALUES OBTAINED WHEN EXCITATION IS RAISED

TEST

SNO PARTICULARS ACTUAL VALUE OBTAINED VALUE

1 VREF 996 100

2 VACT 997 1003

3 IFACT 735 765

4 IGACT 703 707

5 ACTIVE

POWER

703 705

6 REACTIVE

POWER

101 142

7 POWER

FACTOR

099 IND 098 IND

8 ACTIVE

CURRENT(IR)

705 703

9 REACTIVE

CURRENT(IX)

102 138

10 POWER

ANGLE

566 546

11 FIRING

ANGLE

640 632

VALUES OBTAINED WHEN EXCITATION IS LOWERED

SNO PARTICULARS ACTUAL

VALUE

OBTAINED VALUE

1 VREF 100 997

2 VACT 100 997

3 IFACT 787 761

4 IGACT 839 837

5 ACTIVE

POWER

839 830

6 REACTIVE

POWER

155 110

7 POWER

FACTOR

098 IND 099 IND

8 ACTIVE

CURRENT(IR)

835 830

9 REACTIVE

CURRENT(IX)

140 94

10 POWER

ANGLE

613 DEG 631 DEG

11 FIRING

ANGLE

601 DEG 599 DEG

CONCLUSION

For generating the EMF in stator winding excitation is required to the rotor of a

generator There are two types of excitation

1 Static excitation system

2 Brushless excitation system

A certain disadvantage in brushless excitation system is the slow response time of

the field in case of fast load changes specified No slip-rings and brushes direct

measurements of the field parameters not possible

To avoid all loses static excitation is used Since it does not have any rotating

parts mechanical loses and windage loses This system has fast response and speed

control While preferring this excitation system there are no limitations for the

redundancy of Thyristor bridge circuits

Static excitation has fast field discharge by resistor and inverter operation direct

measurement of field quantity is possible The meaning of excitation is nothing but

continuous supply of DC current (ie field current) to the rotor to buildup required

output voltage in the stator

Field current is changed with respect to the change of load so the digital

automatic voltage regulator (DAVR) is used to regulate the output voltage according to

the load variations

So we conclude that static excitation system with DAVR is preferred since it is

having excellent dynamic performance and better options for R amp M

systems The AUTOMATIC channel the programmable controls and the Excitation The

actual values measured from AUTOMATIC channel and MANUAL channel are The

AUTOMATIC channel and the MANUAL channel each have their own The digital

voltage regulator is broken down into several autonomous microprocessor The inputs and

outputs of the processor systems are directed across voltage-isolating The Interrupt

Generator also uses the 3-phase Ug signal to generate the 12 themselves contain a limited

number of hardware inputs and outputs with fixed equipment Whenever addresses from

this range are called up the signal processing module can results the processor is then

able to derive further operating parameters such as the load angle the active power etc

The functions of all microprocessor systems other than the programmable controls

have been accomplished in firmware The non-varying standard function modules can be

configured to the design desired for plant-specific purposes using software switches

(KFlags) Thus for example the stored status of a K-Flag determines whether or not a

Limiter is active and whether the de-excitation or the excitation limiters take precedence

Because these K-flags determine the software Scope of Supply for the installation they

cannot be changed permanently via the Micro-Terminal In this way they differ from

such setting data as the values of the parameters for the PID filter of the voltage regulator

or the set-points for the limiters These values can be permanently changed using the

Micro-Terminal Communication is possible with each of the processor systems via the

Micro-Terminal by plugging on the connecting cable In this way signals within the

processor and setting parameters can be viewed analog signals can be issued and the set

parameters can be altered temporarily (F range) or permanently (C range) Unlike the

other processor systems the programmable controls do not include any firmware for

realization of the functions They have been designed so that the designer can adapt and

change their functions easily using the ldquoFunctional Block Programming Language P10

Digital and analog functions can be implemented in practically any degree of complexity

desired using the P10 functional blocks The control variable of the voltage regulator

(AUTOMATIC channel) and the control variable of the field current regulator

(MANUAL channel) are each processed in separate Gate Control Unit and formed into a

chain of pulses at the appropriate firing angle The pulses of the active channel are

directed to the pulse bus via the associated Intermediate Pulse Stage The pulses for each

converter block are amplified sufficiently in Final Pulse Stage to fire the Thyristor

231 General Information

The functions of the automatic voltage regulator AVR are

1 to regulate the generator voltage

2 to regulate the effect of the reactive andor active current on the voltage

3 to limit VoltHz

4 to limit max and min field current

5 to limit inductive stator current

6 to limit capacitive stator current

7 to limit the load angle

8 to stabilize the power system

Block Diagram shows the software structure of AUTOMATIC channel The

generator limiters not provided for the installation in question (optional equipment) are

identified in this overview as ldquoNot Suppliedrdquo The parameter values signal values and

software switches (flags) marked with addresses (hexadecimal numbers) can be viewed

and altered via the Micro-Terminal The values selected are displayed in sec pu Hz

etc and can where necessary be changed directly in these formats The plant-specific

settings of the variables and the flags can be obtained from the Test and Commissioning

Report This block diagram provides information about the important functions and

possible settings of the AUTOMATIC channel For the sake of clarity no detailed

presentation has been given of special functions such as tracking circuits initializations

etc The page heading cross-refers this overview to the various sheets of the schematic

diagram Binary signals are shown in broken lines analog signals in solid lines The

corresponding text designations in the schematic diagram can be used for identification of

the input signals (hardware inputs) The only analog output signal from the automatic

voltage regulator control variable Ucontr is sent via the data bus (CRU bus) to the Gate

Control Unit Most of the binary messages (outputs) from the AVR are of no interest

functionally and they have been omitted for the sake of clarity The basic structure of the

digital voltage regulator and the limiters is simple This is necessary in order that the

behavior of the regulatorslimiters will remain calculable and understandable in all

operating situations and that there will be no problem in adjusting and optimizing them

The central PID filter in the digital voltage regulator defines the dynamic response of the

closed-loop controls both in the voltage regulator mode and after limiters have

intervened The ldquocontrol deviationrdquo at the input to the PID filter is either the control

deviation for voltage the control deviation of a de-excitation limiter (the value

determined by minimum value selection) or the control deviation of an excitation limiter

(the value determined by maximum value selection) Flag F730 (ldquoPRIORrdquo) is used to

determine whether the exciting (Min value) or the de-exciting signal takes precedence on

the minmax value limiter (normally F730 = 1111 ie the de-exciting signal takes

precedence) With the exception of the Minimum Field Current Limiter all other limiters

have variable factoring multipliers of the signal outputs so that they can be adjusted

individually together with the common PID filter which has been optimized for voltage

regulation The setting parameters for this PID filter are as follows

Vo = KR Static amplification

1

Ta = ---- Integration time constant

Tc1

Vp Proportional amplification

1

Tb = ---- Differential time constant

Tc2

Vinfin Amplification of high frequencies

The BODE diagram below shows the assignment of settings in accordance with

DINIEC standards based on a typical example

The PID filter amplifications Vo Vp and Vinfin can be adjusted in pu values But

the ldquoceiling factorrdquo pl+ must be adjusted correctly with parameter F310 if the total

amplification (circuit amplification) of the control circuit is actually to conform to the

pu settings This factor must agree with the ldquoexternalrdquo amplification ie with the

ceiling value of the transformer- converter circuit

Ceiling factor(pl+) = Ufmax Ufo

in which Ufmax = ceiling field voltage

Ufo = no-load field voltage

To attain a suitable response of the AVR when starting excitation

(ldquoEXCITATION ONrdquo) it may be necessary to change the proportional amplification of

the regulator during this phase Vp2 (transiently activated) and Vp1 (permanently

activated) can be adjusted for this purpose For example the value of Vp2 takes effect

immediately once the excitation is switched on and remains effective for a period as set at

F30C Once the period F30C (eg 5 sec) has expired Vp shifts over to Vp1 (becomes

the steady-state Vp) at the rate of change set The standard operating mode for the PID

filter is voltage regulation for which the discrepancy between the voltage set-point and

the current value for generator voltage Ug (the control deviation) is supplied at the input

To compensate for the voltage drop in the block transformer or whenever several

generators are operating to the same distributing bus the generator voltage must be

varied in proportion to the measured generator current (droop influence) To accomplish

this the voltage set-point is varied as a function of the measured reactive current IX

andor active current IR Flag F712 enables the IX droop Flag F710 the IR droop The

desired compensation is set in F282 and F286 respectively Flags F284 and F288 are used

to select whether this droop influence is to increase the voltage or to reduce it

(compensation) Combined influence of the active and reactive currents is attained by

enabling both droops IX and IR Flag F716 activates a so-called ldquoSoft-Startrdquo at the

starting of excitation This ldquoSoft-Startrdquo ensures that the voltage set-point integrates from

0 to 100 within the time set on F290 when the excitation is switched on

(ldquoEXCITATION ONrdquo) A ldquosmoothrdquo excitation of the generator can be achieved in this

way whenever there is no demand for a quick excitation

232 Voltage Set-Point

Various signals and settings control and limit the voltage set-point F270 For

example the values of F254 and F252 define the normal operating range possible for set-

point adjustment (eg 90 110) using external control commands (control room local

operatorrsquos panel superposed control system) The effective set-point adjustment rate is

governed The set-point can be set at the values of F250 and F256 by activating

appropriate control commands for ldquoSETrdquo input Enabling Flag F71A and activating a

binary input prior to switching on the excitation (ldquoEXCITATION OFFrdquo) sets the Ug set-

point at the value of UAUX This makes it possible for example to ensure that the

generator voltage will agree exactly with the network voltage after the voltage build-up

An external value with variable amplification can be added to the Ug set point by

enabling F724 (for example for stability tests)

233 Regulator Tracking in MANUAL Operation

Whenever the AUTOMATIC channel is not in operation (the MANUAL channel

is ON) a follow-up equipment ensures a smooth switch-back to the AUTOMATIC

mode will always be possible To track the voltage set-point is shifted by means of

RAISELOWER pulses from the Gate Control Unit so that control variable Ucontr at the

output from the PID filter is held steady and identical to the control variable Ucontr from

the MANUAL channel Because this tracking must react slowly resultant transient

control deviations resulting from the amplification in the PID filter might cause severe

interference with control variable Ucontr

To prevent this the follow-up equipment intervenes on the regulators mixing

point with a corresponding compensation signal

234 Ugf Limiter

At under frequency the Ugf Limiter reduces the generator voltage so as to

prevent saturation effects in the supply and measuring transformers To adjust this

limiter the max permissible generator voltage at rated frequency is defined and set

When any under-frequency occurs the generator voltage is thus reduced in proportion to

that setting

235 Field Current Maximum Limiter

The Field Current Maximum Limiter is provided to protect the generator rotor

from s occurring in steady-state and transient operation High field currents are normally

the result of a sharp drop in network voltage or of an improper raising of the voltage set-

point by the operating staff The field current is held steady at the value TH1 ie at the

maximum thermal value permissible for the excitation circuit and the rotor In order that

the generator can support the power network with its transient overload capacity during

brief collapses in voltage a temporary switch-over is made to the transient limit MAX1

(a higher setting) When the generator or the converter is operating at a reduced capacity

These limits TH1MAX1 can be switched over to the lower settings TH2MAX2 by

activating the corresponding binary signals The switch-over from the thermal limit

TH12 to the transient limit MAX12 can be configured in one of three ways

a) Depending on the over current with -dUdt ENABLE

Flag programming F418 = any setting desired F41A = 0000

This variant enables the transient value MAX12 whenever a collapse of voltage

in the network is detected The ENABLE time is fixed and can be set The example

below shows the typical behavior of the limiter configured in this way

b) Dependent on the time integral with -dUdt ENABLE

Flag programming F418 = inactive F41A = 1111

This variant likewise enables the transient value only when a collapse of network

voltage has been detected However the switch-back to the thermal limit is not made

dependent upon the time itself but on the calculated time integral intisup2dt of the The setting

on Parameter F414 in spu takes into account the time the rotor needs to cool down ie

the rate of temperature change in the case of intermittent operation The example below

shows how the timing of the switch-back to the thermal limit depends on the present

value for intisup2dt

The time integral is based on the formula

Example The setting of =isup2dt equivalent to Version a (F416) at a constant 16 times the

nominal field current for 10 seconds (with TH12 = 105) is

c) Dependent on the time integral without any preconditions

Flag programming F418 = 1111 F41A = 1111

In this variant the transient becomes available without any prior conditions

(without a -dUdt ENABLE) with the time integral intisup2dt

237 Inductive Stator Current Limiter

The Inductive Stator Current Limiter holds the stator current Ig within permissible

limits while the generator is in the ldquoover-excitedrdquo operating range by reducing the field

current accordingly The setting TH (thermal limit) provides the limit against stationary s

that might occur To take advantage of the generatorrsquos transient overload capacity a

switch-over is made to the higher setting MAX The principle of operation of this switch-

over to the value MAX permissible only transiently is identical to that employed for the

field current limiter (refer to the description above) When the drive output from the

turbine is very high stator current may exceed permissible limits even while inductive

loading of the generator is low In this case if the stator current limiter is not kept from

influencing the field current the control circuit will oscillate back and forth between the

Inductive Stator Current Limiter (de-

exciting) and the Capacitive Stator Current Limiter (exciting)The output signal of that

function then dominates the control variable of the Ig-dependent limiter via a maximum

value selection

238 Capacitive Stator Current Limiter

239 Load Angle Limiter

The Load Angle Limiter prevents the synchronous machine from slipping out of

phase due to slippage of the rotor The load angle δ the difference in phase between the

rotor and the stator rotating field results mainly from the driving torque (active power P)

acting on the generator and the level of rotor current (field current) If the driving torque

remains constant a increase in the field current reduces the load angle δ The current load

angle δ at any moment is obtained from the generator current and generator voltage based

on a simplified model of the generator Whenever this calculated load angle δ exceeds the

preset limit angle the limiter increases the field current until the load angle has dropped

back to its permissible value The quadrature reactance Xq of the generator and the

network reactance Xe during normal operation must be adjusted on the regulator in order

to obtain the load angle δ The graph below shows the Power Chart for a salient-pole

machine with typical limiter characteristics

The purpose of a Power System Stabilizer is to use the generator excitation to

damp electromechanical oscillations between the network and the generator Depending

on the design of the generator and the requirements imposed for network stability its

main function will be either to damp the oscillations originating in the machine or those

from the network A synchronous generator working in a combined power network is in

principle an oscillating structure In order to produce a torque the magnetic field of the

rotor and the stator must form a given angle (referred to as the rotor displacement or load

angle δ) The electrical torque ME increases as the angle δ increases just as with a

torsion spring Because the ME of the generator and the mechanical driving torque MA

from the turbine are in equilibrium during steady-state operation the angle δ remains in a

given position Whenever this state of equilibrium between MA and ME is disturbed the

load angle slips of this rest position and change thereby the electrical torque ME The

torque attempts to restore the load angle to a stationary position Due to the mass inertia

of the turbinegenerator rotor however this can only take place aperiodically It does so

in the form of more or less effectively damped oscillations (again similar to the effect of

mass inertia on a torsion spring) In order to damp the oscillations there must be a

damping torque produced depending not on the electrical torque ME associated with the

angle but on the difference in frequency (Df) between the rotor and the stator rotating

field ie on the slippage This torque is produced mainly by the so-called damper

winding in the rotor but the dimensioning of this is subject to limits imposed by

considerations of design and economy Some further action is therefore needed to

increase the damping effect The following drastically simplified formula shows the

parameters upon which the amount of active power PE supplied by the generator

depends

PE = active power

It can be seen from the above relationship that the active power that the generator

transfers depends not only on the load angle δ but also on the field current If That means

that a transient change can be made in the active power PE and with that in the effective

electrical torque ME by varying the field current The principle of operation of the DVR

Power System Stabilizer becomes clear from a consideration of the oscillations in power

output and frequency (ΔPE Δf) and the vector diagram If it is assumed that oscillations

in the network frequency generates load oscillations with the mass inertia of the rotor

then the active load of the generator (eg MW-measured) is influenced with a sinusoidal

value -ΔPE (ME-MA = -ΔPE) By inversion of -ΔPE one obtains the fluctuation in

power provided by the rotor +ΔPE As is known the slip signal Δf follows +ΔPE with a

phase delayed by 90deg The +ME produced by the periodic changes in the load angle δ is

in phase with +ΔPE A good damping is attained if ME is varied in phase with the slip

Δf However this signal must also be advanced somewhat to compensate for the time

constants in the excitation circuit and the generator

As mentioned above the electrical torque ME can be influenced by varying the

field current To accomplish this a suitable control signal referred to as variable

disturbance compensation must be imposed upon the voltage set-point or the converter

control variable Ucontr As can be seen from the vector diagram by applying proper

weighting factors (K1 K2) and then adding together the signals -ΔPE and Δf an overall

stabilization signal can be produced that rotates in advance of the Df signal by any angle

desired between 0deg and 90deg Because the amplitude of -DPE remains proportional to the

amplitude of Δf a constant angle in advance of Δf results for the compensation of the

time constants referred to above The optimum weighting factors K1 and K2 for a

synchronous generator working to a power network depend on its operating point at any

moment and the external reactance of the network Normally the selection of a

compromise setting is good enough to attain stability in all operating points and for all

external reactance For special demands these settings must be parameterized as a

function of the external reactance (which means optional equipment Xe-Identification)

The Power System Stabilizer PSS is a section of the AVR computer program and is

processed once per network cycle The voltage at the generator terminals and the

generator current are measured in order to define the signals ΔPE and Δf The calculated

signals for _P_ (=PE) and Δf are then sent across DC filters ldquoDrdquo (real differentiators) that

transmit only the dynamic portion of the signals The ΔPE and Δf signals obtained in this

way are then weighted (multiplied by) with the factors K1 and K2 and sent to the

summing point of the voltage regulator

The PSS stabilization signal is imposed on the automatic voltage regulator only if

the following prerequisites are met

bull Generator on line

bull Generator power output gt the value F338

bull Generator voltage in a range between F33C and F33A

The stabilization signal is limited at the output from the PSS to the lower and

upper limits Flag defines whether the stabilization signal is introduced before or after the

PID filter (usually before the filter) Because the PID filter as noted above already takes

the ceiling factor Vp1 into account the PSS signal needs to be multiplied by Vp1 if it is

added to the voltage regulator following the PID filter (divider at the input to the

minmax limiter) This precaution prevents the DC filter ldquoDrdquo in the P-channel from

producing an unnecessary ldquostabilizationrdquo effect in the case of rapid changes in turbine

load As an alternative for the AVRrsquos Power System Stabilizer a stabilization signal from

an outside system can be imposed by activating the binary input ldquoPSS-SIGNEXTrdquo Flag

F340 can be used to select between an analog and a 12-bit signal and F33E to select the

polarity desired for that signal

24 The MANUAL Channel

241 Summary

The MANUAL channel (Channel 2) has been built as a simple field current

regulator

without additional limiters Its main function is to maintain the excitation of the generator

even if the AUTOMATIC channel becomes non-operational The MANUAL channel

also performs valuable service for purposes of testing commissioning and preventive

maintenance Its measurements regulator generation of firing pulses and power supply

are physically separate from those on the AUTOMATIC channel

242 Principle of Operation

All the functions of the MANUAL channel including the generation of firing

pulses have been implemented in a single electronic module the Gate Control Unit The

control variable Ucontr of voltage regulator is used as the reference value for generating

firing pulses on the principle known as ldquoramp controlrdquo (Comparison of Ucontr with

Usynsynchronous sawtooth signal) For further processing in the UN 0096 Intermediate

Pulse Stage the Gate Control Unit supplies six firing pulses at its output whose phase

position with respect to the synchronous voltage Usyn is in accordance with control

variable Ucontr An internal linearization ensures that the field voltage produced via the

firing pulses remains proportional to the control variable Ucontr throughout the entire

range As a result the circuit amplification of the control remains constant over the entire

range Whenever excitation is switched ON the set-point for Generator Voltage is set

automatically at the preset - ref Value This provision ensures that the generator voltage

always attains approximately its nominal value after the field flashing The Gate Control

Unit can be refunctioned ( by pre-selection with a switch ) for purposes of testing to act

as a purely firing pulse control In this case the control variable Ucontr is adjusted

directly using the RAISELOWER push buttons on the front of the module In this way

for example the relationship between the phase position of the firing pulses and the

control variable Ucontr can be checked easily

CHAPTER 3

PULSE SECTION

31 Pulse Generation and Amplification

The Gate Control Units of both AUTOMATIC channel and MANUAL channel

each supply six firing pulses for operating the 6-pulse thyristor bridges The low-power

pulse signals from these Gate Control Units are then amplified in the Intermediate Pulse

Stage galvanically isolated and then sent to the common pulse bus On the output end

the Intermediate Pulse Stage of the non-active channel) is always blocked The Gate

Control Units generate the pulses based on microprocessor control The reference voltage

used for the firing pulse phase location is the output voltage from the excitation

transformer (Usyn1 Usyn2) The commutation spikes of the synchronous voltage caused

by the converter are calculated prior to use of the voltage as a reference value and are

deliberately filtered out The lower limit for the firing pulses (double pulses) which are

offset from one another by 60deg is defined by the limit rectifier position (αmin) and the

upper limit by the limit inverter position (αmax) for the firing angle αmin and αmax can

be adjusted on the Gate Control Units using BCD (Binary Coded Decimal) switches

αmin ensures that the firing pulses will not be issued (premature firing) until there is

sufficient positive phase voltage on the thyristor involved αmax prevents a dangerous

ldquotippingrdquo of the thyristor bridge into the rectifier mode if the firing angle α is too large

(ldquolate firingrdquo) The critical factors determining αmax are the overlap time uumlmax (max

commutation time) and the ldquorecovery timerdquo of the thyristors (αmax lt 180deg - uumlmax - γ )

An external control signal can force the firing pulses into their inverter limit position

Other binary inputs can block or direct the firing pulses of the Gate Control Units so as to

produce freewheeling on the thyristor bridge During freewheeling the firing pulses for

the thyristor pair R and S are blocked and the pulse signals T+T- are engaged with

chains of pulses Both Gate Control Units (for the MANUAL amp AUTOMATIC channels)

contain a field current monitor that blocks the firing pulses immediately whenever the

current exceeds a preset threshold level In this case the field circuit-breaker is also

tripped via an output contact The purpose of these provisions is to prevent damage to

thyristors and thyristor fuses in case of a slip-ring short-circuit or to keep any damage

that does occur to a minimum The pulse signals are galvanically separated at the outputs

from the Intermediate Pulse Stage (with pulse transmitters) and are then directed to the

common pulse bus This transmission of the pulse signals to the pulse bus via passive

transmitters ensures a high degree of active channel autonomy Practically no possible

malfunctions on the inactive channel (including for example sustained pulses) affect the

active channel

32 Pulse Monitoring

The ldquoPulse Busrdquo and the pulse signals of the AUTOMATIC channel are

monitored This monitoring device consists of potential isolating stages and the common

monitor If the pulse monitoring of the ldquoPulse Busrdquo responds a switch-over is made to

MANUAL channel The function of the potential isolating stages is to couple the pulse

monitoring device to the pulse circuits without any feedback effect The pulse monitoring

checks the six pulse lines for the following malfunctions continuous or periodic failure

of one or more pulses Periodic occurrence of synchronous or asynchronous false pulses

Continuous pulses the pulse monitoring device can be tested while the machine is in

operation

CHAPTER 4

CONVERTER

Thyristor

The term thyristor usually refers to a family of four layer solid state device having

turn on characteristics that can be externally controlled by either current or voltage They

are also referred to as breakdown device because their working depends on avalanche

breakdown Thyristors have only two stages OFF and ON Thyristors have a similar

function to Uni-junctions they act as switches Thyristors use current flow as a switch

Thyristors have three states

1 Reverse blocking mode mdash Voltage is applied in the direction that would be

blocked by a diode

2 Forward blocking mode mdash Voltage is applied in the direction that would cause

a diode to conduct but the thyristor has not yet been triggered into conduction

3 Forward conducting mode mdash The thyristor has been triggered into conduction

and will remain conducting until the forward current drops below a threshold value

known as the holding current Converter is a semiconductor device which converts ac

input voltage into a constant dc output voltage In present excitation system three phase

fully controlled thyristor converter is used

Because of the following advantages thyristor converters are used

a) Thyristors are used for high power applications ie up to 10Kv3500A1KHz

b) Having high reliability and low losses

c) Uni-directional device like diode

d) Itrsquos operation as a rectifier which are low resistance in forward conduction

mode and high resistance in reverse conduction mode

PROTECTION OF THYRISTORS

For reliable operation of a thyristor demands that its specified ratings are not

exceeded When Subjected to or over voltages During the turn - on of SCR didt

prohibitively large False triggering of SCR by high value of dvdt andSpurious signals

between gate and cathode may leads to unwanted turn ndash on

DIDT AND PROTECTION

When thyristor starts conducting in forward conduction mode and is turned on by

gate pulse The anode current increases rapidly whole area of the gate to Cathode

junction then hot spots will be formed near the gate connection this locality of heating

destroys the thyristor Thyristor thermal time is constant The causes due to faults and

short circuits or surge currents Electronic crowbar protection is used against the over

voltages The rate rise of anode current must be kept at the time of turn on below the

rated or specified limiting value The didt value maintained below limited value by using

a inductor also called ldquodidt inductorrdquo in series with anode circuit The locality of heating

is avoided by applying gate current but not greater the maximum gate current

DVDT AND OVER VOLTAGE PROTECTION

With forward voltage across the anode and cathode of a thyristor the two outer

junctions are forward biased but the inner junction is reverse biased This reverse biased

junction J2 has the characteristics of a capacitor due to charges existing across the

junction In other words space-charges exist in the depletion region around junction J2

and therefore junction J2 behaves like a capacitance If the entire anode to cathode

forward voltage Va appears across J2 junction and the charge is denoted by Q then a

charging current i given by Eq (46) follows

i = dQdt =d(Cj Va )dt

= Cj (d Va dt) + Va(d Cj dt) helliphelliphelliphellip(46 a)

As Cj the capacitance of junction J2 is almost constant the current is given by

i = Cj (d Va dt) helliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip(46 b)

If the rate of rise of forward voltage dVadt is high the charging current i will be

more This charging current plays the role of gate current and turns on the SCR even

when gate signal is zero Such phenomena of turning-on a thyristor called dvdt turn-on

must be avoided as it leads to false operation of the thyristor circuit

For controllable operation of the thyristor the rate of rise of forward anode to

cathode voltage dVadt must be kept below the specified rated limit Typical values of

dvdt are 20 ndash 500 Vμsec False turn-on of a thyristor by large dvdt can be prevented by

using a snubber circuit in parallel with the device thyristor are very sensitive for over

voltage than the semiconductor devices

Over voltage transients are perhaps the main cause of thyristor failure

In thyristor there are mainly two types

1 Internal over voltages

Due to the commutation of the thyristors large voltages are generated internally

Because of the series inductance of the SCR circuit the large transient voltages L didt

produced This voltage several times the break over voltage of the device then thyristor

destroys permanently

2 External over voltages

External over voltages are caused due to the interruptions of current flow in an

inductive circuit and also due to the lightening strokes on the lines feeding the thyristor

system For the reliable operation of thyristor the over voltages must be suppressed by

adopting suitable techniques

Suppression of over voltages

The RC circuit called snubber circuit is connected across the device to protect In

order to keep the protective components to a minimum the thyristors are chosen with

their peak voltages ratings are 25 to 3 times of the normal peak working voltage

ldquoselenium thyrector diodes metal oxide varistors or avalanche diode suppressers are

commonly employed for protecting the thyristor circuit against the over voltages

Gate protection

Gate circuit should also be protected against the over voltages and surges Over

voltage at gate circuit can cause false triggering of the SCR may rises the junction

temperature behind specified limit leading to its damage Protection against over

voltage can be achieved by connecting a ZD across the gate circuit and a resister is

connected in series with gate circuit to protect against the s A capacitor and resister are

connected across gate to cathode to by pass the noise

41 Final Pulse Stages

The Final Pulse Stages adapt the output pulses from the Intermediate Pulse Stage

(pulses on the pulse bus) to the gate currents needed for the thyristors Each thyristor

bridge is equipped with its own Final Pulse Stage Each Final Pulse Stages is provided

with a power supply module The amplified output pulses from the Final Pulse Stages

start as a short strong steep pulse with an amplitude approx 2frac12 times that of the main

pulse This initial pulse edge assures proper firing of the thyristors being triggered

Subsequently the weaker part of main pulse keeps firing conditions steady As already

mentioned the Final Pulse Stages and their associated thyristor bridges form single units

All six pulse outputs from a Final Pulse Stage can be blocked by an external control

signal so that all thyristors in the associated thyristor bridge will block the current A

blocking of the pulses is initiated whenever there is a malfunction in the associated

thyristor bridge

42 Converter Power Section

The thyristor converter consists of three independent parallel rectifier blocks TY1

to TY3 which are all in service Even if one block fails the remaining blocks take over

automatically the full design current of the excitation circuit During normal operation

(with ideal current share) and all three bridges in operation each of these blocks has to

carry only (n-2)n (ie33)of its design current If 2 thyristor bridges fail the excitation

is limited Only when all three bridges fail the excitation is switched off Each thyristor

bridge arm is equipped with current flow monitoring CTrsquos Failure of conduction in any

arm is identified by a Current flow monitoring module

43 Converter Cooling

A cooling system is needed to dissipate heat losses in the converter blocks and

electronics Each converter block has therefore been equipped with a fan supplied with

power from the converterrsquos primary voltage (via transformer ndashT8 in field flashing

cubicle) The fans are protected with motor protection circuit breakers An air flow

monitoring unit is provided for monitoring the air flow through the thyristor bridge If a

circuit breaker failure is detected or if the air flow monitor drops off at one of the

thyristor bridges the bridge involved is immediately set out of operation by blocking its

firing pulses

44 Thyristor Converter Monitoring

A thyristor bridge in which defects occur that could threaten the safety of

operation or cause secondary damage is switched off automatically ie its firing pulses

are blocked This happens whenever A thyristor fuse is blown The fuses are monitored

individually with micro switches The Final Pulse Stage fails which is detected by

internal monitors (supply voltage sustained pulse short-circuit on the output end) The

power supply to the fan fails fan air flow as monitored by the Air flow monitor fails or

is insufficient Isolator on ACDC side is open

CHAPTER 5

Field Current Circuit Excitation Transformer

51 Field Circuit Breaker

The circuit-breaker in the field circuit is used to isolate the field circuit from the

converter It is capable of switching off the synchronous machine from full load under the

maximum conditions of a 3-phase short-circuit In addition to its main contacts the field

circuit-breaker also has a de-excitation contact with which the field energy stored in the

field can be dissipated across the de-excitation resistor The de-excitation contact closes

shortly before the main contacts open so as to ensure proper commutation of the field

current from the main contacts to the de-excitation contact when the breaker is switched

off The field circuit-breaker is switched on by electromagnetic force and is kept switched

on by a mechanical latch When the latch is released by a trip coil the circuit-breaker

opens The circuit-breaker also has auxiliary contacts that report its status

52 Field Flashing

In shunt supplied excitation circuits (excitation transformer connected to the

generator terminals) the generator does not have enough remnant voltage for a generator

voltage build-up via the converter In this case a field flashing circuit is provided It

consists of the field flashing contactor the diode bridge and a transformer used to adapt

the auxiliary input voltage to the voltage needed for field flashing when power is

supplied from the auxiliaries network

Fig Field Flashing

Because the field flashing contactor is not able to switch off the energy stored in

the field the control ensures that the contactor can only reopen if the field circuit breaker

has already been opened (generating the TRIP order) or in a normal field flashing

sequence when the converter has taken over the field current Field flashing occurs in the

following stages

1048729The excitation is switched on closing the field flashing contactor ( Field

Circuit Breaker is already closed )

1048729The start-up excitation current flows through the rotor driving the generator

voltage up to approx 15 U

1048729After about 10 U the firing pulses to the converter are released and it begins

to excite the generator to its rated voltage

1048729After about 30 U the field flashing contactor opens (with no current since

the converter is now supplying the current)

The diode bridge at the input to the field flashing contactor prevents a feed-back

from the converter to the source of field flashing while the contactor is still closed

53 De-excitation

When malfunctions occur the stored field energy must be dissipated as quickly

and safely as possible to protect the generator This is done by the converter the field

circuit-breaker and the de-excitation (discharge) resistor

De-excitation (with opening of the field circuit-breaker) takes place in the following

stages

1048729The converter drives to its inverter limit position (negative ceiling voltage)

recovers a portion of the field energy into the network A trip command is given to the

field circuit breaker

1048729The de-excitation contact closes diverting the field voltage to the de-excitation

resistor

1048729Then immediately the main contacts open building voltage The field voltage

commutates to the de-excitation resistor

1048729The current diminishes at a given time constant TE

(With linear resistance TE = Lf (Rf + Re))

Due to the reversal of the field voltage by the converter the field current

commutates from the main contacts of the field circuit-breaker to the de-excitation

resistor in a very early phase This reversal of the field voltage prevents burn-off on the

main contacts and provides effective protection for the field circuit-breaker Depending

on the operating policy an operational shut-down of the excitation can also be effected

with the field circuit-breaker closed This method is useful mainly when the excitation is

switched on and off frequently In this case the converter is merely driven into the

inverter limit position so that the field energy is recovered into the network The

converter then blocks since it is supplying positive current only

54 Excitation Transformer

The excitation transformer matches the generator voltage to the field voltage

(required ceiling voltage) It also serves as a commutation reactance for the thyristor

converter and as a potential isolator between the network and the excitation circuit In

addition the transformer functions as a current limiter in that it makes it possible to keep

any short circuits in the excitation circuit under better control The excitation transformer

is equipped with temperature monitoring probes which set off an alarm when the

temperature exceeds a first max limit and then trips the excitation if the temperature

continues rising to a second (higher)limit

CHAPTER 6

Monitoring and Protection

61 Excitation Monitoring

611 General Information

The main goal of Excitation Monitoring is to make optimum use of the

redundancies provided in the excitation system and to give alarm whenever a malfunction

makes these redundancies unavailable The field current is monitored to see that it does

not exceed a maximum level and if necessary a switch-over to the MANUAL channel is

initiated In addition the criterion for switching off the field flashing is generated The

excitation Monitoring consists of an autonomous processor system

612 over current Alarms

In the Excitation Monitoring the limits for are set at higher levels than the

settings on the Field Current Maximum Limiter Whenever the current exceeds 110 of

the nominal field current contact R1 and the binary output associated with it are

activated immediately If field current remains gt 110 then after a preset inverse-time

has lapsed relay R2 and - after a further delay - relay R0 and the binary outputs

associated with them are activated Parameters match the measurements for If1 and If2 to

the nominal value for field current so that the internal values can be processed and read as

pu values It can be used to falsify the actual value of the field current If (to raise it) so

as to cause a response from the alarm limits for purposes of testing The processed If

signal is always taken from on the active channel (CHANNEL I OR CHANNEL II) As

long as the field current If is above the threshold value 11 Ifn its peak value is

measured This is stored (until RESET) and can be read at any time on the Micro-

Terminal Once the value of If exceeds 11 Ifn integration of this value starts Whenever

the integrated time-current value (intisup2dt) exceeds the preselected reference value the

alarm OVER CURRENT INVERSE-TIME is set off and a command is simultaneously

issued to switch over to the stand by AUTO channel Software switch F758 enables the

three over current alarm functions (R0 R1 R2) and selects one of three possible inverse-

time curves T1 T2 or T3 Within the characteristic curve (T1 T2 T3) selected the

desired limit curve for response is set using the factor F216

613 Switch-Off Criterion for Field Flashing

The Excitation Monitoring supplies the criterion for switching off the field

flashing Whether this criterion is activated based on the actual value for generator voltage

Ug or for field current If or both depends on the settings of the two threshold values

F200F202 (0 setting means that the output is always ldquological 1rdquo) The measurements

Ug12 and If12 are switched over depending on the present status of the channels

(Channel 1 or Channel 2 ON) Whenever Flag F750 is not activated the binary output is

fixed at ldquological 1rdquo

614 Storage of Alarm Status

The outputs of the over current alarms (R1 R2 R0) and the messages NO

FAILURE MONITORING PARAMETERS CHANGED are stored messages can be

erased by activating the input ldquoGENERAL RESETrdquo or by using the RESET button on the

front of the module Erasure with the input ldquoGENERAL RESETrdquo is effective only if the

situation causing the alarm or the malfunction is no longer present Whenever the self-

diagnosis equipment in the processor detects a malfunction the output NO FAILURE of

MONITORING is set at ldquological 0rdquo (= alarm) The alarm ldquoPARAMETERS CHANGEDrdquo

is activated whenever parameters or settings of software switches have been changed via

the Micro-Terminal

615 Actual Value Monitoring

The actual values for generator voltage Ug synchronous voltage Usyn and field

current If are monitored for malfunctions This monitoring is active regardless of whether

or not the generator is in operation Essentially when the generator is in operation the

measurements are monitored by comparing the signals (the smaller signal reading is

detected as incorrect) When the generator is not in operation the measured data are

monitored for extreme values The percentage of deviation permissible in the

measurement signals being compared

(Ug1ampUsyn1 Ug1ampUg2 Ug2ampUsyn2 If1ampIf2) is defined by parameters F208 and

F20AIf the excitation transformer is being supplied from an auxiliary power source (no

shunt operation) the values of Ug and Usyn will be different in some operational

conditions

In that case Flag F75A can be used to deactivate comparative measurements Ug1 Usyn1

and Ug2 Usyn2 The ldquoprimary monitoringrdquo compares Ug1 with Ug2 and generates the

messages that Channel 1 or Channel 2 has suffered a malfunction Whenever Ug1 lt Ug2

and the binary message from CH1 reports no malfunction a malfunction on

Measurement Channel 1 is reported (Ug1Usyn1 FAILURE) A similar malfunction is

also present whenever the binary message CH1 DISTURBANCE is reported and a

discrepancy is detected between Ug1ampUg2 The generation of the alarm ldquoUg2 Usyn2

FAILURErdquo is analogous to that for Channel 1 The ldquosecondary monitoringrdquo compares

Ug1 with Usyn1 amp Ug2 with Usyn2 This is enabled whenever the binary message of the

comparison channel reports a malfunction or whenever both binary messages report no

malfunction - but both secondary monitors report a malfunction As long as the secondary

monitoring is blocked the differences Ug1neUsyn1 or Ug2neUsyn2 trigger malfunction

signals for the measurement channel involved (suspicion that there is a corresponding

error in Usyn) The measurement channel malfunctions are enabled operationally

whenever after excitation has been switched on generator voltage Ug exceeds the value

set on F204 The voltages Ug1 and Ug2 are checked 16 seconds after the excitation is

switched off to see that they do not exceed the limit value F210 that applies to both of

them At the same time g1Ug2Usyn1Usyn2 are checked for extreme values (gt or lt

the operating range) Monitoring for extreme values is likewise enabled during normal

operation ( Excitation ON and Ug gt F204 ) Flag F754 is used to enable or block the

malfunction signals to the binary outputs Basically the monitoring of the actual values

for If1If2 functions like that of the Ug1Ug2 monitoring

62 Excitation Protection

621 General Information

The Excitation Protection switches off the excitation (and de-excites the machine

rapidly) whenever a danger arises that threatens the excitation transformer the converter

or the generator Generally limiter or monitoring functions precede the emergency trips

and these normally respond before the Excitation Protection must initiate a trip

Protective trip commands are issued directly to the field circuit-breaker from potential

free contacts of the board via the trip relays They are directed redundantly to the

operative field circuit-breaker ldquoOFFrdquo command

622 Protection against Excitation Transformer Overheating

This equipment monitors the excitation transformer for overheating in the

windings that could result from over current short-circuits or inadequate cooling The

monitoring uses temperature monitoring modules in conjunction with temperature

sensors built into the windings Normally the temperature is monitored in two stages the

first stage sets off an alarm the second causes a trip of the excitation

623 Rotor Over voltage Protection

Malfunctions in the generator circuit (eg terminal short-circuit failed

synchronization asynchronous operation) cause induced negative field currents that

produce high voltages in the field circuit These must be restricted to a level with a

sufficient safety margin below the insulation capacity of the field winding (test voltage)

and also below the peak blocking voltage of the converter thyristors The crow bar

employs spark gap elements to detect over voltages in the field circuit Whenever they

respond the associated thyristors are fired immediately switching the de-excitation

resistor parallel to the field The de-excitation current generated thereby initiates an

excitation trip via a supervision circuit causing an immediate opening of the field circuit-

breaker The malfunction isets off an alarm and an internal malfunction is indicated at the

cubicle

TEST VALUES OBTAINED WHEN EXCITATION IS RAISED

TEST

SNO PARTICULARS ACTUAL VALUE OBTAINED VALUE

1 VREF 996 100

2 VACT 997 1003

3 IFACT 735 765

4 IGACT 703 707

5 ACTIVE

POWER

703 705

6 REACTIVE

POWER

101 142

7 POWER

FACTOR

099 IND 098 IND

8 ACTIVE

CURRENT(IR)

705 703

9 REACTIVE

CURRENT(IX)

102 138

10 POWER

ANGLE

566 546

11 FIRING

ANGLE

640 632

VALUES OBTAINED WHEN EXCITATION IS LOWERED

SNO PARTICULARS ACTUAL

VALUE

OBTAINED VALUE

1 VREF 100 997

2 VACT 100 997

3 IFACT 787 761

4 IGACT 839 837

5 ACTIVE

POWER

839 830

6 REACTIVE

POWER

155 110

7 POWER

FACTOR

098 IND 099 IND

8 ACTIVE

CURRENT(IR)

835 830

9 REACTIVE

CURRENT(IX)

140 94

10 POWER

ANGLE

613 DEG 631 DEG

11 FIRING

ANGLE

601 DEG 599 DEG

CONCLUSION

For generating the EMF in stator winding excitation is required to the rotor of a

generator There are two types of excitation

1 Static excitation system

2 Brushless excitation system

A certain disadvantage in brushless excitation system is the slow response time of

the field in case of fast load changes specified No slip-rings and brushes direct

measurements of the field parameters not possible

To avoid all loses static excitation is used Since it does not have any rotating

parts mechanical loses and windage loses This system has fast response and speed

control While preferring this excitation system there are no limitations for the

redundancy of Thyristor bridge circuits

Static excitation has fast field discharge by resistor and inverter operation direct

measurement of field quantity is possible The meaning of excitation is nothing but

continuous supply of DC current (ie field current) to the rotor to buildup required

output voltage in the stator

Field current is changed with respect to the change of load so the digital

automatic voltage regulator (DAVR) is used to regulate the output voltage according to

the load variations

So we conclude that static excitation system with DAVR is preferred since it is

having excellent dynamic performance and better options for R amp M

231 General Information

The functions of the automatic voltage regulator AVR are

1 to regulate the generator voltage

2 to regulate the effect of the reactive andor active current on the voltage

3 to limit VoltHz

4 to limit max and min field current

5 to limit inductive stator current

6 to limit capacitive stator current

7 to limit the load angle

8 to stabilize the power system

Block Diagram shows the software structure of AUTOMATIC channel The

generator limiters not provided for the installation in question (optional equipment) are

identified in this overview as ldquoNot Suppliedrdquo The parameter values signal values and

software switches (flags) marked with addresses (hexadecimal numbers) can be viewed

and altered via the Micro-Terminal The values selected are displayed in sec pu Hz

etc and can where necessary be changed directly in these formats The plant-specific

settings of the variables and the flags can be obtained from the Test and Commissioning

Report This block diagram provides information about the important functions and

possible settings of the AUTOMATIC channel For the sake of clarity no detailed

presentation has been given of special functions such as tracking circuits initializations

etc The page heading cross-refers this overview to the various sheets of the schematic

diagram Binary signals are shown in broken lines analog signals in solid lines The

corresponding text designations in the schematic diagram can be used for identification of

the input signals (hardware inputs) The only analog output signal from the automatic

voltage regulator control variable Ucontr is sent via the data bus (CRU bus) to the Gate

Control Unit Most of the binary messages (outputs) from the AVR are of no interest

functionally and they have been omitted for the sake of clarity The basic structure of the

digital voltage regulator and the limiters is simple This is necessary in order that the

behavior of the regulatorslimiters will remain calculable and understandable in all

operating situations and that there will be no problem in adjusting and optimizing them

The central PID filter in the digital voltage regulator defines the dynamic response of the

closed-loop controls both in the voltage regulator mode and after limiters have

intervened The ldquocontrol deviationrdquo at the input to the PID filter is either the control

deviation for voltage the control deviation of a de-excitation limiter (the value

determined by minimum value selection) or the control deviation of an excitation limiter

(the value determined by maximum value selection) Flag F730 (ldquoPRIORrdquo) is used to

determine whether the exciting (Min value) or the de-exciting signal takes precedence on

the minmax value limiter (normally F730 = 1111 ie the de-exciting signal takes

precedence) With the exception of the Minimum Field Current Limiter all other limiters

have variable factoring multipliers of the signal outputs so that they can be adjusted

individually together with the common PID filter which has been optimized for voltage

regulation The setting parameters for this PID filter are as follows

Vo = KR Static amplification

1

Ta = ---- Integration time constant

Tc1

Vp Proportional amplification

1

Tb = ---- Differential time constant

Tc2

Vinfin Amplification of high frequencies

The BODE diagram below shows the assignment of settings in accordance with

DINIEC standards based on a typical example

The PID filter amplifications Vo Vp and Vinfin can be adjusted in pu values But

the ldquoceiling factorrdquo pl+ must be adjusted correctly with parameter F310 if the total

amplification (circuit amplification) of the control circuit is actually to conform to the

pu settings This factor must agree with the ldquoexternalrdquo amplification ie with the

ceiling value of the transformer- converter circuit

Ceiling factor(pl+) = Ufmax Ufo

in which Ufmax = ceiling field voltage

Ufo = no-load field voltage

To attain a suitable response of the AVR when starting excitation

(ldquoEXCITATION ONrdquo) it may be necessary to change the proportional amplification of

the regulator during this phase Vp2 (transiently activated) and Vp1 (permanently

activated) can be adjusted for this purpose For example the value of Vp2 takes effect

immediately once the excitation is switched on and remains effective for a period as set at

F30C Once the period F30C (eg 5 sec) has expired Vp shifts over to Vp1 (becomes

the steady-state Vp) at the rate of change set The standard operating mode for the PID

filter is voltage regulation for which the discrepancy between the voltage set-point and

the current value for generator voltage Ug (the control deviation) is supplied at the input

To compensate for the voltage drop in the block transformer or whenever several

generators are operating to the same distributing bus the generator voltage must be

varied in proportion to the measured generator current (droop influence) To accomplish

this the voltage set-point is varied as a function of the measured reactive current IX

andor active current IR Flag F712 enables the IX droop Flag F710 the IR droop The

desired compensation is set in F282 and F286 respectively Flags F284 and F288 are used

to select whether this droop influence is to increase the voltage or to reduce it

(compensation) Combined influence of the active and reactive currents is attained by

enabling both droops IX and IR Flag F716 activates a so-called ldquoSoft-Startrdquo at the

starting of excitation This ldquoSoft-Startrdquo ensures that the voltage set-point integrates from

0 to 100 within the time set on F290 when the excitation is switched on

(ldquoEXCITATION ONrdquo) A ldquosmoothrdquo excitation of the generator can be achieved in this

way whenever there is no demand for a quick excitation

232 Voltage Set-Point

Various signals and settings control and limit the voltage set-point F270 For

example the values of F254 and F252 define the normal operating range possible for set-

point adjustment (eg 90 110) using external control commands (control room local

operatorrsquos panel superposed control system) The effective set-point adjustment rate is

governed The set-point can be set at the values of F250 and F256 by activating

appropriate control commands for ldquoSETrdquo input Enabling Flag F71A and activating a

binary input prior to switching on the excitation (ldquoEXCITATION OFFrdquo) sets the Ug set-

point at the value of UAUX This makes it possible for example to ensure that the

generator voltage will agree exactly with the network voltage after the voltage build-up

An external value with variable amplification can be added to the Ug set point by

enabling F724 (for example for stability tests)

233 Regulator Tracking in MANUAL Operation

Whenever the AUTOMATIC channel is not in operation (the MANUAL channel

is ON) a follow-up equipment ensures a smooth switch-back to the AUTOMATIC

mode will always be possible To track the voltage set-point is shifted by means of

RAISELOWER pulses from the Gate Control Unit so that control variable Ucontr at the

output from the PID filter is held steady and identical to the control variable Ucontr from

the MANUAL channel Because this tracking must react slowly resultant transient

control deviations resulting from the amplification in the PID filter might cause severe

interference with control variable Ucontr

To prevent this the follow-up equipment intervenes on the regulators mixing

point with a corresponding compensation signal

234 Ugf Limiter

At under frequency the Ugf Limiter reduces the generator voltage so as to

prevent saturation effects in the supply and measuring transformers To adjust this

limiter the max permissible generator voltage at rated frequency is defined and set

When any under-frequency occurs the generator voltage is thus reduced in proportion to

that setting

235 Field Current Maximum Limiter

The Field Current Maximum Limiter is provided to protect the generator rotor

from s occurring in steady-state and transient operation High field currents are normally

the result of a sharp drop in network voltage or of an improper raising of the voltage set-

point by the operating staff The field current is held steady at the value TH1 ie at the

maximum thermal value permissible for the excitation circuit and the rotor In order that

the generator can support the power network with its transient overload capacity during

brief collapses in voltage a temporary switch-over is made to the transient limit MAX1

(a higher setting) When the generator or the converter is operating at a reduced capacity

These limits TH1MAX1 can be switched over to the lower settings TH2MAX2 by

activating the corresponding binary signals The switch-over from the thermal limit

TH12 to the transient limit MAX12 can be configured in one of three ways

a) Depending on the over current with -dUdt ENABLE

Flag programming F418 = any setting desired F41A = 0000

This variant enables the transient value MAX12 whenever a collapse of voltage

in the network is detected The ENABLE time is fixed and can be set The example

below shows the typical behavior of the limiter configured in this way

b) Dependent on the time integral with -dUdt ENABLE

Flag programming F418 = inactive F41A = 1111

This variant likewise enables the transient value only when a collapse of network

voltage has been detected However the switch-back to the thermal limit is not made

dependent upon the time itself but on the calculated time integral intisup2dt of the The setting

on Parameter F414 in spu takes into account the time the rotor needs to cool down ie

the rate of temperature change in the case of intermittent operation The example below

shows how the timing of the switch-back to the thermal limit depends on the present

value for intisup2dt

The time integral is based on the formula

Example The setting of =isup2dt equivalent to Version a (F416) at a constant 16 times the

nominal field current for 10 seconds (with TH12 = 105) is

c) Dependent on the time integral without any preconditions

Flag programming F418 = 1111 F41A = 1111

In this variant the transient becomes available without any prior conditions

(without a -dUdt ENABLE) with the time integral intisup2dt

237 Inductive Stator Current Limiter

The Inductive Stator Current Limiter holds the stator current Ig within permissible

limits while the generator is in the ldquoover-excitedrdquo operating range by reducing the field

current accordingly The setting TH (thermal limit) provides the limit against stationary s

that might occur To take advantage of the generatorrsquos transient overload capacity a

switch-over is made to the higher setting MAX The principle of operation of this switch-

over to the value MAX permissible only transiently is identical to that employed for the

field current limiter (refer to the description above) When the drive output from the

turbine is very high stator current may exceed permissible limits even while inductive

loading of the generator is low In this case if the stator current limiter is not kept from

influencing the field current the control circuit will oscillate back and forth between the

Inductive Stator Current Limiter (de-

exciting) and the Capacitive Stator Current Limiter (exciting)The output signal of that

function then dominates the control variable of the Ig-dependent limiter via a maximum

value selection

238 Capacitive Stator Current Limiter

239 Load Angle Limiter

The Load Angle Limiter prevents the synchronous machine from slipping out of

phase due to slippage of the rotor The load angle δ the difference in phase between the

rotor and the stator rotating field results mainly from the driving torque (active power P)

acting on the generator and the level of rotor current (field current) If the driving torque

remains constant a increase in the field current reduces the load angle δ The current load

angle δ at any moment is obtained from the generator current and generator voltage based

on a simplified model of the generator Whenever this calculated load angle δ exceeds the

preset limit angle the limiter increases the field current until the load angle has dropped

back to its permissible value The quadrature reactance Xq of the generator and the

network reactance Xe during normal operation must be adjusted on the regulator in order

to obtain the load angle δ The graph below shows the Power Chart for a salient-pole

machine with typical limiter characteristics

The purpose of a Power System Stabilizer is to use the generator excitation to

damp electromechanical oscillations between the network and the generator Depending

on the design of the generator and the requirements imposed for network stability its

main function will be either to damp the oscillations originating in the machine or those

from the network A synchronous generator working in a combined power network is in

principle an oscillating structure In order to produce a torque the magnetic field of the

rotor and the stator must form a given angle (referred to as the rotor displacement or load

angle δ) The electrical torque ME increases as the angle δ increases just as with a

torsion spring Because the ME of the generator and the mechanical driving torque MA

from the turbine are in equilibrium during steady-state operation the angle δ remains in a

given position Whenever this state of equilibrium between MA and ME is disturbed the

load angle slips of this rest position and change thereby the electrical torque ME The

torque attempts to restore the load angle to a stationary position Due to the mass inertia

of the turbinegenerator rotor however this can only take place aperiodically It does so

in the form of more or less effectively damped oscillations (again similar to the effect of

mass inertia on a torsion spring) In order to damp the oscillations there must be a

damping torque produced depending not on the electrical torque ME associated with the

angle but on the difference in frequency (Df) between the rotor and the stator rotating

field ie on the slippage This torque is produced mainly by the so-called damper

winding in the rotor but the dimensioning of this is subject to limits imposed by

considerations of design and economy Some further action is therefore needed to

increase the damping effect The following drastically simplified formula shows the

parameters upon which the amount of active power PE supplied by the generator

depends

PE = active power

It can be seen from the above relationship that the active power that the generator

transfers depends not only on the load angle δ but also on the field current If That means

that a transient change can be made in the active power PE and with that in the effective

electrical torque ME by varying the field current The principle of operation of the DVR

Power System Stabilizer becomes clear from a consideration of the oscillations in power

output and frequency (ΔPE Δf) and the vector diagram If it is assumed that oscillations

in the network frequency generates load oscillations with the mass inertia of the rotor

then the active load of the generator (eg MW-measured) is influenced with a sinusoidal

value -ΔPE (ME-MA = -ΔPE) By inversion of -ΔPE one obtains the fluctuation in

power provided by the rotor +ΔPE As is known the slip signal Δf follows +ΔPE with a

phase delayed by 90deg The +ME produced by the periodic changes in the load angle δ is

in phase with +ΔPE A good damping is attained if ME is varied in phase with the slip

Δf However this signal must also be advanced somewhat to compensate for the time

constants in the excitation circuit and the generator

As mentioned above the electrical torque ME can be influenced by varying the

field current To accomplish this a suitable control signal referred to as variable

disturbance compensation must be imposed upon the voltage set-point or the converter

control variable Ucontr As can be seen from the vector diagram by applying proper

weighting factors (K1 K2) and then adding together the signals -ΔPE and Δf an overall

stabilization signal can be produced that rotates in advance of the Df signal by any angle

desired between 0deg and 90deg Because the amplitude of -DPE remains proportional to the

amplitude of Δf a constant angle in advance of Δf results for the compensation of the

time constants referred to above The optimum weighting factors K1 and K2 for a

synchronous generator working to a power network depend on its operating point at any

moment and the external reactance of the network Normally the selection of a

compromise setting is good enough to attain stability in all operating points and for all

external reactance For special demands these settings must be parameterized as a

function of the external reactance (which means optional equipment Xe-Identification)

The Power System Stabilizer PSS is a section of the AVR computer program and is

processed once per network cycle The voltage at the generator terminals and the

generator current are measured in order to define the signals ΔPE and Δf The calculated

signals for _P_ (=PE) and Δf are then sent across DC filters ldquoDrdquo (real differentiators) that

transmit only the dynamic portion of the signals The ΔPE and Δf signals obtained in this

way are then weighted (multiplied by) with the factors K1 and K2 and sent to the

summing point of the voltage regulator

The PSS stabilization signal is imposed on the automatic voltage regulator only if

the following prerequisites are met

bull Generator on line

bull Generator power output gt the value F338

bull Generator voltage in a range between F33C and F33A

The stabilization signal is limited at the output from the PSS to the lower and

upper limits Flag defines whether the stabilization signal is introduced before or after the

PID filter (usually before the filter) Because the PID filter as noted above already takes

the ceiling factor Vp1 into account the PSS signal needs to be multiplied by Vp1 if it is

added to the voltage regulator following the PID filter (divider at the input to the

minmax limiter) This precaution prevents the DC filter ldquoDrdquo in the P-channel from

producing an unnecessary ldquostabilizationrdquo effect in the case of rapid changes in turbine

load As an alternative for the AVRrsquos Power System Stabilizer a stabilization signal from

an outside system can be imposed by activating the binary input ldquoPSS-SIGNEXTrdquo Flag

F340 can be used to select between an analog and a 12-bit signal and F33E to select the

polarity desired for that signal

24 The MANUAL Channel

241 Summary

The MANUAL channel (Channel 2) has been built as a simple field current

regulator

without additional limiters Its main function is to maintain the excitation of the generator

even if the AUTOMATIC channel becomes non-operational The MANUAL channel

also performs valuable service for purposes of testing commissioning and preventive

maintenance Its measurements regulator generation of firing pulses and power supply

are physically separate from those on the AUTOMATIC channel

242 Principle of Operation

All the functions of the MANUAL channel including the generation of firing

pulses have been implemented in a single electronic module the Gate Control Unit The

control variable Ucontr of voltage regulator is used as the reference value for generating

firing pulses on the principle known as ldquoramp controlrdquo (Comparison of Ucontr with

Usynsynchronous sawtooth signal) For further processing in the UN 0096 Intermediate

Pulse Stage the Gate Control Unit supplies six firing pulses at its output whose phase

position with respect to the synchronous voltage Usyn is in accordance with control

variable Ucontr An internal linearization ensures that the field voltage produced via the

firing pulses remains proportional to the control variable Ucontr throughout the entire

range As a result the circuit amplification of the control remains constant over the entire

range Whenever excitation is switched ON the set-point for Generator Voltage is set

automatically at the preset - ref Value This provision ensures that the generator voltage

always attains approximately its nominal value after the field flashing The Gate Control

Unit can be refunctioned ( by pre-selection with a switch ) for purposes of testing to act

as a purely firing pulse control In this case the control variable Ucontr is adjusted

directly using the RAISELOWER push buttons on the front of the module In this way

for example the relationship between the phase position of the firing pulses and the

control variable Ucontr can be checked easily

CHAPTER 3

PULSE SECTION

31 Pulse Generation and Amplification

The Gate Control Units of both AUTOMATIC channel and MANUAL channel

each supply six firing pulses for operating the 6-pulse thyristor bridges The low-power

pulse signals from these Gate Control Units are then amplified in the Intermediate Pulse

Stage galvanically isolated and then sent to the common pulse bus On the output end

the Intermediate Pulse Stage of the non-active channel) is always blocked The Gate

Control Units generate the pulses based on microprocessor control The reference voltage

used for the firing pulse phase location is the output voltage from the excitation

transformer (Usyn1 Usyn2) The commutation spikes of the synchronous voltage caused

by the converter are calculated prior to use of the voltage as a reference value and are

deliberately filtered out The lower limit for the firing pulses (double pulses) which are

offset from one another by 60deg is defined by the limit rectifier position (αmin) and the

upper limit by the limit inverter position (αmax) for the firing angle αmin and αmax can

be adjusted on the Gate Control Units using BCD (Binary Coded Decimal) switches

αmin ensures that the firing pulses will not be issued (premature firing) until there is

sufficient positive phase voltage on the thyristor involved αmax prevents a dangerous

ldquotippingrdquo of the thyristor bridge into the rectifier mode if the firing angle α is too large

(ldquolate firingrdquo) The critical factors determining αmax are the overlap time uumlmax (max

commutation time) and the ldquorecovery timerdquo of the thyristors (αmax lt 180deg - uumlmax - γ )

An external control signal can force the firing pulses into their inverter limit position

Other binary inputs can block or direct the firing pulses of the Gate Control Units so as to

produce freewheeling on the thyristor bridge During freewheeling the firing pulses for

the thyristor pair R and S are blocked and the pulse signals T+T- are engaged with

chains of pulses Both Gate Control Units (for the MANUAL amp AUTOMATIC channels)

contain a field current monitor that blocks the firing pulses immediately whenever the

current exceeds a preset threshold level In this case the field circuit-breaker is also

tripped via an output contact The purpose of these provisions is to prevent damage to

thyristors and thyristor fuses in case of a slip-ring short-circuit or to keep any damage

that does occur to a minimum The pulse signals are galvanically separated at the outputs

from the Intermediate Pulse Stage (with pulse transmitters) and are then directed to the

common pulse bus This transmission of the pulse signals to the pulse bus via passive

transmitters ensures a high degree of active channel autonomy Practically no possible

malfunctions on the inactive channel (including for example sustained pulses) affect the

active channel

32 Pulse Monitoring

The ldquoPulse Busrdquo and the pulse signals of the AUTOMATIC channel are

monitored This monitoring device consists of potential isolating stages and the common

monitor If the pulse monitoring of the ldquoPulse Busrdquo responds a switch-over is made to

MANUAL channel The function of the potential isolating stages is to couple the pulse

monitoring device to the pulse circuits without any feedback effect The pulse monitoring

checks the six pulse lines for the following malfunctions continuous or periodic failure

of one or more pulses Periodic occurrence of synchronous or asynchronous false pulses

Continuous pulses the pulse monitoring device can be tested while the machine is in

operation

CHAPTER 4

CONVERTER

Thyristor

The term thyristor usually refers to a family of four layer solid state device having

turn on characteristics that can be externally controlled by either current or voltage They

are also referred to as breakdown device because their working depends on avalanche

breakdown Thyristors have only two stages OFF and ON Thyristors have a similar

function to Uni-junctions they act as switches Thyristors use current flow as a switch

Thyristors have three states

1 Reverse blocking mode mdash Voltage is applied in the direction that would be

blocked by a diode

2 Forward blocking mode mdash Voltage is applied in the direction that would cause

a diode to conduct but the thyristor has not yet been triggered into conduction

3 Forward conducting mode mdash The thyristor has been triggered into conduction

and will remain conducting until the forward current drops below a threshold value

known as the holding current Converter is a semiconductor device which converts ac

input voltage into a constant dc output voltage In present excitation system three phase

fully controlled thyristor converter is used

Because of the following advantages thyristor converters are used

a) Thyristors are used for high power applications ie up to 10Kv3500A1KHz

b) Having high reliability and low losses

c) Uni-directional device like diode

d) Itrsquos operation as a rectifier which are low resistance in forward conduction

mode and high resistance in reverse conduction mode

PROTECTION OF THYRISTORS

For reliable operation of a thyristor demands that its specified ratings are not

exceeded When Subjected to or over voltages During the turn - on of SCR didt

prohibitively large False triggering of SCR by high value of dvdt andSpurious signals

between gate and cathode may leads to unwanted turn ndash on

DIDT AND PROTECTION

When thyristor starts conducting in forward conduction mode and is turned on by

gate pulse The anode current increases rapidly whole area of the gate to Cathode

junction then hot spots will be formed near the gate connection this locality of heating

destroys the thyristor Thyristor thermal time is constant The causes due to faults and

short circuits or surge currents Electronic crowbar protection is used against the over

voltages The rate rise of anode current must be kept at the time of turn on below the

rated or specified limiting value The didt value maintained below limited value by using

a inductor also called ldquodidt inductorrdquo in series with anode circuit The locality of heating

is avoided by applying gate current but not greater the maximum gate current

DVDT AND OVER VOLTAGE PROTECTION

With forward voltage across the anode and cathode of a thyristor the two outer

junctions are forward biased but the inner junction is reverse biased This reverse biased

junction J2 has the characteristics of a capacitor due to charges existing across the

junction In other words space-charges exist in the depletion region around junction J2

and therefore junction J2 behaves like a capacitance If the entire anode to cathode

forward voltage Va appears across J2 junction and the charge is denoted by Q then a

charging current i given by Eq (46) follows

i = dQdt =d(Cj Va )dt

= Cj (d Va dt) + Va(d Cj dt) helliphelliphelliphellip(46 a)

As Cj the capacitance of junction J2 is almost constant the current is given by

i = Cj (d Va dt) helliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip(46 b)

If the rate of rise of forward voltage dVadt is high the charging current i will be

more This charging current plays the role of gate current and turns on the SCR even

when gate signal is zero Such phenomena of turning-on a thyristor called dvdt turn-on

must be avoided as it leads to false operation of the thyristor circuit

For controllable operation of the thyristor the rate of rise of forward anode to

cathode voltage dVadt must be kept below the specified rated limit Typical values of

dvdt are 20 ndash 500 Vμsec False turn-on of a thyristor by large dvdt can be prevented by

using a snubber circuit in parallel with the device thyristor are very sensitive for over

voltage than the semiconductor devices

Over voltage transients are perhaps the main cause of thyristor failure

In thyristor there are mainly two types

1 Internal over voltages

Due to the commutation of the thyristors large voltages are generated internally

Because of the series inductance of the SCR circuit the large transient voltages L didt

produced This voltage several times the break over voltage of the device then thyristor

destroys permanently

2 External over voltages

External over voltages are caused due to the interruptions of current flow in an

inductive circuit and also due to the lightening strokes on the lines feeding the thyristor

system For the reliable operation of thyristor the over voltages must be suppressed by

adopting suitable techniques

Suppression of over voltages

The RC circuit called snubber circuit is connected across the device to protect In

order to keep the protective components to a minimum the thyristors are chosen with

their peak voltages ratings are 25 to 3 times of the normal peak working voltage

ldquoselenium thyrector diodes metal oxide varistors or avalanche diode suppressers are

commonly employed for protecting the thyristor circuit against the over voltages

Gate protection

Gate circuit should also be protected against the over voltages and surges Over

voltage at gate circuit can cause false triggering of the SCR may rises the junction

temperature behind specified limit leading to its damage Protection against over

voltage can be achieved by connecting a ZD across the gate circuit and a resister is

connected in series with gate circuit to protect against the s A capacitor and resister are

connected across gate to cathode to by pass the noise

41 Final Pulse Stages

The Final Pulse Stages adapt the output pulses from the Intermediate Pulse Stage

(pulses on the pulse bus) to the gate currents needed for the thyristors Each thyristor

bridge is equipped with its own Final Pulse Stage Each Final Pulse Stages is provided

with a power supply module The amplified output pulses from the Final Pulse Stages

start as a short strong steep pulse with an amplitude approx 2frac12 times that of the main

pulse This initial pulse edge assures proper firing of the thyristors being triggered

Subsequently the weaker part of main pulse keeps firing conditions steady As already

mentioned the Final Pulse Stages and their associated thyristor bridges form single units

All six pulse outputs from a Final Pulse Stage can be blocked by an external control

signal so that all thyristors in the associated thyristor bridge will block the current A

blocking of the pulses is initiated whenever there is a malfunction in the associated

thyristor bridge

42 Converter Power Section

The thyristor converter consists of three independent parallel rectifier blocks TY1

to TY3 which are all in service Even if one block fails the remaining blocks take over

automatically the full design current of the excitation circuit During normal operation

(with ideal current share) and all three bridges in operation each of these blocks has to

carry only (n-2)n (ie33)of its design current If 2 thyristor bridges fail the excitation

is limited Only when all three bridges fail the excitation is switched off Each thyristor

bridge arm is equipped with current flow monitoring CTrsquos Failure of conduction in any

arm is identified by a Current flow monitoring module

43 Converter Cooling

A cooling system is needed to dissipate heat losses in the converter blocks and

electronics Each converter block has therefore been equipped with a fan supplied with

power from the converterrsquos primary voltage (via transformer ndashT8 in field flashing

cubicle) The fans are protected with motor protection circuit breakers An air flow

monitoring unit is provided for monitoring the air flow through the thyristor bridge If a

circuit breaker failure is detected or if the air flow monitor drops off at one of the

thyristor bridges the bridge involved is immediately set out of operation by blocking its

firing pulses

44 Thyristor Converter Monitoring

A thyristor bridge in which defects occur that could threaten the safety of

operation or cause secondary damage is switched off automatically ie its firing pulses

are blocked This happens whenever A thyristor fuse is blown The fuses are monitored

individually with micro switches The Final Pulse Stage fails which is detected by

internal monitors (supply voltage sustained pulse short-circuit on the output end) The

power supply to the fan fails fan air flow as monitored by the Air flow monitor fails or

is insufficient Isolator on ACDC side is open

CHAPTER 5

Field Current Circuit Excitation Transformer

51 Field Circuit Breaker

The circuit-breaker in the field circuit is used to isolate the field circuit from the

converter It is capable of switching off the synchronous machine from full load under the

maximum conditions of a 3-phase short-circuit In addition to its main contacts the field

circuit-breaker also has a de-excitation contact with which the field energy stored in the

field can be dissipated across the de-excitation resistor The de-excitation contact closes

shortly before the main contacts open so as to ensure proper commutation of the field

current from the main contacts to the de-excitation contact when the breaker is switched

off The field circuit-breaker is switched on by electromagnetic force and is kept switched

on by a mechanical latch When the latch is released by a trip coil the circuit-breaker

opens The circuit-breaker also has auxiliary contacts that report its status

52 Field Flashing

In shunt supplied excitation circuits (excitation transformer connected to the

generator terminals) the generator does not have enough remnant voltage for a generator

voltage build-up via the converter In this case a field flashing circuit is provided It

consists of the field flashing contactor the diode bridge and a transformer used to adapt

the auxiliary input voltage to the voltage needed for field flashing when power is

supplied from the auxiliaries network

Fig Field Flashing

Because the field flashing contactor is not able to switch off the energy stored in

the field the control ensures that the contactor can only reopen if the field circuit breaker

has already been opened (generating the TRIP order) or in a normal field flashing

sequence when the converter has taken over the field current Field flashing occurs in the

following stages

1048729The excitation is switched on closing the field flashing contactor ( Field

Circuit Breaker is already closed )

1048729The start-up excitation current flows through the rotor driving the generator

voltage up to approx 15 U

1048729After about 10 U the firing pulses to the converter are released and it begins

to excite the generator to its rated voltage

1048729After about 30 U the field flashing contactor opens (with no current since

the converter is now supplying the current)

The diode bridge at the input to the field flashing contactor prevents a feed-back

from the converter to the source of field flashing while the contactor is still closed

53 De-excitation

When malfunctions occur the stored field energy must be dissipated as quickly

and safely as possible to protect the generator This is done by the converter the field

circuit-breaker and the de-excitation (discharge) resistor

De-excitation (with opening of the field circuit-breaker) takes place in the following

stages

1048729The converter drives to its inverter limit position (negative ceiling voltage)

recovers a portion of the field energy into the network A trip command is given to the

field circuit breaker

1048729The de-excitation contact closes diverting the field voltage to the de-excitation

resistor

1048729Then immediately the main contacts open building voltage The field voltage

commutates to the de-excitation resistor

1048729The current diminishes at a given time constant TE

(With linear resistance TE = Lf (Rf + Re))

Due to the reversal of the field voltage by the converter the field current

commutates from the main contacts of the field circuit-breaker to the de-excitation

resistor in a very early phase This reversal of the field voltage prevents burn-off on the

main contacts and provides effective protection for the field circuit-breaker Depending

on the operating policy an operational shut-down of the excitation can also be effected

with the field circuit-breaker closed This method is useful mainly when the excitation is

switched on and off frequently In this case the converter is merely driven into the

inverter limit position so that the field energy is recovered into the network The

converter then blocks since it is supplying positive current only

54 Excitation Transformer

The excitation transformer matches the generator voltage to the field voltage

(required ceiling voltage) It also serves as a commutation reactance for the thyristor

converter and as a potential isolator between the network and the excitation circuit In

addition the transformer functions as a current limiter in that it makes it possible to keep

any short circuits in the excitation circuit under better control The excitation transformer

is equipped with temperature monitoring probes which set off an alarm when the

temperature exceeds a first max limit and then trips the excitation if the temperature

continues rising to a second (higher)limit

CHAPTER 6

Monitoring and Protection

61 Excitation Monitoring

611 General Information

The main goal of Excitation Monitoring is to make optimum use of the

redundancies provided in the excitation system and to give alarm whenever a malfunction

makes these redundancies unavailable The field current is monitored to see that it does

not exceed a maximum level and if necessary a switch-over to the MANUAL channel is

initiated In addition the criterion for switching off the field flashing is generated The

excitation Monitoring consists of an autonomous processor system

612 over current Alarms

In the Excitation Monitoring the limits for are set at higher levels than the

settings on the Field Current Maximum Limiter Whenever the current exceeds 110 of

the nominal field current contact R1 and the binary output associated with it are

activated immediately If field current remains gt 110 then after a preset inverse-time

has lapsed relay R2 and - after a further delay - relay R0 and the binary outputs

associated with them are activated Parameters match the measurements for If1 and If2 to

the nominal value for field current so that the internal values can be processed and read as

pu values It can be used to falsify the actual value of the field current If (to raise it) so

as to cause a response from the alarm limits for purposes of testing The processed If

signal is always taken from on the active channel (CHANNEL I OR CHANNEL II) As

long as the field current If is above the threshold value 11 Ifn its peak value is

measured This is stored (until RESET) and can be read at any time on the Micro-

Terminal Once the value of If exceeds 11 Ifn integration of this value starts Whenever

the integrated time-current value (intisup2dt) exceeds the preselected reference value the

alarm OVER CURRENT INVERSE-TIME is set off and a command is simultaneously

issued to switch over to the stand by AUTO channel Software switch F758 enables the

three over current alarm functions (R0 R1 R2) and selects one of three possible inverse-

time curves T1 T2 or T3 Within the characteristic curve (T1 T2 T3) selected the

desired limit curve for response is set using the factor F216

613 Switch-Off Criterion for Field Flashing

The Excitation Monitoring supplies the criterion for switching off the field

flashing Whether this criterion is activated based on the actual value for generator voltage

Ug or for field current If or both depends on the settings of the two threshold values

F200F202 (0 setting means that the output is always ldquological 1rdquo) The measurements

Ug12 and If12 are switched over depending on the present status of the channels

(Channel 1 or Channel 2 ON) Whenever Flag F750 is not activated the binary output is

fixed at ldquological 1rdquo

614 Storage of Alarm Status

The outputs of the over current alarms (R1 R2 R0) and the messages NO

FAILURE MONITORING PARAMETERS CHANGED are stored messages can be

erased by activating the input ldquoGENERAL RESETrdquo or by using the RESET button on the

front of the module Erasure with the input ldquoGENERAL RESETrdquo is effective only if the

situation causing the alarm or the malfunction is no longer present Whenever the self-

diagnosis equipment in the processor detects a malfunction the output NO FAILURE of

MONITORING is set at ldquological 0rdquo (= alarm) The alarm ldquoPARAMETERS CHANGEDrdquo

is activated whenever parameters or settings of software switches have been changed via

the Micro-Terminal

615 Actual Value Monitoring

The actual values for generator voltage Ug synchronous voltage Usyn and field

current If are monitored for malfunctions This monitoring is active regardless of whether

or not the generator is in operation Essentially when the generator is in operation the

measurements are monitored by comparing the signals (the smaller signal reading is

detected as incorrect) When the generator is not in operation the measured data are

monitored for extreme values The percentage of deviation permissible in the

measurement signals being compared

(Ug1ampUsyn1 Ug1ampUg2 Ug2ampUsyn2 If1ampIf2) is defined by parameters F208 and

F20AIf the excitation transformer is being supplied from an auxiliary power source (no

shunt operation) the values of Ug and Usyn will be different in some operational

conditions

In that case Flag F75A can be used to deactivate comparative measurements Ug1 Usyn1

and Ug2 Usyn2 The ldquoprimary monitoringrdquo compares Ug1 with Ug2 and generates the

messages that Channel 1 or Channel 2 has suffered a malfunction Whenever Ug1 lt Ug2

and the binary message from CH1 reports no malfunction a malfunction on

Measurement Channel 1 is reported (Ug1Usyn1 FAILURE) A similar malfunction is

also present whenever the binary message CH1 DISTURBANCE is reported and a

discrepancy is detected between Ug1ampUg2 The generation of the alarm ldquoUg2 Usyn2

FAILURErdquo is analogous to that for Channel 1 The ldquosecondary monitoringrdquo compares

Ug1 with Usyn1 amp Ug2 with Usyn2 This is enabled whenever the binary message of the

comparison channel reports a malfunction or whenever both binary messages report no

malfunction - but both secondary monitors report a malfunction As long as the secondary

monitoring is blocked the differences Ug1neUsyn1 or Ug2neUsyn2 trigger malfunction

signals for the measurement channel involved (suspicion that there is a corresponding

error in Usyn) The measurement channel malfunctions are enabled operationally

whenever after excitation has been switched on generator voltage Ug exceeds the value

set on F204 The voltages Ug1 and Ug2 are checked 16 seconds after the excitation is

switched off to see that they do not exceed the limit value F210 that applies to both of

them At the same time g1Ug2Usyn1Usyn2 are checked for extreme values (gt or lt

the operating range) Monitoring for extreme values is likewise enabled during normal

operation ( Excitation ON and Ug gt F204 ) Flag F754 is used to enable or block the

malfunction signals to the binary outputs Basically the monitoring of the actual values

for If1If2 functions like that of the Ug1Ug2 monitoring

62 Excitation Protection

621 General Information

The Excitation Protection switches off the excitation (and de-excites the machine

rapidly) whenever a danger arises that threatens the excitation transformer the converter

or the generator Generally limiter or monitoring functions precede the emergency trips

and these normally respond before the Excitation Protection must initiate a trip

Protective trip commands are issued directly to the field circuit-breaker from potential

free contacts of the board via the trip relays They are directed redundantly to the

operative field circuit-breaker ldquoOFFrdquo command

622 Protection against Excitation Transformer Overheating

This equipment monitors the excitation transformer for overheating in the

windings that could result from over current short-circuits or inadequate cooling The

monitoring uses temperature monitoring modules in conjunction with temperature

sensors built into the windings Normally the temperature is monitored in two stages the

first stage sets off an alarm the second causes a trip of the excitation

623 Rotor Over voltage Protection

Malfunctions in the generator circuit (eg terminal short-circuit failed

synchronization asynchronous operation) cause induced negative field currents that

produce high voltages in the field circuit These must be restricted to a level with a

sufficient safety margin below the insulation capacity of the field winding (test voltage)

and also below the peak blocking voltage of the converter thyristors The crow bar

employs spark gap elements to detect over voltages in the field circuit Whenever they

respond the associated thyristors are fired immediately switching the de-excitation

resistor parallel to the field The de-excitation current generated thereby initiates an

excitation trip via a supervision circuit causing an immediate opening of the field circuit-

breaker The malfunction isets off an alarm and an internal malfunction is indicated at the

cubicle

TEST VALUES OBTAINED WHEN EXCITATION IS RAISED

TEST

SNO PARTICULARS ACTUAL VALUE OBTAINED VALUE

1 VREF 996 100

2 VACT 997 1003

3 IFACT 735 765

4 IGACT 703 707

5 ACTIVE

POWER

703 705

6 REACTIVE

POWER

101 142

7 POWER

FACTOR

099 IND 098 IND

8 ACTIVE

CURRENT(IR)

705 703

9 REACTIVE

CURRENT(IX)

102 138

10 POWER

ANGLE

566 546

11 FIRING

ANGLE

640 632

VALUES OBTAINED WHEN EXCITATION IS LOWERED

SNO PARTICULARS ACTUAL

VALUE

OBTAINED VALUE

1 VREF 100 997

2 VACT 100 997

3 IFACT 787 761

4 IGACT 839 837

5 ACTIVE

POWER

839 830

6 REACTIVE

POWER

155 110

7 POWER

FACTOR

098 IND 099 IND

8 ACTIVE

CURRENT(IR)

835 830

9 REACTIVE

CURRENT(IX)

140 94

10 POWER

ANGLE

613 DEG 631 DEG

11 FIRING

ANGLE

601 DEG 599 DEG

CONCLUSION

For generating the EMF in stator winding excitation is required to the rotor of a

generator There are two types of excitation

1 Static excitation system

2 Brushless excitation system

A certain disadvantage in brushless excitation system is the slow response time of

the field in case of fast load changes specified No slip-rings and brushes direct

measurements of the field parameters not possible

To avoid all loses static excitation is used Since it does not have any rotating

parts mechanical loses and windage loses This system has fast response and speed

control While preferring this excitation system there are no limitations for the

redundancy of Thyristor bridge circuits

Static excitation has fast field discharge by resistor and inverter operation direct

measurement of field quantity is possible The meaning of excitation is nothing but

continuous supply of DC current (ie field current) to the rotor to buildup required

output voltage in the stator

Field current is changed with respect to the change of load so the digital

automatic voltage regulator (DAVR) is used to regulate the output voltage according to

the load variations

So we conclude that static excitation system with DAVR is preferred since it is

having excellent dynamic performance and better options for R amp M

intervened The ldquocontrol deviationrdquo at the input to the PID filter is either the control

deviation for voltage the control deviation of a de-excitation limiter (the value

determined by minimum value selection) or the control deviation of an excitation limiter

(the value determined by maximum value selection) Flag F730 (ldquoPRIORrdquo) is used to

determine whether the exciting (Min value) or the de-exciting signal takes precedence on

the minmax value limiter (normally F730 = 1111 ie the de-exciting signal takes

precedence) With the exception of the Minimum Field Current Limiter all other limiters

have variable factoring multipliers of the signal outputs so that they can be adjusted

individually together with the common PID filter which has been optimized for voltage

regulation The setting parameters for this PID filter are as follows

Vo = KR Static amplification

1

Ta = ---- Integration time constant

Tc1

Vp Proportional amplification

1

Tb = ---- Differential time constant

Tc2

Vinfin Amplification of high frequencies

The BODE diagram below shows the assignment of settings in accordance with

DINIEC standards based on a typical example

The PID filter amplifications Vo Vp and Vinfin can be adjusted in pu values But

the ldquoceiling factorrdquo pl+ must be adjusted correctly with parameter F310 if the total

amplification (circuit amplification) of the control circuit is actually to conform to the

pu settings This factor must agree with the ldquoexternalrdquo amplification ie with the

ceiling value of the transformer- converter circuit

Ceiling factor(pl+) = Ufmax Ufo

in which Ufmax = ceiling field voltage

Ufo = no-load field voltage

To attain a suitable response of the AVR when starting excitation

(ldquoEXCITATION ONrdquo) it may be necessary to change the proportional amplification of

the regulator during this phase Vp2 (transiently activated) and Vp1 (permanently

activated) can be adjusted for this purpose For example the value of Vp2 takes effect

immediately once the excitation is switched on and remains effective for a period as set at

F30C Once the period F30C (eg 5 sec) has expired Vp shifts over to Vp1 (becomes

the steady-state Vp) at the rate of change set The standard operating mode for the PID

filter is voltage regulation for which the discrepancy between the voltage set-point and

the current value for generator voltage Ug (the control deviation) is supplied at the input

To compensate for the voltage drop in the block transformer or whenever several

generators are operating to the same distributing bus the generator voltage must be

varied in proportion to the measured generator current (droop influence) To accomplish

this the voltage set-point is varied as a function of the measured reactive current IX

andor active current IR Flag F712 enables the IX droop Flag F710 the IR droop The

desired compensation is set in F282 and F286 respectively Flags F284 and F288 are used

to select whether this droop influence is to increase the voltage or to reduce it

(compensation) Combined influence of the active and reactive currents is attained by

enabling both droops IX and IR Flag F716 activates a so-called ldquoSoft-Startrdquo at the

starting of excitation This ldquoSoft-Startrdquo ensures that the voltage set-point integrates from

0 to 100 within the time set on F290 when the excitation is switched on

(ldquoEXCITATION ONrdquo) A ldquosmoothrdquo excitation of the generator can be achieved in this

way whenever there is no demand for a quick excitation

232 Voltage Set-Point

Various signals and settings control and limit the voltage set-point F270 For

example the values of F254 and F252 define the normal operating range possible for set-

point adjustment (eg 90 110) using external control commands (control room local

operatorrsquos panel superposed control system) The effective set-point adjustment rate is

governed The set-point can be set at the values of F250 and F256 by activating

appropriate control commands for ldquoSETrdquo input Enabling Flag F71A and activating a

binary input prior to switching on the excitation (ldquoEXCITATION OFFrdquo) sets the Ug set-

point at the value of UAUX This makes it possible for example to ensure that the

generator voltage will agree exactly with the network voltage after the voltage build-up

An external value with variable amplification can be added to the Ug set point by

enabling F724 (for example for stability tests)

233 Regulator Tracking in MANUAL Operation

Whenever the AUTOMATIC channel is not in operation (the MANUAL channel

is ON) a follow-up equipment ensures a smooth switch-back to the AUTOMATIC

mode will always be possible To track the voltage set-point is shifted by means of

RAISELOWER pulses from the Gate Control Unit so that control variable Ucontr at the

output from the PID filter is held steady and identical to the control variable Ucontr from

the MANUAL channel Because this tracking must react slowly resultant transient

control deviations resulting from the amplification in the PID filter might cause severe

interference with control variable Ucontr

To prevent this the follow-up equipment intervenes on the regulators mixing

point with a corresponding compensation signal

234 Ugf Limiter

At under frequency the Ugf Limiter reduces the generator voltage so as to

prevent saturation effects in the supply and measuring transformers To adjust this

limiter the max permissible generator voltage at rated frequency is defined and set

When any under-frequency occurs the generator voltage is thus reduced in proportion to

that setting

235 Field Current Maximum Limiter

The Field Current Maximum Limiter is provided to protect the generator rotor

from s occurring in steady-state and transient operation High field currents are normally

the result of a sharp drop in network voltage or of an improper raising of the voltage set-

point by the operating staff The field current is held steady at the value TH1 ie at the

maximum thermal value permissible for the excitation circuit and the rotor In order that

the generator can support the power network with its transient overload capacity during

brief collapses in voltage a temporary switch-over is made to the transient limit MAX1

(a higher setting) When the generator or the converter is operating at a reduced capacity

These limits TH1MAX1 can be switched over to the lower settings TH2MAX2 by

activating the corresponding binary signals The switch-over from the thermal limit

TH12 to the transient limit MAX12 can be configured in one of three ways

a) Depending on the over current with -dUdt ENABLE

Flag programming F418 = any setting desired F41A = 0000

This variant enables the transient value MAX12 whenever a collapse of voltage

in the network is detected The ENABLE time is fixed and can be set The example

below shows the typical behavior of the limiter configured in this way

b) Dependent on the time integral with -dUdt ENABLE

Flag programming F418 = inactive F41A = 1111

This variant likewise enables the transient value only when a collapse of network

voltage has been detected However the switch-back to the thermal limit is not made

dependent upon the time itself but on the calculated time integral intisup2dt of the The setting

on Parameter F414 in spu takes into account the time the rotor needs to cool down ie

the rate of temperature change in the case of intermittent operation The example below

shows how the timing of the switch-back to the thermal limit depends on the present

value for intisup2dt

The time integral is based on the formula

Example The setting of =isup2dt equivalent to Version a (F416) at a constant 16 times the

nominal field current for 10 seconds (with TH12 = 105) is

c) Dependent on the time integral without any preconditions

Flag programming F418 = 1111 F41A = 1111

In this variant the transient becomes available without any prior conditions

(without a -dUdt ENABLE) with the time integral intisup2dt

237 Inductive Stator Current Limiter

The Inductive Stator Current Limiter holds the stator current Ig within permissible

limits while the generator is in the ldquoover-excitedrdquo operating range by reducing the field

current accordingly The setting TH (thermal limit) provides the limit against stationary s

that might occur To take advantage of the generatorrsquos transient overload capacity a

switch-over is made to the higher setting MAX The principle of operation of this switch-

over to the value MAX permissible only transiently is identical to that employed for the

field current limiter (refer to the description above) When the drive output from the

turbine is very high stator current may exceed permissible limits even while inductive

loading of the generator is low In this case if the stator current limiter is not kept from

influencing the field current the control circuit will oscillate back and forth between the

Inductive Stator Current Limiter (de-

exciting) and the Capacitive Stator Current Limiter (exciting)The output signal of that

function then dominates the control variable of the Ig-dependent limiter via a maximum

value selection

238 Capacitive Stator Current Limiter

239 Load Angle Limiter

The Load Angle Limiter prevents the synchronous machine from slipping out of

phase due to slippage of the rotor The load angle δ the difference in phase between the

rotor and the stator rotating field results mainly from the driving torque (active power P)

acting on the generator and the level of rotor current (field current) If the driving torque

remains constant a increase in the field current reduces the load angle δ The current load

angle δ at any moment is obtained from the generator current and generator voltage based

on a simplified model of the generator Whenever this calculated load angle δ exceeds the

preset limit angle the limiter increases the field current until the load angle has dropped

back to its permissible value The quadrature reactance Xq of the generator and the

network reactance Xe during normal operation must be adjusted on the regulator in order

to obtain the load angle δ The graph below shows the Power Chart for a salient-pole

machine with typical limiter characteristics

The purpose of a Power System Stabilizer is to use the generator excitation to

damp electromechanical oscillations between the network and the generator Depending

on the design of the generator and the requirements imposed for network stability its

main function will be either to damp the oscillations originating in the machine or those

from the network A synchronous generator working in a combined power network is in

principle an oscillating structure In order to produce a torque the magnetic field of the

rotor and the stator must form a given angle (referred to as the rotor displacement or load

angle δ) The electrical torque ME increases as the angle δ increases just as with a

torsion spring Because the ME of the generator and the mechanical driving torque MA

from the turbine are in equilibrium during steady-state operation the angle δ remains in a

given position Whenever this state of equilibrium between MA and ME is disturbed the

load angle slips of this rest position and change thereby the electrical torque ME The

torque attempts to restore the load angle to a stationary position Due to the mass inertia

of the turbinegenerator rotor however this can only take place aperiodically It does so

in the form of more or less effectively damped oscillations (again similar to the effect of

mass inertia on a torsion spring) In order to damp the oscillations there must be a

damping torque produced depending not on the electrical torque ME associated with the

angle but on the difference in frequency (Df) between the rotor and the stator rotating

field ie on the slippage This torque is produced mainly by the so-called damper

winding in the rotor but the dimensioning of this is subject to limits imposed by

considerations of design and economy Some further action is therefore needed to

increase the damping effect The following drastically simplified formula shows the

parameters upon which the amount of active power PE supplied by the generator

depends

PE = active power

It can be seen from the above relationship that the active power that the generator

transfers depends not only on the load angle δ but also on the field current If That means

that a transient change can be made in the active power PE and with that in the effective

electrical torque ME by varying the field current The principle of operation of the DVR

Power System Stabilizer becomes clear from a consideration of the oscillations in power

output and frequency (ΔPE Δf) and the vector diagram If it is assumed that oscillations

in the network frequency generates load oscillations with the mass inertia of the rotor

then the active load of the generator (eg MW-measured) is influenced with a sinusoidal

value -ΔPE (ME-MA = -ΔPE) By inversion of -ΔPE one obtains the fluctuation in

power provided by the rotor +ΔPE As is known the slip signal Δf follows +ΔPE with a

phase delayed by 90deg The +ME produced by the periodic changes in the load angle δ is

in phase with +ΔPE A good damping is attained if ME is varied in phase with the slip

Δf However this signal must also be advanced somewhat to compensate for the time

constants in the excitation circuit and the generator

As mentioned above the electrical torque ME can be influenced by varying the

field current To accomplish this a suitable control signal referred to as variable

disturbance compensation must be imposed upon the voltage set-point or the converter

control variable Ucontr As can be seen from the vector diagram by applying proper

weighting factors (K1 K2) and then adding together the signals -ΔPE and Δf an overall

stabilization signal can be produced that rotates in advance of the Df signal by any angle

desired between 0deg and 90deg Because the amplitude of -DPE remains proportional to the

amplitude of Δf a constant angle in advance of Δf results for the compensation of the

time constants referred to above The optimum weighting factors K1 and K2 for a

synchronous generator working to a power network depend on its operating point at any

moment and the external reactance of the network Normally the selection of a

compromise setting is good enough to attain stability in all operating points and for all

external reactance For special demands these settings must be parameterized as a

function of the external reactance (which means optional equipment Xe-Identification)

The Power System Stabilizer PSS is a section of the AVR computer program and is

processed once per network cycle The voltage at the generator terminals and the

generator current are measured in order to define the signals ΔPE and Δf The calculated

signals for _P_ (=PE) and Δf are then sent across DC filters ldquoDrdquo (real differentiators) that

transmit only the dynamic portion of the signals The ΔPE and Δf signals obtained in this

way are then weighted (multiplied by) with the factors K1 and K2 and sent to the

summing point of the voltage regulator

The PSS stabilization signal is imposed on the automatic voltage regulator only if

the following prerequisites are met

bull Generator on line

bull Generator power output gt the value F338

bull Generator voltage in a range between F33C and F33A

The stabilization signal is limited at the output from the PSS to the lower and

upper limits Flag defines whether the stabilization signal is introduced before or after the

PID filter (usually before the filter) Because the PID filter as noted above already takes

the ceiling factor Vp1 into account the PSS signal needs to be multiplied by Vp1 if it is

added to the voltage regulator following the PID filter (divider at the input to the

minmax limiter) This precaution prevents the DC filter ldquoDrdquo in the P-channel from

producing an unnecessary ldquostabilizationrdquo effect in the case of rapid changes in turbine

load As an alternative for the AVRrsquos Power System Stabilizer a stabilization signal from

an outside system can be imposed by activating the binary input ldquoPSS-SIGNEXTrdquo Flag

F340 can be used to select between an analog and a 12-bit signal and F33E to select the

polarity desired for that signal

24 The MANUAL Channel

241 Summary

The MANUAL channel (Channel 2) has been built as a simple field current

regulator

without additional limiters Its main function is to maintain the excitation of the generator

even if the AUTOMATIC channel becomes non-operational The MANUAL channel

also performs valuable service for purposes of testing commissioning and preventive

maintenance Its measurements regulator generation of firing pulses and power supply

are physically separate from those on the AUTOMATIC channel

242 Principle of Operation

All the functions of the MANUAL channel including the generation of firing

pulses have been implemented in a single electronic module the Gate Control Unit The

control variable Ucontr of voltage regulator is used as the reference value for generating

firing pulses on the principle known as ldquoramp controlrdquo (Comparison of Ucontr with

Usynsynchronous sawtooth signal) For further processing in the UN 0096 Intermediate

Pulse Stage the Gate Control Unit supplies six firing pulses at its output whose phase

position with respect to the synchronous voltage Usyn is in accordance with control

variable Ucontr An internal linearization ensures that the field voltage produced via the

firing pulses remains proportional to the control variable Ucontr throughout the entire

range As a result the circuit amplification of the control remains constant over the entire

range Whenever excitation is switched ON the set-point for Generator Voltage is set

automatically at the preset - ref Value This provision ensures that the generator voltage

always attains approximately its nominal value after the field flashing The Gate Control

Unit can be refunctioned ( by pre-selection with a switch ) for purposes of testing to act

as a purely firing pulse control In this case the control variable Ucontr is adjusted

directly using the RAISELOWER push buttons on the front of the module In this way

for example the relationship between the phase position of the firing pulses and the

control variable Ucontr can be checked easily

CHAPTER 3

PULSE SECTION

31 Pulse Generation and Amplification

The Gate Control Units of both AUTOMATIC channel and MANUAL channel

each supply six firing pulses for operating the 6-pulse thyristor bridges The low-power

pulse signals from these Gate Control Units are then amplified in the Intermediate Pulse

Stage galvanically isolated and then sent to the common pulse bus On the output end

the Intermediate Pulse Stage of the non-active channel) is always blocked The Gate

Control Units generate the pulses based on microprocessor control The reference voltage

used for the firing pulse phase location is the output voltage from the excitation

transformer (Usyn1 Usyn2) The commutation spikes of the synchronous voltage caused

by the converter are calculated prior to use of the voltage as a reference value and are

deliberately filtered out The lower limit for the firing pulses (double pulses) which are

offset from one another by 60deg is defined by the limit rectifier position (αmin) and the

upper limit by the limit inverter position (αmax) for the firing angle αmin and αmax can

be adjusted on the Gate Control Units using BCD (Binary Coded Decimal) switches

αmin ensures that the firing pulses will not be issued (premature firing) until there is

sufficient positive phase voltage on the thyristor involved αmax prevents a dangerous

ldquotippingrdquo of the thyristor bridge into the rectifier mode if the firing angle α is too large

(ldquolate firingrdquo) The critical factors determining αmax are the overlap time uumlmax (max

commutation time) and the ldquorecovery timerdquo of the thyristors (αmax lt 180deg - uumlmax - γ )

An external control signal can force the firing pulses into their inverter limit position

Other binary inputs can block or direct the firing pulses of the Gate Control Units so as to

produce freewheeling on the thyristor bridge During freewheeling the firing pulses for

the thyristor pair R and S are blocked and the pulse signals T+T- are engaged with

chains of pulses Both Gate Control Units (for the MANUAL amp AUTOMATIC channels)

contain a field current monitor that blocks the firing pulses immediately whenever the

current exceeds a preset threshold level In this case the field circuit-breaker is also

tripped via an output contact The purpose of these provisions is to prevent damage to

thyristors and thyristor fuses in case of a slip-ring short-circuit or to keep any damage

that does occur to a minimum The pulse signals are galvanically separated at the outputs

from the Intermediate Pulse Stage (with pulse transmitters) and are then directed to the

common pulse bus This transmission of the pulse signals to the pulse bus via passive

transmitters ensures a high degree of active channel autonomy Practically no possible

malfunctions on the inactive channel (including for example sustained pulses) affect the

active channel

32 Pulse Monitoring

The ldquoPulse Busrdquo and the pulse signals of the AUTOMATIC channel are

monitored This monitoring device consists of potential isolating stages and the common

monitor If the pulse monitoring of the ldquoPulse Busrdquo responds a switch-over is made to

MANUAL channel The function of the potential isolating stages is to couple the pulse

monitoring device to the pulse circuits without any feedback effect The pulse monitoring

checks the six pulse lines for the following malfunctions continuous or periodic failure

of one or more pulses Periodic occurrence of synchronous or asynchronous false pulses

Continuous pulses the pulse monitoring device can be tested while the machine is in

operation

CHAPTER 4

CONVERTER

Thyristor

The term thyristor usually refers to a family of four layer solid state device having

turn on characteristics that can be externally controlled by either current or voltage They

are also referred to as breakdown device because their working depends on avalanche

breakdown Thyristors have only two stages OFF and ON Thyristors have a similar

function to Uni-junctions they act as switches Thyristors use current flow as a switch

Thyristors have three states

1 Reverse blocking mode mdash Voltage is applied in the direction that would be

blocked by a diode

2 Forward blocking mode mdash Voltage is applied in the direction that would cause

a diode to conduct but the thyristor has not yet been triggered into conduction

3 Forward conducting mode mdash The thyristor has been triggered into conduction

and will remain conducting until the forward current drops below a threshold value

known as the holding current Converter is a semiconductor device which converts ac

input voltage into a constant dc output voltage In present excitation system three phase

fully controlled thyristor converter is used

Because of the following advantages thyristor converters are used

a) Thyristors are used for high power applications ie up to 10Kv3500A1KHz

b) Having high reliability and low losses

c) Uni-directional device like diode

d) Itrsquos operation as a rectifier which are low resistance in forward conduction

mode and high resistance in reverse conduction mode

PROTECTION OF THYRISTORS

For reliable operation of a thyristor demands that its specified ratings are not

exceeded When Subjected to or over voltages During the turn - on of SCR didt

prohibitively large False triggering of SCR by high value of dvdt andSpurious signals

between gate and cathode may leads to unwanted turn ndash on

DIDT AND PROTECTION

When thyristor starts conducting in forward conduction mode and is turned on by

gate pulse The anode current increases rapidly whole area of the gate to Cathode

junction then hot spots will be formed near the gate connection this locality of heating

destroys the thyristor Thyristor thermal time is constant The causes due to faults and

short circuits or surge currents Electronic crowbar protection is used against the over

voltages The rate rise of anode current must be kept at the time of turn on below the

rated or specified limiting value The didt value maintained below limited value by using

a inductor also called ldquodidt inductorrdquo in series with anode circuit The locality of heating

is avoided by applying gate current but not greater the maximum gate current

DVDT AND OVER VOLTAGE PROTECTION

With forward voltage across the anode and cathode of a thyristor the two outer

junctions are forward biased but the inner junction is reverse biased This reverse biased

junction J2 has the characteristics of a capacitor due to charges existing across the

junction In other words space-charges exist in the depletion region around junction J2

and therefore junction J2 behaves like a capacitance If the entire anode to cathode

forward voltage Va appears across J2 junction and the charge is denoted by Q then a

charging current i given by Eq (46) follows

i = dQdt =d(Cj Va )dt

= Cj (d Va dt) + Va(d Cj dt) helliphelliphelliphellip(46 a)

As Cj the capacitance of junction J2 is almost constant the current is given by

i = Cj (d Va dt) helliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip(46 b)

If the rate of rise of forward voltage dVadt is high the charging current i will be

more This charging current plays the role of gate current and turns on the SCR even

when gate signal is zero Such phenomena of turning-on a thyristor called dvdt turn-on

must be avoided as it leads to false operation of the thyristor circuit

For controllable operation of the thyristor the rate of rise of forward anode to

cathode voltage dVadt must be kept below the specified rated limit Typical values of

dvdt are 20 ndash 500 Vμsec False turn-on of a thyristor by large dvdt can be prevented by

using a snubber circuit in parallel with the device thyristor are very sensitive for over

voltage than the semiconductor devices

Over voltage transients are perhaps the main cause of thyristor failure

In thyristor there are mainly two types

1 Internal over voltages

Due to the commutation of the thyristors large voltages are generated internally

Because of the series inductance of the SCR circuit the large transient voltages L didt

produced This voltage several times the break over voltage of the device then thyristor

destroys permanently

2 External over voltages

External over voltages are caused due to the interruptions of current flow in an

inductive circuit and also due to the lightening strokes on the lines feeding the thyristor

system For the reliable operation of thyristor the over voltages must be suppressed by

adopting suitable techniques

Suppression of over voltages

The RC circuit called snubber circuit is connected across the device to protect In

order to keep the protective components to a minimum the thyristors are chosen with

their peak voltages ratings are 25 to 3 times of the normal peak working voltage

ldquoselenium thyrector diodes metal oxide varistors or avalanche diode suppressers are

commonly employed for protecting the thyristor circuit against the over voltages

Gate protection

Gate circuit should also be protected against the over voltages and surges Over

voltage at gate circuit can cause false triggering of the SCR may rises the junction

temperature behind specified limit leading to its damage Protection against over

voltage can be achieved by connecting a ZD across the gate circuit and a resister is

connected in series with gate circuit to protect against the s A capacitor and resister are

connected across gate to cathode to by pass the noise

41 Final Pulse Stages

The Final Pulse Stages adapt the output pulses from the Intermediate Pulse Stage

(pulses on the pulse bus) to the gate currents needed for the thyristors Each thyristor

bridge is equipped with its own Final Pulse Stage Each Final Pulse Stages is provided

with a power supply module The amplified output pulses from the Final Pulse Stages

start as a short strong steep pulse with an amplitude approx 2frac12 times that of the main

pulse This initial pulse edge assures proper firing of the thyristors being triggered

Subsequently the weaker part of main pulse keeps firing conditions steady As already

mentioned the Final Pulse Stages and their associated thyristor bridges form single units

All six pulse outputs from a Final Pulse Stage can be blocked by an external control

signal so that all thyristors in the associated thyristor bridge will block the current A

blocking of the pulses is initiated whenever there is a malfunction in the associated

thyristor bridge

42 Converter Power Section

The thyristor converter consists of three independent parallel rectifier blocks TY1

to TY3 which are all in service Even if one block fails the remaining blocks take over

automatically the full design current of the excitation circuit During normal operation

(with ideal current share) and all three bridges in operation each of these blocks has to

carry only (n-2)n (ie33)of its design current If 2 thyristor bridges fail the excitation

is limited Only when all three bridges fail the excitation is switched off Each thyristor

bridge arm is equipped with current flow monitoring CTrsquos Failure of conduction in any

arm is identified by a Current flow monitoring module

43 Converter Cooling

A cooling system is needed to dissipate heat losses in the converter blocks and

electronics Each converter block has therefore been equipped with a fan supplied with

power from the converterrsquos primary voltage (via transformer ndashT8 in field flashing

cubicle) The fans are protected with motor protection circuit breakers An air flow

monitoring unit is provided for monitoring the air flow through the thyristor bridge If a

circuit breaker failure is detected or if the air flow monitor drops off at one of the

thyristor bridges the bridge involved is immediately set out of operation by blocking its

firing pulses

44 Thyristor Converter Monitoring

A thyristor bridge in which defects occur that could threaten the safety of

operation or cause secondary damage is switched off automatically ie its firing pulses

are blocked This happens whenever A thyristor fuse is blown The fuses are monitored

individually with micro switches The Final Pulse Stage fails which is detected by

internal monitors (supply voltage sustained pulse short-circuit on the output end) The

power supply to the fan fails fan air flow as monitored by the Air flow monitor fails or

is insufficient Isolator on ACDC side is open

CHAPTER 5

Field Current Circuit Excitation Transformer

51 Field Circuit Breaker

The circuit-breaker in the field circuit is used to isolate the field circuit from the

converter It is capable of switching off the synchronous machine from full load under the

maximum conditions of a 3-phase short-circuit In addition to its main contacts the field

circuit-breaker also has a de-excitation contact with which the field energy stored in the

field can be dissipated across the de-excitation resistor The de-excitation contact closes

shortly before the main contacts open so as to ensure proper commutation of the field

current from the main contacts to the de-excitation contact when the breaker is switched

off The field circuit-breaker is switched on by electromagnetic force and is kept switched

on by a mechanical latch When the latch is released by a trip coil the circuit-breaker

opens The circuit-breaker also has auxiliary contacts that report its status

52 Field Flashing

In shunt supplied excitation circuits (excitation transformer connected to the

generator terminals) the generator does not have enough remnant voltage for a generator

voltage build-up via the converter In this case a field flashing circuit is provided It

consists of the field flashing contactor the diode bridge and a transformer used to adapt

the auxiliary input voltage to the voltage needed for field flashing when power is

supplied from the auxiliaries network

Fig Field Flashing

Because the field flashing contactor is not able to switch off the energy stored in

the field the control ensures that the contactor can only reopen if the field circuit breaker

has already been opened (generating the TRIP order) or in a normal field flashing

sequence when the converter has taken over the field current Field flashing occurs in the

following stages

1048729The excitation is switched on closing the field flashing contactor ( Field

Circuit Breaker is already closed )

1048729The start-up excitation current flows through the rotor driving the generator

voltage up to approx 15 U

1048729After about 10 U the firing pulses to the converter are released and it begins

to excite the generator to its rated voltage

1048729After about 30 U the field flashing contactor opens (with no current since

the converter is now supplying the current)

The diode bridge at the input to the field flashing contactor prevents a feed-back

from the converter to the source of field flashing while the contactor is still closed

53 De-excitation

When malfunctions occur the stored field energy must be dissipated as quickly

and safely as possible to protect the generator This is done by the converter the field

circuit-breaker and the de-excitation (discharge) resistor

De-excitation (with opening of the field circuit-breaker) takes place in the following

stages

1048729The converter drives to its inverter limit position (negative ceiling voltage)

recovers a portion of the field energy into the network A trip command is given to the

field circuit breaker

1048729The de-excitation contact closes diverting the field voltage to the de-excitation

resistor

1048729Then immediately the main contacts open building voltage The field voltage

commutates to the de-excitation resistor

1048729The current diminishes at a given time constant TE

(With linear resistance TE = Lf (Rf + Re))

Due to the reversal of the field voltage by the converter the field current

commutates from the main contacts of the field circuit-breaker to the de-excitation

resistor in a very early phase This reversal of the field voltage prevents burn-off on the

main contacts and provides effective protection for the field circuit-breaker Depending

on the operating policy an operational shut-down of the excitation can also be effected

with the field circuit-breaker closed This method is useful mainly when the excitation is

switched on and off frequently In this case the converter is merely driven into the

inverter limit position so that the field energy is recovered into the network The

converter then blocks since it is supplying positive current only

54 Excitation Transformer

The excitation transformer matches the generator voltage to the field voltage

(required ceiling voltage) It also serves as a commutation reactance for the thyristor

converter and as a potential isolator between the network and the excitation circuit In

addition the transformer functions as a current limiter in that it makes it possible to keep

any short circuits in the excitation circuit under better control The excitation transformer

is equipped with temperature monitoring probes which set off an alarm when the

temperature exceeds a first max limit and then trips the excitation if the temperature

continues rising to a second (higher)limit

CHAPTER 6

Monitoring and Protection

61 Excitation Monitoring

611 General Information

The main goal of Excitation Monitoring is to make optimum use of the

redundancies provided in the excitation system and to give alarm whenever a malfunction

makes these redundancies unavailable The field current is monitored to see that it does

not exceed a maximum level and if necessary a switch-over to the MANUAL channel is

initiated In addition the criterion for switching off the field flashing is generated The

excitation Monitoring consists of an autonomous processor system

612 over current Alarms

In the Excitation Monitoring the limits for are set at higher levels than the

settings on the Field Current Maximum Limiter Whenever the current exceeds 110 of

the nominal field current contact R1 and the binary output associated with it are

activated immediately If field current remains gt 110 then after a preset inverse-time

has lapsed relay R2 and - after a further delay - relay R0 and the binary outputs

associated with them are activated Parameters match the measurements for If1 and If2 to

the nominal value for field current so that the internal values can be processed and read as

pu values It can be used to falsify the actual value of the field current If (to raise it) so

as to cause a response from the alarm limits for purposes of testing The processed If

signal is always taken from on the active channel (CHANNEL I OR CHANNEL II) As

long as the field current If is above the threshold value 11 Ifn its peak value is

measured This is stored (until RESET) and can be read at any time on the Micro-

Terminal Once the value of If exceeds 11 Ifn integration of this value starts Whenever

the integrated time-current value (intisup2dt) exceeds the preselected reference value the

alarm OVER CURRENT INVERSE-TIME is set off and a command is simultaneously

issued to switch over to the stand by AUTO channel Software switch F758 enables the

three over current alarm functions (R0 R1 R2) and selects one of three possible inverse-

time curves T1 T2 or T3 Within the characteristic curve (T1 T2 T3) selected the

desired limit curve for response is set using the factor F216

613 Switch-Off Criterion for Field Flashing

The Excitation Monitoring supplies the criterion for switching off the field

flashing Whether this criterion is activated based on the actual value for generator voltage

Ug or for field current If or both depends on the settings of the two threshold values

F200F202 (0 setting means that the output is always ldquological 1rdquo) The measurements

Ug12 and If12 are switched over depending on the present status of the channels

(Channel 1 or Channel 2 ON) Whenever Flag F750 is not activated the binary output is

fixed at ldquological 1rdquo

614 Storage of Alarm Status

The outputs of the over current alarms (R1 R2 R0) and the messages NO

FAILURE MONITORING PARAMETERS CHANGED are stored messages can be

erased by activating the input ldquoGENERAL RESETrdquo or by using the RESET button on the

front of the module Erasure with the input ldquoGENERAL RESETrdquo is effective only if the

situation causing the alarm or the malfunction is no longer present Whenever the self-

diagnosis equipment in the processor detects a malfunction the output NO FAILURE of

MONITORING is set at ldquological 0rdquo (= alarm) The alarm ldquoPARAMETERS CHANGEDrdquo

is activated whenever parameters or settings of software switches have been changed via

the Micro-Terminal

615 Actual Value Monitoring

The actual values for generator voltage Ug synchronous voltage Usyn and field

current If are monitored for malfunctions This monitoring is active regardless of whether

or not the generator is in operation Essentially when the generator is in operation the

measurements are monitored by comparing the signals (the smaller signal reading is

detected as incorrect) When the generator is not in operation the measured data are

monitored for extreme values The percentage of deviation permissible in the

measurement signals being compared

(Ug1ampUsyn1 Ug1ampUg2 Ug2ampUsyn2 If1ampIf2) is defined by parameters F208 and

F20AIf the excitation transformer is being supplied from an auxiliary power source (no

shunt operation) the values of Ug and Usyn will be different in some operational

conditions

In that case Flag F75A can be used to deactivate comparative measurements Ug1 Usyn1

and Ug2 Usyn2 The ldquoprimary monitoringrdquo compares Ug1 with Ug2 and generates the

messages that Channel 1 or Channel 2 has suffered a malfunction Whenever Ug1 lt Ug2

and the binary message from CH1 reports no malfunction a malfunction on

Measurement Channel 1 is reported (Ug1Usyn1 FAILURE) A similar malfunction is

also present whenever the binary message CH1 DISTURBANCE is reported and a

discrepancy is detected between Ug1ampUg2 The generation of the alarm ldquoUg2 Usyn2

FAILURErdquo is analogous to that for Channel 1 The ldquosecondary monitoringrdquo compares

Ug1 with Usyn1 amp Ug2 with Usyn2 This is enabled whenever the binary message of the

comparison channel reports a malfunction or whenever both binary messages report no

malfunction - but both secondary monitors report a malfunction As long as the secondary

monitoring is blocked the differences Ug1neUsyn1 or Ug2neUsyn2 trigger malfunction

signals for the measurement channel involved (suspicion that there is a corresponding

error in Usyn) The measurement channel malfunctions are enabled operationally

whenever after excitation has been switched on generator voltage Ug exceeds the value

set on F204 The voltages Ug1 and Ug2 are checked 16 seconds after the excitation is

switched off to see that they do not exceed the limit value F210 that applies to both of

them At the same time g1Ug2Usyn1Usyn2 are checked for extreme values (gt or lt

the operating range) Monitoring for extreme values is likewise enabled during normal

operation ( Excitation ON and Ug gt F204 ) Flag F754 is used to enable or block the

malfunction signals to the binary outputs Basically the monitoring of the actual values

for If1If2 functions like that of the Ug1Ug2 monitoring

62 Excitation Protection

621 General Information

The Excitation Protection switches off the excitation (and de-excites the machine

rapidly) whenever a danger arises that threatens the excitation transformer the converter

or the generator Generally limiter or monitoring functions precede the emergency trips

and these normally respond before the Excitation Protection must initiate a trip

Protective trip commands are issued directly to the field circuit-breaker from potential

free contacts of the board via the trip relays They are directed redundantly to the

operative field circuit-breaker ldquoOFFrdquo command

622 Protection against Excitation Transformer Overheating

This equipment monitors the excitation transformer for overheating in the

windings that could result from over current short-circuits or inadequate cooling The

monitoring uses temperature monitoring modules in conjunction with temperature

sensors built into the windings Normally the temperature is monitored in two stages the

first stage sets off an alarm the second causes a trip of the excitation

623 Rotor Over voltage Protection

Malfunctions in the generator circuit (eg terminal short-circuit failed

synchronization asynchronous operation) cause induced negative field currents that

produce high voltages in the field circuit These must be restricted to a level with a

sufficient safety margin below the insulation capacity of the field winding (test voltage)

and also below the peak blocking voltage of the converter thyristors The crow bar

employs spark gap elements to detect over voltages in the field circuit Whenever they

respond the associated thyristors are fired immediately switching the de-excitation

resistor parallel to the field The de-excitation current generated thereby initiates an

excitation trip via a supervision circuit causing an immediate opening of the field circuit-

breaker The malfunction isets off an alarm and an internal malfunction is indicated at the

cubicle

TEST VALUES OBTAINED WHEN EXCITATION IS RAISED

TEST

SNO PARTICULARS ACTUAL VALUE OBTAINED VALUE

1 VREF 996 100

2 VACT 997 1003

3 IFACT 735 765

4 IGACT 703 707

5 ACTIVE

POWER

703 705

6 REACTIVE

POWER

101 142

7 POWER

FACTOR

099 IND 098 IND

8 ACTIVE

CURRENT(IR)

705 703

9 REACTIVE

CURRENT(IX)

102 138

10 POWER

ANGLE

566 546

11 FIRING

ANGLE

640 632

VALUES OBTAINED WHEN EXCITATION IS LOWERED

SNO PARTICULARS ACTUAL

VALUE

OBTAINED VALUE

1 VREF 100 997

2 VACT 100 997

3 IFACT 787 761

4 IGACT 839 837

5 ACTIVE

POWER

839 830

6 REACTIVE

POWER

155 110

7 POWER

FACTOR

098 IND 099 IND

8 ACTIVE

CURRENT(IR)

835 830

9 REACTIVE

CURRENT(IX)

140 94

10 POWER

ANGLE

613 DEG 631 DEG

11 FIRING

ANGLE

601 DEG 599 DEG

CONCLUSION

For generating the EMF in stator winding excitation is required to the rotor of a

generator There are two types of excitation

1 Static excitation system

2 Brushless excitation system

A certain disadvantage in brushless excitation system is the slow response time of

the field in case of fast load changes specified No slip-rings and brushes direct

measurements of the field parameters not possible

To avoid all loses static excitation is used Since it does not have any rotating

parts mechanical loses and windage loses This system has fast response and speed

control While preferring this excitation system there are no limitations for the

redundancy of Thyristor bridge circuits

Static excitation has fast field discharge by resistor and inverter operation direct

measurement of field quantity is possible The meaning of excitation is nothing but

continuous supply of DC current (ie field current) to the rotor to buildup required

output voltage in the stator

Field current is changed with respect to the change of load so the digital

automatic voltage regulator (DAVR) is used to regulate the output voltage according to

the load variations

So we conclude that static excitation system with DAVR is preferred since it is

having excellent dynamic performance and better options for R amp M

The BODE diagram below shows the assignment of settings in accordance with

DINIEC standards based on a typical example

The PID filter amplifications Vo Vp and Vinfin can be adjusted in pu values But

the ldquoceiling factorrdquo pl+ must be adjusted correctly with parameter F310 if the total

amplification (circuit amplification) of the control circuit is actually to conform to the

pu settings This factor must agree with the ldquoexternalrdquo amplification ie with the

ceiling value of the transformer- converter circuit

Ceiling factor(pl+) = Ufmax Ufo

in which Ufmax = ceiling field voltage

Ufo = no-load field voltage

To attain a suitable response of the AVR when starting excitation

(ldquoEXCITATION ONrdquo) it may be necessary to change the proportional amplification of

the regulator during this phase Vp2 (transiently activated) and Vp1 (permanently

activated) can be adjusted for this purpose For example the value of Vp2 takes effect

immediately once the excitation is switched on and remains effective for a period as set at

F30C Once the period F30C (eg 5 sec) has expired Vp shifts over to Vp1 (becomes

the steady-state Vp) at the rate of change set The standard operating mode for the PID

filter is voltage regulation for which the discrepancy between the voltage set-point and

the current value for generator voltage Ug (the control deviation) is supplied at the input

To compensate for the voltage drop in the block transformer or whenever several

generators are operating to the same distributing bus the generator voltage must be

varied in proportion to the measured generator current (droop influence) To accomplish

this the voltage set-point is varied as a function of the measured reactive current IX

andor active current IR Flag F712 enables the IX droop Flag F710 the IR droop The

desired compensation is set in F282 and F286 respectively Flags F284 and F288 are used

to select whether this droop influence is to increase the voltage or to reduce it

(compensation) Combined influence of the active and reactive currents is attained by

enabling both droops IX and IR Flag F716 activates a so-called ldquoSoft-Startrdquo at the

starting of excitation This ldquoSoft-Startrdquo ensures that the voltage set-point integrates from

0 to 100 within the time set on F290 when the excitation is switched on

(ldquoEXCITATION ONrdquo) A ldquosmoothrdquo excitation of the generator can be achieved in this

way whenever there is no demand for a quick excitation

232 Voltage Set-Point

Various signals and settings control and limit the voltage set-point F270 For

example the values of F254 and F252 define the normal operating range possible for set-

point adjustment (eg 90 110) using external control commands (control room local

operatorrsquos panel superposed control system) The effective set-point adjustment rate is

governed The set-point can be set at the values of F250 and F256 by activating

appropriate control commands for ldquoSETrdquo input Enabling Flag F71A and activating a

binary input prior to switching on the excitation (ldquoEXCITATION OFFrdquo) sets the Ug set-

point at the value of UAUX This makes it possible for example to ensure that the

generator voltage will agree exactly with the network voltage after the voltage build-up

An external value with variable amplification can be added to the Ug set point by

enabling F724 (for example for stability tests)

233 Regulator Tracking in MANUAL Operation

Whenever the AUTOMATIC channel is not in operation (the MANUAL channel

is ON) a follow-up equipment ensures a smooth switch-back to the AUTOMATIC

mode will always be possible To track the voltage set-point is shifted by means of

RAISELOWER pulses from the Gate Control Unit so that control variable Ucontr at the

output from the PID filter is held steady and identical to the control variable Ucontr from

the MANUAL channel Because this tracking must react slowly resultant transient

control deviations resulting from the amplification in the PID filter might cause severe

interference with control variable Ucontr

To prevent this the follow-up equipment intervenes on the regulators mixing

point with a corresponding compensation signal

234 Ugf Limiter

At under frequency the Ugf Limiter reduces the generator voltage so as to

prevent saturation effects in the supply and measuring transformers To adjust this

limiter the max permissible generator voltage at rated frequency is defined and set

When any under-frequency occurs the generator voltage is thus reduced in proportion to

that setting

235 Field Current Maximum Limiter

The Field Current Maximum Limiter is provided to protect the generator rotor

from s occurring in steady-state and transient operation High field currents are normally

the result of a sharp drop in network voltage or of an improper raising of the voltage set-

point by the operating staff The field current is held steady at the value TH1 ie at the

maximum thermal value permissible for the excitation circuit and the rotor In order that

the generator can support the power network with its transient overload capacity during

brief collapses in voltage a temporary switch-over is made to the transient limit MAX1

(a higher setting) When the generator or the converter is operating at a reduced capacity

These limits TH1MAX1 can be switched over to the lower settings TH2MAX2 by

activating the corresponding binary signals The switch-over from the thermal limit

TH12 to the transient limit MAX12 can be configured in one of three ways

a) Depending on the over current with -dUdt ENABLE

Flag programming F418 = any setting desired F41A = 0000

This variant enables the transient value MAX12 whenever a collapse of voltage

in the network is detected The ENABLE time is fixed and can be set The example

below shows the typical behavior of the limiter configured in this way

b) Dependent on the time integral with -dUdt ENABLE

Flag programming F418 = inactive F41A = 1111

This variant likewise enables the transient value only when a collapse of network

voltage has been detected However the switch-back to the thermal limit is not made

dependent upon the time itself but on the calculated time integral intisup2dt of the The setting

on Parameter F414 in spu takes into account the time the rotor needs to cool down ie

the rate of temperature change in the case of intermittent operation The example below

shows how the timing of the switch-back to the thermal limit depends on the present

value for intisup2dt

The time integral is based on the formula

Example The setting of =isup2dt equivalent to Version a (F416) at a constant 16 times the

nominal field current for 10 seconds (with TH12 = 105) is

c) Dependent on the time integral without any preconditions

Flag programming F418 = 1111 F41A = 1111

In this variant the transient becomes available without any prior conditions

(without a -dUdt ENABLE) with the time integral intisup2dt

237 Inductive Stator Current Limiter

The Inductive Stator Current Limiter holds the stator current Ig within permissible

limits while the generator is in the ldquoover-excitedrdquo operating range by reducing the field

current accordingly The setting TH (thermal limit) provides the limit against stationary s

that might occur To take advantage of the generatorrsquos transient overload capacity a

switch-over is made to the higher setting MAX The principle of operation of this switch-

over to the value MAX permissible only transiently is identical to that employed for the

field current limiter (refer to the description above) When the drive output from the

turbine is very high stator current may exceed permissible limits even while inductive

loading of the generator is low In this case if the stator current limiter is not kept from

influencing the field current the control circuit will oscillate back and forth between the

Inductive Stator Current Limiter (de-

exciting) and the Capacitive Stator Current Limiter (exciting)The output signal of that

function then dominates the control variable of the Ig-dependent limiter via a maximum

value selection

238 Capacitive Stator Current Limiter

239 Load Angle Limiter

The Load Angle Limiter prevents the synchronous machine from slipping out of

phase due to slippage of the rotor The load angle δ the difference in phase between the

rotor and the stator rotating field results mainly from the driving torque (active power P)

acting on the generator and the level of rotor current (field current) If the driving torque

remains constant a increase in the field current reduces the load angle δ The current load

angle δ at any moment is obtained from the generator current and generator voltage based

on a simplified model of the generator Whenever this calculated load angle δ exceeds the

preset limit angle the limiter increases the field current until the load angle has dropped

back to its permissible value The quadrature reactance Xq of the generator and the

network reactance Xe during normal operation must be adjusted on the regulator in order

to obtain the load angle δ The graph below shows the Power Chart for a salient-pole

machine with typical limiter characteristics

The purpose of a Power System Stabilizer is to use the generator excitation to

damp electromechanical oscillations between the network and the generator Depending

on the design of the generator and the requirements imposed for network stability its

main function will be either to damp the oscillations originating in the machine or those

from the network A synchronous generator working in a combined power network is in

principle an oscillating structure In order to produce a torque the magnetic field of the

rotor and the stator must form a given angle (referred to as the rotor displacement or load

angle δ) The electrical torque ME increases as the angle δ increases just as with a

torsion spring Because the ME of the generator and the mechanical driving torque MA

from the turbine are in equilibrium during steady-state operation the angle δ remains in a

given position Whenever this state of equilibrium between MA and ME is disturbed the

load angle slips of this rest position and change thereby the electrical torque ME The

torque attempts to restore the load angle to a stationary position Due to the mass inertia

of the turbinegenerator rotor however this can only take place aperiodically It does so

in the form of more or less effectively damped oscillations (again similar to the effect of

mass inertia on a torsion spring) In order to damp the oscillations there must be a

damping torque produced depending not on the electrical torque ME associated with the

angle but on the difference in frequency (Df) between the rotor and the stator rotating

field ie on the slippage This torque is produced mainly by the so-called damper

winding in the rotor but the dimensioning of this is subject to limits imposed by

considerations of design and economy Some further action is therefore needed to

increase the damping effect The following drastically simplified formula shows the

parameters upon which the amount of active power PE supplied by the generator

depends

PE = active power

It can be seen from the above relationship that the active power that the generator

transfers depends not only on the load angle δ but also on the field current If That means

that a transient change can be made in the active power PE and with that in the effective

electrical torque ME by varying the field current The principle of operation of the DVR

Power System Stabilizer becomes clear from a consideration of the oscillations in power

output and frequency (ΔPE Δf) and the vector diagram If it is assumed that oscillations

in the network frequency generates load oscillations with the mass inertia of the rotor

then the active load of the generator (eg MW-measured) is influenced with a sinusoidal

value -ΔPE (ME-MA = -ΔPE) By inversion of -ΔPE one obtains the fluctuation in

power provided by the rotor +ΔPE As is known the slip signal Δf follows +ΔPE with a

phase delayed by 90deg The +ME produced by the periodic changes in the load angle δ is

in phase with +ΔPE A good damping is attained if ME is varied in phase with the slip

Δf However this signal must also be advanced somewhat to compensate for the time

constants in the excitation circuit and the generator

As mentioned above the electrical torque ME can be influenced by varying the

field current To accomplish this a suitable control signal referred to as variable

disturbance compensation must be imposed upon the voltage set-point or the converter

control variable Ucontr As can be seen from the vector diagram by applying proper

weighting factors (K1 K2) and then adding together the signals -ΔPE and Δf an overall

stabilization signal can be produced that rotates in advance of the Df signal by any angle

desired between 0deg and 90deg Because the amplitude of -DPE remains proportional to the

amplitude of Δf a constant angle in advance of Δf results for the compensation of the

time constants referred to above The optimum weighting factors K1 and K2 for a

synchronous generator working to a power network depend on its operating point at any

moment and the external reactance of the network Normally the selection of a

compromise setting is good enough to attain stability in all operating points and for all

external reactance For special demands these settings must be parameterized as a

function of the external reactance (which means optional equipment Xe-Identification)

The Power System Stabilizer PSS is a section of the AVR computer program and is

processed once per network cycle The voltage at the generator terminals and the

generator current are measured in order to define the signals ΔPE and Δf The calculated

signals for _P_ (=PE) and Δf are then sent across DC filters ldquoDrdquo (real differentiators) that

transmit only the dynamic portion of the signals The ΔPE and Δf signals obtained in this

way are then weighted (multiplied by) with the factors K1 and K2 and sent to the

summing point of the voltage regulator

The PSS stabilization signal is imposed on the automatic voltage regulator only if

the following prerequisites are met

bull Generator on line

bull Generator power output gt the value F338

bull Generator voltage in a range between F33C and F33A

The stabilization signal is limited at the output from the PSS to the lower and

upper limits Flag defines whether the stabilization signal is introduced before or after the

PID filter (usually before the filter) Because the PID filter as noted above already takes

the ceiling factor Vp1 into account the PSS signal needs to be multiplied by Vp1 if it is

added to the voltage regulator following the PID filter (divider at the input to the

minmax limiter) This precaution prevents the DC filter ldquoDrdquo in the P-channel from

producing an unnecessary ldquostabilizationrdquo effect in the case of rapid changes in turbine

load As an alternative for the AVRrsquos Power System Stabilizer a stabilization signal from

an outside system can be imposed by activating the binary input ldquoPSS-SIGNEXTrdquo Flag

F340 can be used to select between an analog and a 12-bit signal and F33E to select the

polarity desired for that signal

24 The MANUAL Channel

241 Summary

The MANUAL channel (Channel 2) has been built as a simple field current

regulator

without additional limiters Its main function is to maintain the excitation of the generator

even if the AUTOMATIC channel becomes non-operational The MANUAL channel

also performs valuable service for purposes of testing commissioning and preventive

maintenance Its measurements regulator generation of firing pulses and power supply

are physically separate from those on the AUTOMATIC channel

242 Principle of Operation

All the functions of the MANUAL channel including the generation of firing

pulses have been implemented in a single electronic module the Gate Control Unit The

control variable Ucontr of voltage regulator is used as the reference value for generating

firing pulses on the principle known as ldquoramp controlrdquo (Comparison of Ucontr with

Usynsynchronous sawtooth signal) For further processing in the UN 0096 Intermediate

Pulse Stage the Gate Control Unit supplies six firing pulses at its output whose phase

position with respect to the synchronous voltage Usyn is in accordance with control

variable Ucontr An internal linearization ensures that the field voltage produced via the

firing pulses remains proportional to the control variable Ucontr throughout the entire

range As a result the circuit amplification of the control remains constant over the entire

range Whenever excitation is switched ON the set-point for Generator Voltage is set

automatically at the preset - ref Value This provision ensures that the generator voltage

always attains approximately its nominal value after the field flashing The Gate Control

Unit can be refunctioned ( by pre-selection with a switch ) for purposes of testing to act

as a purely firing pulse control In this case the control variable Ucontr is adjusted

directly using the RAISELOWER push buttons on the front of the module In this way

for example the relationship between the phase position of the firing pulses and the

control variable Ucontr can be checked easily

CHAPTER 3

PULSE SECTION

31 Pulse Generation and Amplification

The Gate Control Units of both AUTOMATIC channel and MANUAL channel

each supply six firing pulses for operating the 6-pulse thyristor bridges The low-power

pulse signals from these Gate Control Units are then amplified in the Intermediate Pulse

Stage galvanically isolated and then sent to the common pulse bus On the output end

the Intermediate Pulse Stage of the non-active channel) is always blocked The Gate

Control Units generate the pulses based on microprocessor control The reference voltage

used for the firing pulse phase location is the output voltage from the excitation

transformer (Usyn1 Usyn2) The commutation spikes of the synchronous voltage caused

by the converter are calculated prior to use of the voltage as a reference value and are

deliberately filtered out The lower limit for the firing pulses (double pulses) which are

offset from one another by 60deg is defined by the limit rectifier position (αmin) and the

upper limit by the limit inverter position (αmax) for the firing angle αmin and αmax can

be adjusted on the Gate Control Units using BCD (Binary Coded Decimal) switches

αmin ensures that the firing pulses will not be issued (premature firing) until there is

sufficient positive phase voltage on the thyristor involved αmax prevents a dangerous

ldquotippingrdquo of the thyristor bridge into the rectifier mode if the firing angle α is too large

(ldquolate firingrdquo) The critical factors determining αmax are the overlap time uumlmax (max

commutation time) and the ldquorecovery timerdquo of the thyristors (αmax lt 180deg - uumlmax - γ )

An external control signal can force the firing pulses into their inverter limit position

Other binary inputs can block or direct the firing pulses of the Gate Control Units so as to

produce freewheeling on the thyristor bridge During freewheeling the firing pulses for

the thyristor pair R and S are blocked and the pulse signals T+T- are engaged with

chains of pulses Both Gate Control Units (for the MANUAL amp AUTOMATIC channels)

contain a field current monitor that blocks the firing pulses immediately whenever the

current exceeds a preset threshold level In this case the field circuit-breaker is also

tripped via an output contact The purpose of these provisions is to prevent damage to

thyristors and thyristor fuses in case of a slip-ring short-circuit or to keep any damage

that does occur to a minimum The pulse signals are galvanically separated at the outputs

from the Intermediate Pulse Stage (with pulse transmitters) and are then directed to the

common pulse bus This transmission of the pulse signals to the pulse bus via passive

transmitters ensures a high degree of active channel autonomy Practically no possible

malfunctions on the inactive channel (including for example sustained pulses) affect the

active channel

32 Pulse Monitoring

The ldquoPulse Busrdquo and the pulse signals of the AUTOMATIC channel are

monitored This monitoring device consists of potential isolating stages and the common

monitor If the pulse monitoring of the ldquoPulse Busrdquo responds a switch-over is made to

MANUAL channel The function of the potential isolating stages is to couple the pulse

monitoring device to the pulse circuits without any feedback effect The pulse monitoring

checks the six pulse lines for the following malfunctions continuous or periodic failure

of one or more pulses Periodic occurrence of synchronous or asynchronous false pulses

Continuous pulses the pulse monitoring device can be tested while the machine is in

operation

CHAPTER 4

CONVERTER

Thyristor

The term thyristor usually refers to a family of four layer solid state device having

turn on characteristics that can be externally controlled by either current or voltage They

are also referred to as breakdown device because their working depends on avalanche

breakdown Thyristors have only two stages OFF and ON Thyristors have a similar

function to Uni-junctions they act as switches Thyristors use current flow as a switch

Thyristors have three states

1 Reverse blocking mode mdash Voltage is applied in the direction that would be

blocked by a diode

2 Forward blocking mode mdash Voltage is applied in the direction that would cause

a diode to conduct but the thyristor has not yet been triggered into conduction

3 Forward conducting mode mdash The thyristor has been triggered into conduction

and will remain conducting until the forward current drops below a threshold value

known as the holding current Converter is a semiconductor device which converts ac

input voltage into a constant dc output voltage In present excitation system three phase

fully controlled thyristor converter is used

Because of the following advantages thyristor converters are used

a) Thyristors are used for high power applications ie up to 10Kv3500A1KHz

b) Having high reliability and low losses

c) Uni-directional device like diode

d) Itrsquos operation as a rectifier which are low resistance in forward conduction

mode and high resistance in reverse conduction mode

PROTECTION OF THYRISTORS

For reliable operation of a thyristor demands that its specified ratings are not

exceeded When Subjected to or over voltages During the turn - on of SCR didt

prohibitively large False triggering of SCR by high value of dvdt andSpurious signals

between gate and cathode may leads to unwanted turn ndash on

DIDT AND PROTECTION

When thyristor starts conducting in forward conduction mode and is turned on by

gate pulse The anode current increases rapidly whole area of the gate to Cathode

junction then hot spots will be formed near the gate connection this locality of heating

destroys the thyristor Thyristor thermal time is constant The causes due to faults and

short circuits or surge currents Electronic crowbar protection is used against the over

voltages The rate rise of anode current must be kept at the time of turn on below the

rated or specified limiting value The didt value maintained below limited value by using

a inductor also called ldquodidt inductorrdquo in series with anode circuit The locality of heating

is avoided by applying gate current but not greater the maximum gate current

DVDT AND OVER VOLTAGE PROTECTION

With forward voltage across the anode and cathode of a thyristor the two outer

junctions are forward biased but the inner junction is reverse biased This reverse biased

junction J2 has the characteristics of a capacitor due to charges existing across the

junction In other words space-charges exist in the depletion region around junction J2

and therefore junction J2 behaves like a capacitance If the entire anode to cathode

forward voltage Va appears across J2 junction and the charge is denoted by Q then a

charging current i given by Eq (46) follows

i = dQdt =d(Cj Va )dt

= Cj (d Va dt) + Va(d Cj dt) helliphelliphelliphellip(46 a)

As Cj the capacitance of junction J2 is almost constant the current is given by

i = Cj (d Va dt) helliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip(46 b)

If the rate of rise of forward voltage dVadt is high the charging current i will be

more This charging current plays the role of gate current and turns on the SCR even

when gate signal is zero Such phenomena of turning-on a thyristor called dvdt turn-on

must be avoided as it leads to false operation of the thyristor circuit

For controllable operation of the thyristor the rate of rise of forward anode to

cathode voltage dVadt must be kept below the specified rated limit Typical values of

dvdt are 20 ndash 500 Vμsec False turn-on of a thyristor by large dvdt can be prevented by

using a snubber circuit in parallel with the device thyristor are very sensitive for over

voltage than the semiconductor devices

Over voltage transients are perhaps the main cause of thyristor failure

In thyristor there are mainly two types

1 Internal over voltages

Due to the commutation of the thyristors large voltages are generated internally

Because of the series inductance of the SCR circuit the large transient voltages L didt

produced This voltage several times the break over voltage of the device then thyristor

destroys permanently

2 External over voltages

External over voltages are caused due to the interruptions of current flow in an

inductive circuit and also due to the lightening strokes on the lines feeding the thyristor

system For the reliable operation of thyristor the over voltages must be suppressed by

adopting suitable techniques

Suppression of over voltages

The RC circuit called snubber circuit is connected across the device to protect In

order to keep the protective components to a minimum the thyristors are chosen with

their peak voltages ratings are 25 to 3 times of the normal peak working voltage

ldquoselenium thyrector diodes metal oxide varistors or avalanche diode suppressers are

commonly employed for protecting the thyristor circuit against the over voltages

Gate protection

Gate circuit should also be protected against the over voltages and surges Over

voltage at gate circuit can cause false triggering of the SCR may rises the junction

temperature behind specified limit leading to its damage Protection against over

voltage can be achieved by connecting a ZD across the gate circuit and a resister is

connected in series with gate circuit to protect against the s A capacitor and resister are

connected across gate to cathode to by pass the noise

41 Final Pulse Stages

The Final Pulse Stages adapt the output pulses from the Intermediate Pulse Stage

(pulses on the pulse bus) to the gate currents needed for the thyristors Each thyristor

bridge is equipped with its own Final Pulse Stage Each Final Pulse Stages is provided

with a power supply module The amplified output pulses from the Final Pulse Stages

start as a short strong steep pulse with an amplitude approx 2frac12 times that of the main

pulse This initial pulse edge assures proper firing of the thyristors being triggered

Subsequently the weaker part of main pulse keeps firing conditions steady As already

mentioned the Final Pulse Stages and their associated thyristor bridges form single units

All six pulse outputs from a Final Pulse Stage can be blocked by an external control

signal so that all thyristors in the associated thyristor bridge will block the current A

blocking of the pulses is initiated whenever there is a malfunction in the associated

thyristor bridge

42 Converter Power Section

The thyristor converter consists of three independent parallel rectifier blocks TY1

to TY3 which are all in service Even if one block fails the remaining blocks take over

automatically the full design current of the excitation circuit During normal operation

(with ideal current share) and all three bridges in operation each of these blocks has to

carry only (n-2)n (ie33)of its design current If 2 thyristor bridges fail the excitation

is limited Only when all three bridges fail the excitation is switched off Each thyristor

bridge arm is equipped with current flow monitoring CTrsquos Failure of conduction in any

arm is identified by a Current flow monitoring module

43 Converter Cooling

A cooling system is needed to dissipate heat losses in the converter blocks and

electronics Each converter block has therefore been equipped with a fan supplied with

power from the converterrsquos primary voltage (via transformer ndashT8 in field flashing

cubicle) The fans are protected with motor protection circuit breakers An air flow

monitoring unit is provided for monitoring the air flow through the thyristor bridge If a

circuit breaker failure is detected or if the air flow monitor drops off at one of the

thyristor bridges the bridge involved is immediately set out of operation by blocking its

firing pulses

44 Thyristor Converter Monitoring

A thyristor bridge in which defects occur that could threaten the safety of

operation or cause secondary damage is switched off automatically ie its firing pulses

are blocked This happens whenever A thyristor fuse is blown The fuses are monitored

individually with micro switches The Final Pulse Stage fails which is detected by

internal monitors (supply voltage sustained pulse short-circuit on the output end) The

power supply to the fan fails fan air flow as monitored by the Air flow monitor fails or

is insufficient Isolator on ACDC side is open

CHAPTER 5

Field Current Circuit Excitation Transformer

51 Field Circuit Breaker

The circuit-breaker in the field circuit is used to isolate the field circuit from the

converter It is capable of switching off the synchronous machine from full load under the

maximum conditions of a 3-phase short-circuit In addition to its main contacts the field

circuit-breaker also has a de-excitation contact with which the field energy stored in the

field can be dissipated across the de-excitation resistor The de-excitation contact closes

shortly before the main contacts open so as to ensure proper commutation of the field

current from the main contacts to the de-excitation contact when the breaker is switched

off The field circuit-breaker is switched on by electromagnetic force and is kept switched

on by a mechanical latch When the latch is released by a trip coil the circuit-breaker

opens The circuit-breaker also has auxiliary contacts that report its status

52 Field Flashing

In shunt supplied excitation circuits (excitation transformer connected to the

generator terminals) the generator does not have enough remnant voltage for a generator

voltage build-up via the converter In this case a field flashing circuit is provided It

consists of the field flashing contactor the diode bridge and a transformer used to adapt

the auxiliary input voltage to the voltage needed for field flashing when power is

supplied from the auxiliaries network

Fig Field Flashing

Because the field flashing contactor is not able to switch off the energy stored in

the field the control ensures that the contactor can only reopen if the field circuit breaker

has already been opened (generating the TRIP order) or in a normal field flashing

sequence when the converter has taken over the field current Field flashing occurs in the

following stages

1048729The excitation is switched on closing the field flashing contactor ( Field

Circuit Breaker is already closed )

1048729The start-up excitation current flows through the rotor driving the generator

voltage up to approx 15 U

1048729After about 10 U the firing pulses to the converter are released and it begins

to excite the generator to its rated voltage

1048729After about 30 U the field flashing contactor opens (with no current since

the converter is now supplying the current)

The diode bridge at the input to the field flashing contactor prevents a feed-back

from the converter to the source of field flashing while the contactor is still closed

53 De-excitation

When malfunctions occur the stored field energy must be dissipated as quickly

and safely as possible to protect the generator This is done by the converter the field

circuit-breaker and the de-excitation (discharge) resistor

De-excitation (with opening of the field circuit-breaker) takes place in the following

stages

1048729The converter drives to its inverter limit position (negative ceiling voltage)

recovers a portion of the field energy into the network A trip command is given to the

field circuit breaker

1048729The de-excitation contact closes diverting the field voltage to the de-excitation

resistor

1048729Then immediately the main contacts open building voltage The field voltage

commutates to the de-excitation resistor

1048729The current diminishes at a given time constant TE

(With linear resistance TE = Lf (Rf + Re))

Due to the reversal of the field voltage by the converter the field current

commutates from the main contacts of the field circuit-breaker to the de-excitation

resistor in a very early phase This reversal of the field voltage prevents burn-off on the

main contacts and provides effective protection for the field circuit-breaker Depending

on the operating policy an operational shut-down of the excitation can also be effected

with the field circuit-breaker closed This method is useful mainly when the excitation is

switched on and off frequently In this case the converter is merely driven into the

inverter limit position so that the field energy is recovered into the network The

converter then blocks since it is supplying positive current only

54 Excitation Transformer

The excitation transformer matches the generator voltage to the field voltage

(required ceiling voltage) It also serves as a commutation reactance for the thyristor

converter and as a potential isolator between the network and the excitation circuit In

addition the transformer functions as a current limiter in that it makes it possible to keep

any short circuits in the excitation circuit under better control The excitation transformer

is equipped with temperature monitoring probes which set off an alarm when the

temperature exceeds a first max limit and then trips the excitation if the temperature

continues rising to a second (higher)limit

CHAPTER 6

Monitoring and Protection

61 Excitation Monitoring

611 General Information

The main goal of Excitation Monitoring is to make optimum use of the

redundancies provided in the excitation system and to give alarm whenever a malfunction

makes these redundancies unavailable The field current is monitored to see that it does

not exceed a maximum level and if necessary a switch-over to the MANUAL channel is

initiated In addition the criterion for switching off the field flashing is generated The

excitation Monitoring consists of an autonomous processor system

612 over current Alarms

In the Excitation Monitoring the limits for are set at higher levels than the

settings on the Field Current Maximum Limiter Whenever the current exceeds 110 of

the nominal field current contact R1 and the binary output associated with it are

activated immediately If field current remains gt 110 then after a preset inverse-time

has lapsed relay R2 and - after a further delay - relay R0 and the binary outputs

associated with them are activated Parameters match the measurements for If1 and If2 to

the nominal value for field current so that the internal values can be processed and read as

pu values It can be used to falsify the actual value of the field current If (to raise it) so

as to cause a response from the alarm limits for purposes of testing The processed If

signal is always taken from on the active channel (CHANNEL I OR CHANNEL II) As

long as the field current If is above the threshold value 11 Ifn its peak value is

measured This is stored (until RESET) and can be read at any time on the Micro-

Terminal Once the value of If exceeds 11 Ifn integration of this value starts Whenever

the integrated time-current value (intisup2dt) exceeds the preselected reference value the

alarm OVER CURRENT INVERSE-TIME is set off and a command is simultaneously

issued to switch over to the stand by AUTO channel Software switch F758 enables the

three over current alarm functions (R0 R1 R2) and selects one of three possible inverse-

time curves T1 T2 or T3 Within the characteristic curve (T1 T2 T3) selected the

desired limit curve for response is set using the factor F216

613 Switch-Off Criterion for Field Flashing

The Excitation Monitoring supplies the criterion for switching off the field

flashing Whether this criterion is activated based on the actual value for generator voltage

Ug or for field current If or both depends on the settings of the two threshold values

F200F202 (0 setting means that the output is always ldquological 1rdquo) The measurements

Ug12 and If12 are switched over depending on the present status of the channels

(Channel 1 or Channel 2 ON) Whenever Flag F750 is not activated the binary output is

fixed at ldquological 1rdquo

614 Storage of Alarm Status

The outputs of the over current alarms (R1 R2 R0) and the messages NO

FAILURE MONITORING PARAMETERS CHANGED are stored messages can be

erased by activating the input ldquoGENERAL RESETrdquo or by using the RESET button on the

front of the module Erasure with the input ldquoGENERAL RESETrdquo is effective only if the

situation causing the alarm or the malfunction is no longer present Whenever the self-

diagnosis equipment in the processor detects a malfunction the output NO FAILURE of

MONITORING is set at ldquological 0rdquo (= alarm) The alarm ldquoPARAMETERS CHANGEDrdquo

is activated whenever parameters or settings of software switches have been changed via

the Micro-Terminal

615 Actual Value Monitoring

The actual values for generator voltage Ug synchronous voltage Usyn and field

current If are monitored for malfunctions This monitoring is active regardless of whether

or not the generator is in operation Essentially when the generator is in operation the

measurements are monitored by comparing the signals (the smaller signal reading is

detected as incorrect) When the generator is not in operation the measured data are

monitored for extreme values The percentage of deviation permissible in the

measurement signals being compared

(Ug1ampUsyn1 Ug1ampUg2 Ug2ampUsyn2 If1ampIf2) is defined by parameters F208 and

F20AIf the excitation transformer is being supplied from an auxiliary power source (no

shunt operation) the values of Ug and Usyn will be different in some operational

conditions

In that case Flag F75A can be used to deactivate comparative measurements Ug1 Usyn1

and Ug2 Usyn2 The ldquoprimary monitoringrdquo compares Ug1 with Ug2 and generates the

messages that Channel 1 or Channel 2 has suffered a malfunction Whenever Ug1 lt Ug2

and the binary message from CH1 reports no malfunction a malfunction on

Measurement Channel 1 is reported (Ug1Usyn1 FAILURE) A similar malfunction is

also present whenever the binary message CH1 DISTURBANCE is reported and a

discrepancy is detected between Ug1ampUg2 The generation of the alarm ldquoUg2 Usyn2

FAILURErdquo is analogous to that for Channel 1 The ldquosecondary monitoringrdquo compares

Ug1 with Usyn1 amp Ug2 with Usyn2 This is enabled whenever the binary message of the

comparison channel reports a malfunction or whenever both binary messages report no

malfunction - but both secondary monitors report a malfunction As long as the secondary

monitoring is blocked the differences Ug1neUsyn1 or Ug2neUsyn2 trigger malfunction

signals for the measurement channel involved (suspicion that there is a corresponding

error in Usyn) The measurement channel malfunctions are enabled operationally

whenever after excitation has been switched on generator voltage Ug exceeds the value

set on F204 The voltages Ug1 and Ug2 are checked 16 seconds after the excitation is

switched off to see that they do not exceed the limit value F210 that applies to both of

them At the same time g1Ug2Usyn1Usyn2 are checked for extreme values (gt or lt

the operating range) Monitoring for extreme values is likewise enabled during normal

operation ( Excitation ON and Ug gt F204 ) Flag F754 is used to enable or block the

malfunction signals to the binary outputs Basically the monitoring of the actual values

for If1If2 functions like that of the Ug1Ug2 monitoring

62 Excitation Protection

621 General Information

The Excitation Protection switches off the excitation (and de-excites the machine

rapidly) whenever a danger arises that threatens the excitation transformer the converter

or the generator Generally limiter or monitoring functions precede the emergency trips

and these normally respond before the Excitation Protection must initiate a trip

Protective trip commands are issued directly to the field circuit-breaker from potential

free contacts of the board via the trip relays They are directed redundantly to the

operative field circuit-breaker ldquoOFFrdquo command

622 Protection against Excitation Transformer Overheating

This equipment monitors the excitation transformer for overheating in the

windings that could result from over current short-circuits or inadequate cooling The

monitoring uses temperature monitoring modules in conjunction with temperature

sensors built into the windings Normally the temperature is monitored in two stages the

first stage sets off an alarm the second causes a trip of the excitation

623 Rotor Over voltage Protection

Malfunctions in the generator circuit (eg terminal short-circuit failed

synchronization asynchronous operation) cause induced negative field currents that

produce high voltages in the field circuit These must be restricted to a level with a

sufficient safety margin below the insulation capacity of the field winding (test voltage)

and also below the peak blocking voltage of the converter thyristors The crow bar

employs spark gap elements to detect over voltages in the field circuit Whenever they

respond the associated thyristors are fired immediately switching the de-excitation

resistor parallel to the field The de-excitation current generated thereby initiates an

excitation trip via a supervision circuit causing an immediate opening of the field circuit-

breaker The malfunction isets off an alarm and an internal malfunction is indicated at the

cubicle

TEST VALUES OBTAINED WHEN EXCITATION IS RAISED

TEST

SNO PARTICULARS ACTUAL VALUE OBTAINED VALUE

1 VREF 996 100

2 VACT 997 1003

3 IFACT 735 765

4 IGACT 703 707

5 ACTIVE

POWER

703 705

6 REACTIVE

POWER

101 142

7 POWER

FACTOR

099 IND 098 IND

8 ACTIVE

CURRENT(IR)

705 703

9 REACTIVE

CURRENT(IX)

102 138

10 POWER

ANGLE

566 546

11 FIRING

ANGLE

640 632

VALUES OBTAINED WHEN EXCITATION IS LOWERED

SNO PARTICULARS ACTUAL

VALUE

OBTAINED VALUE

1 VREF 100 997

2 VACT 100 997

3 IFACT 787 761

4 IGACT 839 837

5 ACTIVE

POWER

839 830

6 REACTIVE

POWER

155 110

7 POWER

FACTOR

098 IND 099 IND

8 ACTIVE

CURRENT(IR)

835 830

9 REACTIVE

CURRENT(IX)

140 94

10 POWER

ANGLE

613 DEG 631 DEG

11 FIRING

ANGLE

601 DEG 599 DEG

CONCLUSION

For generating the EMF in stator winding excitation is required to the rotor of a

generator There are two types of excitation

1 Static excitation system

2 Brushless excitation system

A certain disadvantage in brushless excitation system is the slow response time of

the field in case of fast load changes specified No slip-rings and brushes direct

measurements of the field parameters not possible

To avoid all loses static excitation is used Since it does not have any rotating

parts mechanical loses and windage loses This system has fast response and speed

control While preferring this excitation system there are no limitations for the

redundancy of Thyristor bridge circuits

Static excitation has fast field discharge by resistor and inverter operation direct

measurement of field quantity is possible The meaning of excitation is nothing but

continuous supply of DC current (ie field current) to the rotor to buildup required

output voltage in the stator

Field current is changed with respect to the change of load so the digital

automatic voltage regulator (DAVR) is used to regulate the output voltage according to

the load variations

So we conclude that static excitation system with DAVR is preferred since it is

having excellent dynamic performance and better options for R amp M

to select whether this droop influence is to increase the voltage or to reduce it

(compensation) Combined influence of the active and reactive currents is attained by

enabling both droops IX and IR Flag F716 activates a so-called ldquoSoft-Startrdquo at the

starting of excitation This ldquoSoft-Startrdquo ensures that the voltage set-point integrates from

0 to 100 within the time set on F290 when the excitation is switched on

(ldquoEXCITATION ONrdquo) A ldquosmoothrdquo excitation of the generator can be achieved in this

way whenever there is no demand for a quick excitation

232 Voltage Set-Point

Various signals and settings control and limit the voltage set-point F270 For

example the values of F254 and F252 define the normal operating range possible for set-

point adjustment (eg 90 110) using external control commands (control room local

operatorrsquos panel superposed control system) The effective set-point adjustment rate is

governed The set-point can be set at the values of F250 and F256 by activating

appropriate control commands for ldquoSETrdquo input Enabling Flag F71A and activating a

binary input prior to switching on the excitation (ldquoEXCITATION OFFrdquo) sets the Ug set-

point at the value of UAUX This makes it possible for example to ensure that the

generator voltage will agree exactly with the network voltage after the voltage build-up

An external value with variable amplification can be added to the Ug set point by

enabling F724 (for example for stability tests)

233 Regulator Tracking in MANUAL Operation

Whenever the AUTOMATIC channel is not in operation (the MANUAL channel

is ON) a follow-up equipment ensures a smooth switch-back to the AUTOMATIC

mode will always be possible To track the voltage set-point is shifted by means of

RAISELOWER pulses from the Gate Control Unit so that control variable Ucontr at the

output from the PID filter is held steady and identical to the control variable Ucontr from

the MANUAL channel Because this tracking must react slowly resultant transient

control deviations resulting from the amplification in the PID filter might cause severe

interference with control variable Ucontr

To prevent this the follow-up equipment intervenes on the regulators mixing

point with a corresponding compensation signal

234 Ugf Limiter

At under frequency the Ugf Limiter reduces the generator voltage so as to

prevent saturation effects in the supply and measuring transformers To adjust this

limiter the max permissible generator voltage at rated frequency is defined and set

When any under-frequency occurs the generator voltage is thus reduced in proportion to

that setting

235 Field Current Maximum Limiter

The Field Current Maximum Limiter is provided to protect the generator rotor

from s occurring in steady-state and transient operation High field currents are normally

the result of a sharp drop in network voltage or of an improper raising of the voltage set-

point by the operating staff The field current is held steady at the value TH1 ie at the

maximum thermal value permissible for the excitation circuit and the rotor In order that

the generator can support the power network with its transient overload capacity during

brief collapses in voltage a temporary switch-over is made to the transient limit MAX1

(a higher setting) When the generator or the converter is operating at a reduced capacity

These limits TH1MAX1 can be switched over to the lower settings TH2MAX2 by

activating the corresponding binary signals The switch-over from the thermal limit

TH12 to the transient limit MAX12 can be configured in one of three ways

a) Depending on the over current with -dUdt ENABLE

Flag programming F418 = any setting desired F41A = 0000

This variant enables the transient value MAX12 whenever a collapse of voltage

in the network is detected The ENABLE time is fixed and can be set The example

below shows the typical behavior of the limiter configured in this way

b) Dependent on the time integral with -dUdt ENABLE

Flag programming F418 = inactive F41A = 1111

This variant likewise enables the transient value only when a collapse of network

voltage has been detected However the switch-back to the thermal limit is not made

dependent upon the time itself but on the calculated time integral intisup2dt of the The setting

on Parameter F414 in spu takes into account the time the rotor needs to cool down ie

the rate of temperature change in the case of intermittent operation The example below

shows how the timing of the switch-back to the thermal limit depends on the present

value for intisup2dt

The time integral is based on the formula

Example The setting of =isup2dt equivalent to Version a (F416) at a constant 16 times the

nominal field current for 10 seconds (with TH12 = 105) is

c) Dependent on the time integral without any preconditions

Flag programming F418 = 1111 F41A = 1111

In this variant the transient becomes available without any prior conditions

(without a -dUdt ENABLE) with the time integral intisup2dt

237 Inductive Stator Current Limiter

The Inductive Stator Current Limiter holds the stator current Ig within permissible

limits while the generator is in the ldquoover-excitedrdquo operating range by reducing the field

current accordingly The setting TH (thermal limit) provides the limit against stationary s

that might occur To take advantage of the generatorrsquos transient overload capacity a

switch-over is made to the higher setting MAX The principle of operation of this switch-

over to the value MAX permissible only transiently is identical to that employed for the

field current limiter (refer to the description above) When the drive output from the

turbine is very high stator current may exceed permissible limits even while inductive

loading of the generator is low In this case if the stator current limiter is not kept from

influencing the field current the control circuit will oscillate back and forth between the

Inductive Stator Current Limiter (de-

exciting) and the Capacitive Stator Current Limiter (exciting)The output signal of that

function then dominates the control variable of the Ig-dependent limiter via a maximum

value selection

238 Capacitive Stator Current Limiter

239 Load Angle Limiter

The Load Angle Limiter prevents the synchronous machine from slipping out of

phase due to slippage of the rotor The load angle δ the difference in phase between the

rotor and the stator rotating field results mainly from the driving torque (active power P)

acting on the generator and the level of rotor current (field current) If the driving torque

remains constant a increase in the field current reduces the load angle δ The current load

angle δ at any moment is obtained from the generator current and generator voltage based

on a simplified model of the generator Whenever this calculated load angle δ exceeds the

preset limit angle the limiter increases the field current until the load angle has dropped

back to its permissible value The quadrature reactance Xq of the generator and the

network reactance Xe during normal operation must be adjusted on the regulator in order

to obtain the load angle δ The graph below shows the Power Chart for a salient-pole

machine with typical limiter characteristics

The purpose of a Power System Stabilizer is to use the generator excitation to

damp electromechanical oscillations between the network and the generator Depending

on the design of the generator and the requirements imposed for network stability its

main function will be either to damp the oscillations originating in the machine or those

from the network A synchronous generator working in a combined power network is in

principle an oscillating structure In order to produce a torque the magnetic field of the

rotor and the stator must form a given angle (referred to as the rotor displacement or load

angle δ) The electrical torque ME increases as the angle δ increases just as with a

torsion spring Because the ME of the generator and the mechanical driving torque MA

from the turbine are in equilibrium during steady-state operation the angle δ remains in a

given position Whenever this state of equilibrium between MA and ME is disturbed the

load angle slips of this rest position and change thereby the electrical torque ME The

torque attempts to restore the load angle to a stationary position Due to the mass inertia

of the turbinegenerator rotor however this can only take place aperiodically It does so

in the form of more or less effectively damped oscillations (again similar to the effect of

mass inertia on a torsion spring) In order to damp the oscillations there must be a

damping torque produced depending not on the electrical torque ME associated with the

angle but on the difference in frequency (Df) between the rotor and the stator rotating

field ie on the slippage This torque is produced mainly by the so-called damper

winding in the rotor but the dimensioning of this is subject to limits imposed by

considerations of design and economy Some further action is therefore needed to

increase the damping effect The following drastically simplified formula shows the

parameters upon which the amount of active power PE supplied by the generator

depends

PE = active power

It can be seen from the above relationship that the active power that the generator

transfers depends not only on the load angle δ but also on the field current If That means

that a transient change can be made in the active power PE and with that in the effective

electrical torque ME by varying the field current The principle of operation of the DVR

Power System Stabilizer becomes clear from a consideration of the oscillations in power

output and frequency (ΔPE Δf) and the vector diagram If it is assumed that oscillations

in the network frequency generates load oscillations with the mass inertia of the rotor

then the active load of the generator (eg MW-measured) is influenced with a sinusoidal

value -ΔPE (ME-MA = -ΔPE) By inversion of -ΔPE one obtains the fluctuation in

power provided by the rotor +ΔPE As is known the slip signal Δf follows +ΔPE with a

phase delayed by 90deg The +ME produced by the periodic changes in the load angle δ is

in phase with +ΔPE A good damping is attained if ME is varied in phase with the slip

Δf However this signal must also be advanced somewhat to compensate for the time

constants in the excitation circuit and the generator

As mentioned above the electrical torque ME can be influenced by varying the

field current To accomplish this a suitable control signal referred to as variable

disturbance compensation must be imposed upon the voltage set-point or the converter

control variable Ucontr As can be seen from the vector diagram by applying proper

weighting factors (K1 K2) and then adding together the signals -ΔPE and Δf an overall

stabilization signal can be produced that rotates in advance of the Df signal by any angle

desired between 0deg and 90deg Because the amplitude of -DPE remains proportional to the

amplitude of Δf a constant angle in advance of Δf results for the compensation of the

time constants referred to above The optimum weighting factors K1 and K2 for a

synchronous generator working to a power network depend on its operating point at any

moment and the external reactance of the network Normally the selection of a

compromise setting is good enough to attain stability in all operating points and for all

external reactance For special demands these settings must be parameterized as a

function of the external reactance (which means optional equipment Xe-Identification)

The Power System Stabilizer PSS is a section of the AVR computer program and is

processed once per network cycle The voltage at the generator terminals and the

generator current are measured in order to define the signals ΔPE and Δf The calculated

signals for _P_ (=PE) and Δf are then sent across DC filters ldquoDrdquo (real differentiators) that

transmit only the dynamic portion of the signals The ΔPE and Δf signals obtained in this

way are then weighted (multiplied by) with the factors K1 and K2 and sent to the

summing point of the voltage regulator

The PSS stabilization signal is imposed on the automatic voltage regulator only if

the following prerequisites are met

bull Generator on line

bull Generator power output gt the value F338

bull Generator voltage in a range between F33C and F33A

The stabilization signal is limited at the output from the PSS to the lower and

upper limits Flag defines whether the stabilization signal is introduced before or after the

PID filter (usually before the filter) Because the PID filter as noted above already takes

the ceiling factor Vp1 into account the PSS signal needs to be multiplied by Vp1 if it is

added to the voltage regulator following the PID filter (divider at the input to the

minmax limiter) This precaution prevents the DC filter ldquoDrdquo in the P-channel from

producing an unnecessary ldquostabilizationrdquo effect in the case of rapid changes in turbine

load As an alternative for the AVRrsquos Power System Stabilizer a stabilization signal from

an outside system can be imposed by activating the binary input ldquoPSS-SIGNEXTrdquo Flag

F340 can be used to select between an analog and a 12-bit signal and F33E to select the

polarity desired for that signal

24 The MANUAL Channel

241 Summary

The MANUAL channel (Channel 2) has been built as a simple field current

regulator

without additional limiters Its main function is to maintain the excitation of the generator

even if the AUTOMATIC channel becomes non-operational The MANUAL channel

also performs valuable service for purposes of testing commissioning and preventive

maintenance Its measurements regulator generation of firing pulses and power supply

are physically separate from those on the AUTOMATIC channel

242 Principle of Operation

All the functions of the MANUAL channel including the generation of firing

pulses have been implemented in a single electronic module the Gate Control Unit The

control variable Ucontr of voltage regulator is used as the reference value for generating

firing pulses on the principle known as ldquoramp controlrdquo (Comparison of Ucontr with

Usynsynchronous sawtooth signal) For further processing in the UN 0096 Intermediate

Pulse Stage the Gate Control Unit supplies six firing pulses at its output whose phase

position with respect to the synchronous voltage Usyn is in accordance with control

variable Ucontr An internal linearization ensures that the field voltage produced via the

firing pulses remains proportional to the control variable Ucontr throughout the entire

range As a result the circuit amplification of the control remains constant over the entire

range Whenever excitation is switched ON the set-point for Generator Voltage is set

automatically at the preset - ref Value This provision ensures that the generator voltage

always attains approximately its nominal value after the field flashing The Gate Control

Unit can be refunctioned ( by pre-selection with a switch ) for purposes of testing to act

as a purely firing pulse control In this case the control variable Ucontr is adjusted

directly using the RAISELOWER push buttons on the front of the module In this way

for example the relationship between the phase position of the firing pulses and the

control variable Ucontr can be checked easily

CHAPTER 3

PULSE SECTION

31 Pulse Generation and Amplification

The Gate Control Units of both AUTOMATIC channel and MANUAL channel

each supply six firing pulses for operating the 6-pulse thyristor bridges The low-power

pulse signals from these Gate Control Units are then amplified in the Intermediate Pulse

Stage galvanically isolated and then sent to the common pulse bus On the output end

the Intermediate Pulse Stage of the non-active channel) is always blocked The Gate

Control Units generate the pulses based on microprocessor control The reference voltage

used for the firing pulse phase location is the output voltage from the excitation

transformer (Usyn1 Usyn2) The commutation spikes of the synchronous voltage caused

by the converter are calculated prior to use of the voltage as a reference value and are

deliberately filtered out The lower limit for the firing pulses (double pulses) which are

offset from one another by 60deg is defined by the limit rectifier position (αmin) and the

upper limit by the limit inverter position (αmax) for the firing angle αmin and αmax can

be adjusted on the Gate Control Units using BCD (Binary Coded Decimal) switches

αmin ensures that the firing pulses will not be issued (premature firing) until there is

sufficient positive phase voltage on the thyristor involved αmax prevents a dangerous

ldquotippingrdquo of the thyristor bridge into the rectifier mode if the firing angle α is too large

(ldquolate firingrdquo) The critical factors determining αmax are the overlap time uumlmax (max

commutation time) and the ldquorecovery timerdquo of the thyristors (αmax lt 180deg - uumlmax - γ )

An external control signal can force the firing pulses into their inverter limit position

Other binary inputs can block or direct the firing pulses of the Gate Control Units so as to

produce freewheeling on the thyristor bridge During freewheeling the firing pulses for

the thyristor pair R and S are blocked and the pulse signals T+T- are engaged with

chains of pulses Both Gate Control Units (for the MANUAL amp AUTOMATIC channels)

contain a field current monitor that blocks the firing pulses immediately whenever the

current exceeds a preset threshold level In this case the field circuit-breaker is also

tripped via an output contact The purpose of these provisions is to prevent damage to

thyristors and thyristor fuses in case of a slip-ring short-circuit or to keep any damage

that does occur to a minimum The pulse signals are galvanically separated at the outputs

from the Intermediate Pulse Stage (with pulse transmitters) and are then directed to the

common pulse bus This transmission of the pulse signals to the pulse bus via passive

transmitters ensures a high degree of active channel autonomy Practically no possible

malfunctions on the inactive channel (including for example sustained pulses) affect the

active channel

32 Pulse Monitoring

The ldquoPulse Busrdquo and the pulse signals of the AUTOMATIC channel are

monitored This monitoring device consists of potential isolating stages and the common

monitor If the pulse monitoring of the ldquoPulse Busrdquo responds a switch-over is made to

MANUAL channel The function of the potential isolating stages is to couple the pulse

monitoring device to the pulse circuits without any feedback effect The pulse monitoring

checks the six pulse lines for the following malfunctions continuous or periodic failure

of one or more pulses Periodic occurrence of synchronous or asynchronous false pulses

Continuous pulses the pulse monitoring device can be tested while the machine is in

operation

CHAPTER 4

CONVERTER

Thyristor

The term thyristor usually refers to a family of four layer solid state device having

turn on characteristics that can be externally controlled by either current or voltage They

are also referred to as breakdown device because their working depends on avalanche

breakdown Thyristors have only two stages OFF and ON Thyristors have a similar

function to Uni-junctions they act as switches Thyristors use current flow as a switch

Thyristors have three states

1 Reverse blocking mode mdash Voltage is applied in the direction that would be

blocked by a diode

2 Forward blocking mode mdash Voltage is applied in the direction that would cause

a diode to conduct but the thyristor has not yet been triggered into conduction

3 Forward conducting mode mdash The thyristor has been triggered into conduction

and will remain conducting until the forward current drops below a threshold value

known as the holding current Converter is a semiconductor device which converts ac

input voltage into a constant dc output voltage In present excitation system three phase

fully controlled thyristor converter is used

Because of the following advantages thyristor converters are used

a) Thyristors are used for high power applications ie up to 10Kv3500A1KHz

b) Having high reliability and low losses

c) Uni-directional device like diode

d) Itrsquos operation as a rectifier which are low resistance in forward conduction

mode and high resistance in reverse conduction mode

PROTECTION OF THYRISTORS

For reliable operation of a thyristor demands that its specified ratings are not

exceeded When Subjected to or over voltages During the turn - on of SCR didt

prohibitively large False triggering of SCR by high value of dvdt andSpurious signals

between gate and cathode may leads to unwanted turn ndash on

DIDT AND PROTECTION

When thyristor starts conducting in forward conduction mode and is turned on by

gate pulse The anode current increases rapidly whole area of the gate to Cathode

junction then hot spots will be formed near the gate connection this locality of heating

destroys the thyristor Thyristor thermal time is constant The causes due to faults and

short circuits or surge currents Electronic crowbar protection is used against the over

voltages The rate rise of anode current must be kept at the time of turn on below the

rated or specified limiting value The didt value maintained below limited value by using

a inductor also called ldquodidt inductorrdquo in series with anode circuit The locality of heating

is avoided by applying gate current but not greater the maximum gate current

DVDT AND OVER VOLTAGE PROTECTION

With forward voltage across the anode and cathode of a thyristor the two outer

junctions are forward biased but the inner junction is reverse biased This reverse biased

junction J2 has the characteristics of a capacitor due to charges existing across the

junction In other words space-charges exist in the depletion region around junction J2

and therefore junction J2 behaves like a capacitance If the entire anode to cathode

forward voltage Va appears across J2 junction and the charge is denoted by Q then a

charging current i given by Eq (46) follows

i = dQdt =d(Cj Va )dt

= Cj (d Va dt) + Va(d Cj dt) helliphelliphelliphellip(46 a)

As Cj the capacitance of junction J2 is almost constant the current is given by

i = Cj (d Va dt) helliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip(46 b)

If the rate of rise of forward voltage dVadt is high the charging current i will be

more This charging current plays the role of gate current and turns on the SCR even

when gate signal is zero Such phenomena of turning-on a thyristor called dvdt turn-on

must be avoided as it leads to false operation of the thyristor circuit

For controllable operation of the thyristor the rate of rise of forward anode to

cathode voltage dVadt must be kept below the specified rated limit Typical values of

dvdt are 20 ndash 500 Vμsec False turn-on of a thyristor by large dvdt can be prevented by

using a snubber circuit in parallel with the device thyristor are very sensitive for over

voltage than the semiconductor devices

Over voltage transients are perhaps the main cause of thyristor failure

In thyristor there are mainly two types

1 Internal over voltages

Due to the commutation of the thyristors large voltages are generated internally

Because of the series inductance of the SCR circuit the large transient voltages L didt

produced This voltage several times the break over voltage of the device then thyristor

destroys permanently

2 External over voltages

External over voltages are caused due to the interruptions of current flow in an

inductive circuit and also due to the lightening strokes on the lines feeding the thyristor

system For the reliable operation of thyristor the over voltages must be suppressed by

adopting suitable techniques

Suppression of over voltages

The RC circuit called snubber circuit is connected across the device to protect In

order to keep the protective components to a minimum the thyristors are chosen with

their peak voltages ratings are 25 to 3 times of the normal peak working voltage

ldquoselenium thyrector diodes metal oxide varistors or avalanche diode suppressers are

commonly employed for protecting the thyristor circuit against the over voltages

Gate protection

Gate circuit should also be protected against the over voltages and surges Over

voltage at gate circuit can cause false triggering of the SCR may rises the junction

temperature behind specified limit leading to its damage Protection against over

voltage can be achieved by connecting a ZD across the gate circuit and a resister is

connected in series with gate circuit to protect against the s A capacitor and resister are

connected across gate to cathode to by pass the noise

41 Final Pulse Stages

The Final Pulse Stages adapt the output pulses from the Intermediate Pulse Stage

(pulses on the pulse bus) to the gate currents needed for the thyristors Each thyristor

bridge is equipped with its own Final Pulse Stage Each Final Pulse Stages is provided

with a power supply module The amplified output pulses from the Final Pulse Stages

start as a short strong steep pulse with an amplitude approx 2frac12 times that of the main

pulse This initial pulse edge assures proper firing of the thyristors being triggered

Subsequently the weaker part of main pulse keeps firing conditions steady As already

mentioned the Final Pulse Stages and their associated thyristor bridges form single units

All six pulse outputs from a Final Pulse Stage can be blocked by an external control

signal so that all thyristors in the associated thyristor bridge will block the current A

blocking of the pulses is initiated whenever there is a malfunction in the associated

thyristor bridge

42 Converter Power Section

The thyristor converter consists of three independent parallel rectifier blocks TY1

to TY3 which are all in service Even if one block fails the remaining blocks take over

automatically the full design current of the excitation circuit During normal operation

(with ideal current share) and all three bridges in operation each of these blocks has to

carry only (n-2)n (ie33)of its design current If 2 thyristor bridges fail the excitation

is limited Only when all three bridges fail the excitation is switched off Each thyristor

bridge arm is equipped with current flow monitoring CTrsquos Failure of conduction in any

arm is identified by a Current flow monitoring module

43 Converter Cooling

A cooling system is needed to dissipate heat losses in the converter blocks and

electronics Each converter block has therefore been equipped with a fan supplied with

power from the converterrsquos primary voltage (via transformer ndashT8 in field flashing

cubicle) The fans are protected with motor protection circuit breakers An air flow

monitoring unit is provided for monitoring the air flow through the thyristor bridge If a

circuit breaker failure is detected or if the air flow monitor drops off at one of the

thyristor bridges the bridge involved is immediately set out of operation by blocking its

firing pulses

44 Thyristor Converter Monitoring

A thyristor bridge in which defects occur that could threaten the safety of

operation or cause secondary damage is switched off automatically ie its firing pulses

are blocked This happens whenever A thyristor fuse is blown The fuses are monitored

individually with micro switches The Final Pulse Stage fails which is detected by

internal monitors (supply voltage sustained pulse short-circuit on the output end) The

power supply to the fan fails fan air flow as monitored by the Air flow monitor fails or

is insufficient Isolator on ACDC side is open

CHAPTER 5

Field Current Circuit Excitation Transformer

51 Field Circuit Breaker

The circuit-breaker in the field circuit is used to isolate the field circuit from the

converter It is capable of switching off the synchronous machine from full load under the

maximum conditions of a 3-phase short-circuit In addition to its main contacts the field

circuit-breaker also has a de-excitation contact with which the field energy stored in the

field can be dissipated across the de-excitation resistor The de-excitation contact closes

shortly before the main contacts open so as to ensure proper commutation of the field

current from the main contacts to the de-excitation contact when the breaker is switched

off The field circuit-breaker is switched on by electromagnetic force and is kept switched

on by a mechanical latch When the latch is released by a trip coil the circuit-breaker

opens The circuit-breaker also has auxiliary contacts that report its status

52 Field Flashing

In shunt supplied excitation circuits (excitation transformer connected to the

generator terminals) the generator does not have enough remnant voltage for a generator

voltage build-up via the converter In this case a field flashing circuit is provided It

consists of the field flashing contactor the diode bridge and a transformer used to adapt

the auxiliary input voltage to the voltage needed for field flashing when power is

supplied from the auxiliaries network

Fig Field Flashing

Because the field flashing contactor is not able to switch off the energy stored in

the field the control ensures that the contactor can only reopen if the field circuit breaker

has already been opened (generating the TRIP order) or in a normal field flashing

sequence when the converter has taken over the field current Field flashing occurs in the

following stages

1048729The excitation is switched on closing the field flashing contactor ( Field

Circuit Breaker is already closed )

1048729The start-up excitation current flows through the rotor driving the generator

voltage up to approx 15 U

1048729After about 10 U the firing pulses to the converter are released and it begins

to excite the generator to its rated voltage

1048729After about 30 U the field flashing contactor opens (with no current since

the converter is now supplying the current)

The diode bridge at the input to the field flashing contactor prevents a feed-back

from the converter to the source of field flashing while the contactor is still closed

53 De-excitation

When malfunctions occur the stored field energy must be dissipated as quickly

and safely as possible to protect the generator This is done by the converter the field

circuit-breaker and the de-excitation (discharge) resistor

De-excitation (with opening of the field circuit-breaker) takes place in the following

stages

1048729The converter drives to its inverter limit position (negative ceiling voltage)

recovers a portion of the field energy into the network A trip command is given to the

field circuit breaker

1048729The de-excitation contact closes diverting the field voltage to the de-excitation

resistor

1048729Then immediately the main contacts open building voltage The field voltage

commutates to the de-excitation resistor

1048729The current diminishes at a given time constant TE

(With linear resistance TE = Lf (Rf + Re))

Due to the reversal of the field voltage by the converter the field current

commutates from the main contacts of the field circuit-breaker to the de-excitation

resistor in a very early phase This reversal of the field voltage prevents burn-off on the

main contacts and provides effective protection for the field circuit-breaker Depending

on the operating policy an operational shut-down of the excitation can also be effected

with the field circuit-breaker closed This method is useful mainly when the excitation is

switched on and off frequently In this case the converter is merely driven into the

inverter limit position so that the field energy is recovered into the network The

converter then blocks since it is supplying positive current only

54 Excitation Transformer

The excitation transformer matches the generator voltage to the field voltage

(required ceiling voltage) It also serves as a commutation reactance for the thyristor

converter and as a potential isolator between the network and the excitation circuit In

addition the transformer functions as a current limiter in that it makes it possible to keep

any short circuits in the excitation circuit under better control The excitation transformer

is equipped with temperature monitoring probes which set off an alarm when the

temperature exceeds a first max limit and then trips the excitation if the temperature

continues rising to a second (higher)limit

CHAPTER 6

Monitoring and Protection

61 Excitation Monitoring

611 General Information

The main goal of Excitation Monitoring is to make optimum use of the

redundancies provided in the excitation system and to give alarm whenever a malfunction

makes these redundancies unavailable The field current is monitored to see that it does

not exceed a maximum level and if necessary a switch-over to the MANUAL channel is

initiated In addition the criterion for switching off the field flashing is generated The

excitation Monitoring consists of an autonomous processor system

612 over current Alarms

In the Excitation Monitoring the limits for are set at higher levels than the

settings on the Field Current Maximum Limiter Whenever the current exceeds 110 of

the nominal field current contact R1 and the binary output associated with it are

activated immediately If field current remains gt 110 then after a preset inverse-time

has lapsed relay R2 and - after a further delay - relay R0 and the binary outputs

associated with them are activated Parameters match the measurements for If1 and If2 to

the nominal value for field current so that the internal values can be processed and read as

pu values It can be used to falsify the actual value of the field current If (to raise it) so

as to cause a response from the alarm limits for purposes of testing The processed If

signal is always taken from on the active channel (CHANNEL I OR CHANNEL II) As

long as the field current If is above the threshold value 11 Ifn its peak value is

measured This is stored (until RESET) and can be read at any time on the Micro-

Terminal Once the value of If exceeds 11 Ifn integration of this value starts Whenever

the integrated time-current value (intisup2dt) exceeds the preselected reference value the

alarm OVER CURRENT INVERSE-TIME is set off and a command is simultaneously

issued to switch over to the stand by AUTO channel Software switch F758 enables the

three over current alarm functions (R0 R1 R2) and selects one of three possible inverse-

time curves T1 T2 or T3 Within the characteristic curve (T1 T2 T3) selected the

desired limit curve for response is set using the factor F216

613 Switch-Off Criterion for Field Flashing

The Excitation Monitoring supplies the criterion for switching off the field

flashing Whether this criterion is activated based on the actual value for generator voltage

Ug or for field current If or both depends on the settings of the two threshold values

F200F202 (0 setting means that the output is always ldquological 1rdquo) The measurements

Ug12 and If12 are switched over depending on the present status of the channels

(Channel 1 or Channel 2 ON) Whenever Flag F750 is not activated the binary output is

fixed at ldquological 1rdquo

614 Storage of Alarm Status

The outputs of the over current alarms (R1 R2 R0) and the messages NO

FAILURE MONITORING PARAMETERS CHANGED are stored messages can be

erased by activating the input ldquoGENERAL RESETrdquo or by using the RESET button on the

front of the module Erasure with the input ldquoGENERAL RESETrdquo is effective only if the

situation causing the alarm or the malfunction is no longer present Whenever the self-

diagnosis equipment in the processor detects a malfunction the output NO FAILURE of

MONITORING is set at ldquological 0rdquo (= alarm) The alarm ldquoPARAMETERS CHANGEDrdquo

is activated whenever parameters or settings of software switches have been changed via

the Micro-Terminal

615 Actual Value Monitoring

The actual values for generator voltage Ug synchronous voltage Usyn and field

current If are monitored for malfunctions This monitoring is active regardless of whether

or not the generator is in operation Essentially when the generator is in operation the

measurements are monitored by comparing the signals (the smaller signal reading is

detected as incorrect) When the generator is not in operation the measured data are

monitored for extreme values The percentage of deviation permissible in the

measurement signals being compared

(Ug1ampUsyn1 Ug1ampUg2 Ug2ampUsyn2 If1ampIf2) is defined by parameters F208 and

F20AIf the excitation transformer is being supplied from an auxiliary power source (no

shunt operation) the values of Ug and Usyn will be different in some operational

conditions

In that case Flag F75A can be used to deactivate comparative measurements Ug1 Usyn1

and Ug2 Usyn2 The ldquoprimary monitoringrdquo compares Ug1 with Ug2 and generates the

messages that Channel 1 or Channel 2 has suffered a malfunction Whenever Ug1 lt Ug2

and the binary message from CH1 reports no malfunction a malfunction on

Measurement Channel 1 is reported (Ug1Usyn1 FAILURE) A similar malfunction is

also present whenever the binary message CH1 DISTURBANCE is reported and a

discrepancy is detected between Ug1ampUg2 The generation of the alarm ldquoUg2 Usyn2

FAILURErdquo is analogous to that for Channel 1 The ldquosecondary monitoringrdquo compares

Ug1 with Usyn1 amp Ug2 with Usyn2 This is enabled whenever the binary message of the

comparison channel reports a malfunction or whenever both binary messages report no

malfunction - but both secondary monitors report a malfunction As long as the secondary

monitoring is blocked the differences Ug1neUsyn1 or Ug2neUsyn2 trigger malfunction

signals for the measurement channel involved (suspicion that there is a corresponding

error in Usyn) The measurement channel malfunctions are enabled operationally

whenever after excitation has been switched on generator voltage Ug exceeds the value

set on F204 The voltages Ug1 and Ug2 are checked 16 seconds after the excitation is

switched off to see that they do not exceed the limit value F210 that applies to both of

them At the same time g1Ug2Usyn1Usyn2 are checked for extreme values (gt or lt

the operating range) Monitoring for extreme values is likewise enabled during normal

operation ( Excitation ON and Ug gt F204 ) Flag F754 is used to enable or block the

malfunction signals to the binary outputs Basically the monitoring of the actual values

for If1If2 functions like that of the Ug1Ug2 monitoring

62 Excitation Protection

621 General Information

The Excitation Protection switches off the excitation (and de-excites the machine

rapidly) whenever a danger arises that threatens the excitation transformer the converter

or the generator Generally limiter or monitoring functions precede the emergency trips

and these normally respond before the Excitation Protection must initiate a trip

Protective trip commands are issued directly to the field circuit-breaker from potential

free contacts of the board via the trip relays They are directed redundantly to the

operative field circuit-breaker ldquoOFFrdquo command

622 Protection against Excitation Transformer Overheating

This equipment monitors the excitation transformer for overheating in the

windings that could result from over current short-circuits or inadequate cooling The

monitoring uses temperature monitoring modules in conjunction with temperature

sensors built into the windings Normally the temperature is monitored in two stages the

first stage sets off an alarm the second causes a trip of the excitation

623 Rotor Over voltage Protection

Malfunctions in the generator circuit (eg terminal short-circuit failed

synchronization asynchronous operation) cause induced negative field currents that

produce high voltages in the field circuit These must be restricted to a level with a

sufficient safety margin below the insulation capacity of the field winding (test voltage)

and also below the peak blocking voltage of the converter thyristors The crow bar

employs spark gap elements to detect over voltages in the field circuit Whenever they

respond the associated thyristors are fired immediately switching the de-excitation

resistor parallel to the field The de-excitation current generated thereby initiates an

excitation trip via a supervision circuit causing an immediate opening of the field circuit-

breaker The malfunction isets off an alarm and an internal malfunction is indicated at the

cubicle

TEST VALUES OBTAINED WHEN EXCITATION IS RAISED

TEST

SNO PARTICULARS ACTUAL VALUE OBTAINED VALUE

1 VREF 996 100

2 VACT 997 1003

3 IFACT 735 765

4 IGACT 703 707

5 ACTIVE

POWER

703 705

6 REACTIVE

POWER

101 142

7 POWER

FACTOR

099 IND 098 IND

8 ACTIVE

CURRENT(IR)

705 703

9 REACTIVE

CURRENT(IX)

102 138

10 POWER

ANGLE

566 546

11 FIRING

ANGLE

640 632

VALUES OBTAINED WHEN EXCITATION IS LOWERED

SNO PARTICULARS ACTUAL

VALUE

OBTAINED VALUE

1 VREF 100 997

2 VACT 100 997

3 IFACT 787 761

4 IGACT 839 837

5 ACTIVE

POWER

839 830

6 REACTIVE

POWER

155 110

7 POWER

FACTOR

098 IND 099 IND

8 ACTIVE

CURRENT(IR)

835 830

9 REACTIVE

CURRENT(IX)

140 94

10 POWER

ANGLE

613 DEG 631 DEG

11 FIRING

ANGLE

601 DEG 599 DEG

CONCLUSION

For generating the EMF in stator winding excitation is required to the rotor of a

generator There are two types of excitation

1 Static excitation system

2 Brushless excitation system

A certain disadvantage in brushless excitation system is the slow response time of

the field in case of fast load changes specified No slip-rings and brushes direct

measurements of the field parameters not possible

To avoid all loses static excitation is used Since it does not have any rotating

parts mechanical loses and windage loses This system has fast response and speed

control While preferring this excitation system there are no limitations for the

redundancy of Thyristor bridge circuits

Static excitation has fast field discharge by resistor and inverter operation direct

measurement of field quantity is possible The meaning of excitation is nothing but

continuous supply of DC current (ie field current) to the rotor to buildup required

output voltage in the stator

Field current is changed with respect to the change of load so the digital

automatic voltage regulator (DAVR) is used to regulate the output voltage according to

the load variations

So we conclude that static excitation system with DAVR is preferred since it is

having excellent dynamic performance and better options for R amp M

234 Ugf Limiter

At under frequency the Ugf Limiter reduces the generator voltage so as to

prevent saturation effects in the supply and measuring transformers To adjust this

limiter the max permissible generator voltage at rated frequency is defined and set

When any under-frequency occurs the generator voltage is thus reduced in proportion to

that setting

235 Field Current Maximum Limiter

The Field Current Maximum Limiter is provided to protect the generator rotor

from s occurring in steady-state and transient operation High field currents are normally

the result of a sharp drop in network voltage or of an improper raising of the voltage set-

point by the operating staff The field current is held steady at the value TH1 ie at the

maximum thermal value permissible for the excitation circuit and the rotor In order that

the generator can support the power network with its transient overload capacity during

brief collapses in voltage a temporary switch-over is made to the transient limit MAX1

(a higher setting) When the generator or the converter is operating at a reduced capacity

These limits TH1MAX1 can be switched over to the lower settings TH2MAX2 by

activating the corresponding binary signals The switch-over from the thermal limit

TH12 to the transient limit MAX12 can be configured in one of three ways

a) Depending on the over current with -dUdt ENABLE

Flag programming F418 = any setting desired F41A = 0000

This variant enables the transient value MAX12 whenever a collapse of voltage

in the network is detected The ENABLE time is fixed and can be set The example

below shows the typical behavior of the limiter configured in this way

b) Dependent on the time integral with -dUdt ENABLE

Flag programming F418 = inactive F41A = 1111

This variant likewise enables the transient value only when a collapse of network

voltage has been detected However the switch-back to the thermal limit is not made

dependent upon the time itself but on the calculated time integral intisup2dt of the The setting

on Parameter F414 in spu takes into account the time the rotor needs to cool down ie

the rate of temperature change in the case of intermittent operation The example below

shows how the timing of the switch-back to the thermal limit depends on the present

value for intisup2dt

The time integral is based on the formula

Example The setting of =isup2dt equivalent to Version a (F416) at a constant 16 times the

nominal field current for 10 seconds (with TH12 = 105) is

c) Dependent on the time integral without any preconditions

Flag programming F418 = 1111 F41A = 1111

In this variant the transient becomes available without any prior conditions

(without a -dUdt ENABLE) with the time integral intisup2dt

237 Inductive Stator Current Limiter

The Inductive Stator Current Limiter holds the stator current Ig within permissible

limits while the generator is in the ldquoover-excitedrdquo operating range by reducing the field

current accordingly The setting TH (thermal limit) provides the limit against stationary s

that might occur To take advantage of the generatorrsquos transient overload capacity a

switch-over is made to the higher setting MAX The principle of operation of this switch-

over to the value MAX permissible only transiently is identical to that employed for the

field current limiter (refer to the description above) When the drive output from the

turbine is very high stator current may exceed permissible limits even while inductive

loading of the generator is low In this case if the stator current limiter is not kept from

influencing the field current the control circuit will oscillate back and forth between the

Inductive Stator Current Limiter (de-

exciting) and the Capacitive Stator Current Limiter (exciting)The output signal of that

function then dominates the control variable of the Ig-dependent limiter via a maximum

value selection

238 Capacitive Stator Current Limiter

239 Load Angle Limiter

The Load Angle Limiter prevents the synchronous machine from slipping out of

phase due to slippage of the rotor The load angle δ the difference in phase between the

rotor and the stator rotating field results mainly from the driving torque (active power P)

acting on the generator and the level of rotor current (field current) If the driving torque

remains constant a increase in the field current reduces the load angle δ The current load

angle δ at any moment is obtained from the generator current and generator voltage based

on a simplified model of the generator Whenever this calculated load angle δ exceeds the

preset limit angle the limiter increases the field current until the load angle has dropped

back to its permissible value The quadrature reactance Xq of the generator and the

network reactance Xe during normal operation must be adjusted on the regulator in order

to obtain the load angle δ The graph below shows the Power Chart for a salient-pole

machine with typical limiter characteristics

The purpose of a Power System Stabilizer is to use the generator excitation to

damp electromechanical oscillations between the network and the generator Depending

on the design of the generator and the requirements imposed for network stability its

main function will be either to damp the oscillations originating in the machine or those

from the network A synchronous generator working in a combined power network is in

principle an oscillating structure In order to produce a torque the magnetic field of the

rotor and the stator must form a given angle (referred to as the rotor displacement or load

angle δ) The electrical torque ME increases as the angle δ increases just as with a

torsion spring Because the ME of the generator and the mechanical driving torque MA

from the turbine are in equilibrium during steady-state operation the angle δ remains in a

given position Whenever this state of equilibrium between MA and ME is disturbed the

load angle slips of this rest position and change thereby the electrical torque ME The

torque attempts to restore the load angle to a stationary position Due to the mass inertia

of the turbinegenerator rotor however this can only take place aperiodically It does so

in the form of more or less effectively damped oscillations (again similar to the effect of

mass inertia on a torsion spring) In order to damp the oscillations there must be a

damping torque produced depending not on the electrical torque ME associated with the

angle but on the difference in frequency (Df) between the rotor and the stator rotating

field ie on the slippage This torque is produced mainly by the so-called damper

winding in the rotor but the dimensioning of this is subject to limits imposed by

considerations of design and economy Some further action is therefore needed to

increase the damping effect The following drastically simplified formula shows the

parameters upon which the amount of active power PE supplied by the generator

depends

PE = active power

It can be seen from the above relationship that the active power that the generator

transfers depends not only on the load angle δ but also on the field current If That means

that a transient change can be made in the active power PE and with that in the effective

electrical torque ME by varying the field current The principle of operation of the DVR

Power System Stabilizer becomes clear from a consideration of the oscillations in power

output and frequency (ΔPE Δf) and the vector diagram If it is assumed that oscillations

in the network frequency generates load oscillations with the mass inertia of the rotor

then the active load of the generator (eg MW-measured) is influenced with a sinusoidal

value -ΔPE (ME-MA = -ΔPE) By inversion of -ΔPE one obtains the fluctuation in

power provided by the rotor +ΔPE As is known the slip signal Δf follows +ΔPE with a

phase delayed by 90deg The +ME produced by the periodic changes in the load angle δ is

in phase with +ΔPE A good damping is attained if ME is varied in phase with the slip

Δf However this signal must also be advanced somewhat to compensate for the time

constants in the excitation circuit and the generator

As mentioned above the electrical torque ME can be influenced by varying the

field current To accomplish this a suitable control signal referred to as variable

disturbance compensation must be imposed upon the voltage set-point or the converter

control variable Ucontr As can be seen from the vector diagram by applying proper

weighting factors (K1 K2) and then adding together the signals -ΔPE and Δf an overall

stabilization signal can be produced that rotates in advance of the Df signal by any angle

desired between 0deg and 90deg Because the amplitude of -DPE remains proportional to the

amplitude of Δf a constant angle in advance of Δf results for the compensation of the

time constants referred to above The optimum weighting factors K1 and K2 for a

synchronous generator working to a power network depend on its operating point at any

moment and the external reactance of the network Normally the selection of a

compromise setting is good enough to attain stability in all operating points and for all

external reactance For special demands these settings must be parameterized as a

function of the external reactance (which means optional equipment Xe-Identification)

The Power System Stabilizer PSS is a section of the AVR computer program and is

processed once per network cycle The voltage at the generator terminals and the

generator current are measured in order to define the signals ΔPE and Δf The calculated

signals for _P_ (=PE) and Δf are then sent across DC filters ldquoDrdquo (real differentiators) that

transmit only the dynamic portion of the signals The ΔPE and Δf signals obtained in this

way are then weighted (multiplied by) with the factors K1 and K2 and sent to the

summing point of the voltage regulator

The PSS stabilization signal is imposed on the automatic voltage regulator only if

the following prerequisites are met

bull Generator on line

bull Generator power output gt the value F338

bull Generator voltage in a range between F33C and F33A

The stabilization signal is limited at the output from the PSS to the lower and

upper limits Flag defines whether the stabilization signal is introduced before or after the

PID filter (usually before the filter) Because the PID filter as noted above already takes

the ceiling factor Vp1 into account the PSS signal needs to be multiplied by Vp1 if it is

added to the voltage regulator following the PID filter (divider at the input to the

minmax limiter) This precaution prevents the DC filter ldquoDrdquo in the P-channel from

producing an unnecessary ldquostabilizationrdquo effect in the case of rapid changes in turbine

load As an alternative for the AVRrsquos Power System Stabilizer a stabilization signal from

an outside system can be imposed by activating the binary input ldquoPSS-SIGNEXTrdquo Flag

F340 can be used to select between an analog and a 12-bit signal and F33E to select the

polarity desired for that signal

24 The MANUAL Channel

241 Summary

The MANUAL channel (Channel 2) has been built as a simple field current

regulator

without additional limiters Its main function is to maintain the excitation of the generator

even if the AUTOMATIC channel becomes non-operational The MANUAL channel

also performs valuable service for purposes of testing commissioning and preventive

maintenance Its measurements regulator generation of firing pulses and power supply

are physically separate from those on the AUTOMATIC channel

242 Principle of Operation

All the functions of the MANUAL channel including the generation of firing

pulses have been implemented in a single electronic module the Gate Control Unit The

control variable Ucontr of voltage regulator is used as the reference value for generating

firing pulses on the principle known as ldquoramp controlrdquo (Comparison of Ucontr with

Usynsynchronous sawtooth signal) For further processing in the UN 0096 Intermediate

Pulse Stage the Gate Control Unit supplies six firing pulses at its output whose phase

position with respect to the synchronous voltage Usyn is in accordance with control

variable Ucontr An internal linearization ensures that the field voltage produced via the

firing pulses remains proportional to the control variable Ucontr throughout the entire

range As a result the circuit amplification of the control remains constant over the entire

range Whenever excitation is switched ON the set-point for Generator Voltage is set

automatically at the preset - ref Value This provision ensures that the generator voltage

always attains approximately its nominal value after the field flashing The Gate Control

Unit can be refunctioned ( by pre-selection with a switch ) for purposes of testing to act

as a purely firing pulse control In this case the control variable Ucontr is adjusted

directly using the RAISELOWER push buttons on the front of the module In this way

for example the relationship between the phase position of the firing pulses and the

control variable Ucontr can be checked easily

CHAPTER 3

PULSE SECTION

31 Pulse Generation and Amplification

The Gate Control Units of both AUTOMATIC channel and MANUAL channel

each supply six firing pulses for operating the 6-pulse thyristor bridges The low-power

pulse signals from these Gate Control Units are then amplified in the Intermediate Pulse

Stage galvanically isolated and then sent to the common pulse bus On the output end

the Intermediate Pulse Stage of the non-active channel) is always blocked The Gate

Control Units generate the pulses based on microprocessor control The reference voltage

used for the firing pulse phase location is the output voltage from the excitation

transformer (Usyn1 Usyn2) The commutation spikes of the synchronous voltage caused

by the converter are calculated prior to use of the voltage as a reference value and are

deliberately filtered out The lower limit for the firing pulses (double pulses) which are

offset from one another by 60deg is defined by the limit rectifier position (αmin) and the

upper limit by the limit inverter position (αmax) for the firing angle αmin and αmax can

be adjusted on the Gate Control Units using BCD (Binary Coded Decimal) switches

αmin ensures that the firing pulses will not be issued (premature firing) until there is

sufficient positive phase voltage on the thyristor involved αmax prevents a dangerous

ldquotippingrdquo of the thyristor bridge into the rectifier mode if the firing angle α is too large

(ldquolate firingrdquo) The critical factors determining αmax are the overlap time uumlmax (max

commutation time) and the ldquorecovery timerdquo of the thyristors (αmax lt 180deg - uumlmax - γ )

An external control signal can force the firing pulses into their inverter limit position

Other binary inputs can block or direct the firing pulses of the Gate Control Units so as to

produce freewheeling on the thyristor bridge During freewheeling the firing pulses for

the thyristor pair R and S are blocked and the pulse signals T+T- are engaged with

chains of pulses Both Gate Control Units (for the MANUAL amp AUTOMATIC channels)

contain a field current monitor that blocks the firing pulses immediately whenever the

current exceeds a preset threshold level In this case the field circuit-breaker is also

tripped via an output contact The purpose of these provisions is to prevent damage to

thyristors and thyristor fuses in case of a slip-ring short-circuit or to keep any damage

that does occur to a minimum The pulse signals are galvanically separated at the outputs

from the Intermediate Pulse Stage (with pulse transmitters) and are then directed to the

common pulse bus This transmission of the pulse signals to the pulse bus via passive

transmitters ensures a high degree of active channel autonomy Practically no possible

malfunctions on the inactive channel (including for example sustained pulses) affect the

active channel

32 Pulse Monitoring

The ldquoPulse Busrdquo and the pulse signals of the AUTOMATIC channel are

monitored This monitoring device consists of potential isolating stages and the common

monitor If the pulse monitoring of the ldquoPulse Busrdquo responds a switch-over is made to

MANUAL channel The function of the potential isolating stages is to couple the pulse

monitoring device to the pulse circuits without any feedback effect The pulse monitoring

checks the six pulse lines for the following malfunctions continuous or periodic failure

of one or more pulses Periodic occurrence of synchronous or asynchronous false pulses

Continuous pulses the pulse monitoring device can be tested while the machine is in

operation

CHAPTER 4

CONVERTER

Thyristor

The term thyristor usually refers to a family of four layer solid state device having

turn on characteristics that can be externally controlled by either current or voltage They

are also referred to as breakdown device because their working depends on avalanche

breakdown Thyristors have only two stages OFF and ON Thyristors have a similar

function to Uni-junctions they act as switches Thyristors use current flow as a switch

Thyristors have three states

1 Reverse blocking mode mdash Voltage is applied in the direction that would be

blocked by a diode

2 Forward blocking mode mdash Voltage is applied in the direction that would cause

a diode to conduct but the thyristor has not yet been triggered into conduction

3 Forward conducting mode mdash The thyristor has been triggered into conduction

and will remain conducting until the forward current drops below a threshold value

known as the holding current Converter is a semiconductor device which converts ac

input voltage into a constant dc output voltage In present excitation system three phase

fully controlled thyristor converter is used

Because of the following advantages thyristor converters are used

a) Thyristors are used for high power applications ie up to 10Kv3500A1KHz

b) Having high reliability and low losses

c) Uni-directional device like diode

d) Itrsquos operation as a rectifier which are low resistance in forward conduction

mode and high resistance in reverse conduction mode

PROTECTION OF THYRISTORS

For reliable operation of a thyristor demands that its specified ratings are not

exceeded When Subjected to or over voltages During the turn - on of SCR didt

prohibitively large False triggering of SCR by high value of dvdt andSpurious signals

between gate and cathode may leads to unwanted turn ndash on

DIDT AND PROTECTION

When thyristor starts conducting in forward conduction mode and is turned on by

gate pulse The anode current increases rapidly whole area of the gate to Cathode

junction then hot spots will be formed near the gate connection this locality of heating

destroys the thyristor Thyristor thermal time is constant The causes due to faults and

short circuits or surge currents Electronic crowbar protection is used against the over

voltages The rate rise of anode current must be kept at the time of turn on below the

rated or specified limiting value The didt value maintained below limited value by using

a inductor also called ldquodidt inductorrdquo in series with anode circuit The locality of heating

is avoided by applying gate current but not greater the maximum gate current

DVDT AND OVER VOLTAGE PROTECTION

With forward voltage across the anode and cathode of a thyristor the two outer

junctions are forward biased but the inner junction is reverse biased This reverse biased

junction J2 has the characteristics of a capacitor due to charges existing across the

junction In other words space-charges exist in the depletion region around junction J2

and therefore junction J2 behaves like a capacitance If the entire anode to cathode

forward voltage Va appears across J2 junction and the charge is denoted by Q then a

charging current i given by Eq (46) follows

i = dQdt =d(Cj Va )dt

= Cj (d Va dt) + Va(d Cj dt) helliphelliphelliphellip(46 a)

As Cj the capacitance of junction J2 is almost constant the current is given by

i = Cj (d Va dt) helliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip(46 b)

If the rate of rise of forward voltage dVadt is high the charging current i will be

more This charging current plays the role of gate current and turns on the SCR even

when gate signal is zero Such phenomena of turning-on a thyristor called dvdt turn-on

must be avoided as it leads to false operation of the thyristor circuit

For controllable operation of the thyristor the rate of rise of forward anode to

cathode voltage dVadt must be kept below the specified rated limit Typical values of

dvdt are 20 ndash 500 Vμsec False turn-on of a thyristor by large dvdt can be prevented by

using a snubber circuit in parallel with the device thyristor are very sensitive for over

voltage than the semiconductor devices

Over voltage transients are perhaps the main cause of thyristor failure

In thyristor there are mainly two types

1 Internal over voltages

Due to the commutation of the thyristors large voltages are generated internally

Because of the series inductance of the SCR circuit the large transient voltages L didt

produced This voltage several times the break over voltage of the device then thyristor

destroys permanently

2 External over voltages

External over voltages are caused due to the interruptions of current flow in an

inductive circuit and also due to the lightening strokes on the lines feeding the thyristor

system For the reliable operation of thyristor the over voltages must be suppressed by

adopting suitable techniques

Suppression of over voltages

The RC circuit called snubber circuit is connected across the device to protect In

order to keep the protective components to a minimum the thyristors are chosen with

their peak voltages ratings are 25 to 3 times of the normal peak working voltage

ldquoselenium thyrector diodes metal oxide varistors or avalanche diode suppressers are

commonly employed for protecting the thyristor circuit against the over voltages

Gate protection

Gate circuit should also be protected against the over voltages and surges Over

voltage at gate circuit can cause false triggering of the SCR may rises the junction

temperature behind specified limit leading to its damage Protection against over

voltage can be achieved by connecting a ZD across the gate circuit and a resister is

connected in series with gate circuit to protect against the s A capacitor and resister are

connected across gate to cathode to by pass the noise

41 Final Pulse Stages

The Final Pulse Stages adapt the output pulses from the Intermediate Pulse Stage

(pulses on the pulse bus) to the gate currents needed for the thyristors Each thyristor

bridge is equipped with its own Final Pulse Stage Each Final Pulse Stages is provided

with a power supply module The amplified output pulses from the Final Pulse Stages

start as a short strong steep pulse with an amplitude approx 2frac12 times that of the main

pulse This initial pulse edge assures proper firing of the thyristors being triggered

Subsequently the weaker part of main pulse keeps firing conditions steady As already

mentioned the Final Pulse Stages and their associated thyristor bridges form single units

All six pulse outputs from a Final Pulse Stage can be blocked by an external control

signal so that all thyristors in the associated thyristor bridge will block the current A

blocking of the pulses is initiated whenever there is a malfunction in the associated

thyristor bridge

42 Converter Power Section

The thyristor converter consists of three independent parallel rectifier blocks TY1

to TY3 which are all in service Even if one block fails the remaining blocks take over

automatically the full design current of the excitation circuit During normal operation

(with ideal current share) and all three bridges in operation each of these blocks has to

carry only (n-2)n (ie33)of its design current If 2 thyristor bridges fail the excitation

is limited Only when all three bridges fail the excitation is switched off Each thyristor

bridge arm is equipped with current flow monitoring CTrsquos Failure of conduction in any

arm is identified by a Current flow monitoring module

43 Converter Cooling

A cooling system is needed to dissipate heat losses in the converter blocks and

electronics Each converter block has therefore been equipped with a fan supplied with

power from the converterrsquos primary voltage (via transformer ndashT8 in field flashing

cubicle) The fans are protected with motor protection circuit breakers An air flow

monitoring unit is provided for monitoring the air flow through the thyristor bridge If a

circuit breaker failure is detected or if the air flow monitor drops off at one of the

thyristor bridges the bridge involved is immediately set out of operation by blocking its

firing pulses

44 Thyristor Converter Monitoring

A thyristor bridge in which defects occur that could threaten the safety of

operation or cause secondary damage is switched off automatically ie its firing pulses

are blocked This happens whenever A thyristor fuse is blown The fuses are monitored

individually with micro switches The Final Pulse Stage fails which is detected by

internal monitors (supply voltage sustained pulse short-circuit on the output end) The

power supply to the fan fails fan air flow as monitored by the Air flow monitor fails or

is insufficient Isolator on ACDC side is open

CHAPTER 5

Field Current Circuit Excitation Transformer

51 Field Circuit Breaker

The circuit-breaker in the field circuit is used to isolate the field circuit from the

converter It is capable of switching off the synchronous machine from full load under the

maximum conditions of a 3-phase short-circuit In addition to its main contacts the field

circuit-breaker also has a de-excitation contact with which the field energy stored in the

field can be dissipated across the de-excitation resistor The de-excitation contact closes

shortly before the main contacts open so as to ensure proper commutation of the field

current from the main contacts to the de-excitation contact when the breaker is switched

off The field circuit-breaker is switched on by electromagnetic force and is kept switched

on by a mechanical latch When the latch is released by a trip coil the circuit-breaker

opens The circuit-breaker also has auxiliary contacts that report its status

52 Field Flashing

In shunt supplied excitation circuits (excitation transformer connected to the

generator terminals) the generator does not have enough remnant voltage for a generator

voltage build-up via the converter In this case a field flashing circuit is provided It

consists of the field flashing contactor the diode bridge and a transformer used to adapt

the auxiliary input voltage to the voltage needed for field flashing when power is

supplied from the auxiliaries network

Fig Field Flashing

Because the field flashing contactor is not able to switch off the energy stored in

the field the control ensures that the contactor can only reopen if the field circuit breaker

has already been opened (generating the TRIP order) or in a normal field flashing

sequence when the converter has taken over the field current Field flashing occurs in the

following stages

1048729The excitation is switched on closing the field flashing contactor ( Field

Circuit Breaker is already closed )

1048729The start-up excitation current flows through the rotor driving the generator

voltage up to approx 15 U

1048729After about 10 U the firing pulses to the converter are released and it begins

to excite the generator to its rated voltage

1048729After about 30 U the field flashing contactor opens (with no current since

the converter is now supplying the current)

The diode bridge at the input to the field flashing contactor prevents a feed-back

from the converter to the source of field flashing while the contactor is still closed

53 De-excitation

When malfunctions occur the stored field energy must be dissipated as quickly

and safely as possible to protect the generator This is done by the converter the field

circuit-breaker and the de-excitation (discharge) resistor

De-excitation (with opening of the field circuit-breaker) takes place in the following

stages

1048729The converter drives to its inverter limit position (negative ceiling voltage)

recovers a portion of the field energy into the network A trip command is given to the

field circuit breaker

1048729The de-excitation contact closes diverting the field voltage to the de-excitation

resistor

1048729Then immediately the main contacts open building voltage The field voltage

commutates to the de-excitation resistor

1048729The current diminishes at a given time constant TE

(With linear resistance TE = Lf (Rf + Re))

Due to the reversal of the field voltage by the converter the field current

commutates from the main contacts of the field circuit-breaker to the de-excitation

resistor in a very early phase This reversal of the field voltage prevents burn-off on the

main contacts and provides effective protection for the field circuit-breaker Depending

on the operating policy an operational shut-down of the excitation can also be effected

with the field circuit-breaker closed This method is useful mainly when the excitation is

switched on and off frequently In this case the converter is merely driven into the

inverter limit position so that the field energy is recovered into the network The

converter then blocks since it is supplying positive current only

54 Excitation Transformer

The excitation transformer matches the generator voltage to the field voltage

(required ceiling voltage) It also serves as a commutation reactance for the thyristor

converter and as a potential isolator between the network and the excitation circuit In

addition the transformer functions as a current limiter in that it makes it possible to keep

any short circuits in the excitation circuit under better control The excitation transformer

is equipped with temperature monitoring probes which set off an alarm when the

temperature exceeds a first max limit and then trips the excitation if the temperature

continues rising to a second (higher)limit

CHAPTER 6

Monitoring and Protection

61 Excitation Monitoring

611 General Information

The main goal of Excitation Monitoring is to make optimum use of the

redundancies provided in the excitation system and to give alarm whenever a malfunction

makes these redundancies unavailable The field current is monitored to see that it does

not exceed a maximum level and if necessary a switch-over to the MANUAL channel is

initiated In addition the criterion for switching off the field flashing is generated The

excitation Monitoring consists of an autonomous processor system

612 over current Alarms

In the Excitation Monitoring the limits for are set at higher levels than the

settings on the Field Current Maximum Limiter Whenever the current exceeds 110 of

the nominal field current contact R1 and the binary output associated with it are

activated immediately If field current remains gt 110 then after a preset inverse-time

has lapsed relay R2 and - after a further delay - relay R0 and the binary outputs

associated with them are activated Parameters match the measurements for If1 and If2 to

the nominal value for field current so that the internal values can be processed and read as

pu values It can be used to falsify the actual value of the field current If (to raise it) so

as to cause a response from the alarm limits for purposes of testing The processed If

signal is always taken from on the active channel (CHANNEL I OR CHANNEL II) As

long as the field current If is above the threshold value 11 Ifn its peak value is

measured This is stored (until RESET) and can be read at any time on the Micro-

Terminal Once the value of If exceeds 11 Ifn integration of this value starts Whenever

the integrated time-current value (intisup2dt) exceeds the preselected reference value the

alarm OVER CURRENT INVERSE-TIME is set off and a command is simultaneously

issued to switch over to the stand by AUTO channel Software switch F758 enables the

three over current alarm functions (R0 R1 R2) and selects one of three possible inverse-

time curves T1 T2 or T3 Within the characteristic curve (T1 T2 T3) selected the

desired limit curve for response is set using the factor F216

613 Switch-Off Criterion for Field Flashing

The Excitation Monitoring supplies the criterion for switching off the field

flashing Whether this criterion is activated based on the actual value for generator voltage

Ug or for field current If or both depends on the settings of the two threshold values

F200F202 (0 setting means that the output is always ldquological 1rdquo) The measurements

Ug12 and If12 are switched over depending on the present status of the channels

(Channel 1 or Channel 2 ON) Whenever Flag F750 is not activated the binary output is

fixed at ldquological 1rdquo

614 Storage of Alarm Status

The outputs of the over current alarms (R1 R2 R0) and the messages NO

FAILURE MONITORING PARAMETERS CHANGED are stored messages can be

erased by activating the input ldquoGENERAL RESETrdquo or by using the RESET button on the

front of the module Erasure with the input ldquoGENERAL RESETrdquo is effective only if the

situation causing the alarm or the malfunction is no longer present Whenever the self-

diagnosis equipment in the processor detects a malfunction the output NO FAILURE of

MONITORING is set at ldquological 0rdquo (= alarm) The alarm ldquoPARAMETERS CHANGEDrdquo

is activated whenever parameters or settings of software switches have been changed via

the Micro-Terminal

615 Actual Value Monitoring

The actual values for generator voltage Ug synchronous voltage Usyn and field

current If are monitored for malfunctions This monitoring is active regardless of whether

or not the generator is in operation Essentially when the generator is in operation the

measurements are monitored by comparing the signals (the smaller signal reading is

detected as incorrect) When the generator is not in operation the measured data are

monitored for extreme values The percentage of deviation permissible in the

measurement signals being compared

(Ug1ampUsyn1 Ug1ampUg2 Ug2ampUsyn2 If1ampIf2) is defined by parameters F208 and

F20AIf the excitation transformer is being supplied from an auxiliary power source (no

shunt operation) the values of Ug and Usyn will be different in some operational

conditions

In that case Flag F75A can be used to deactivate comparative measurements Ug1 Usyn1

and Ug2 Usyn2 The ldquoprimary monitoringrdquo compares Ug1 with Ug2 and generates the

messages that Channel 1 or Channel 2 has suffered a malfunction Whenever Ug1 lt Ug2

and the binary message from CH1 reports no malfunction a malfunction on

Measurement Channel 1 is reported (Ug1Usyn1 FAILURE) A similar malfunction is

also present whenever the binary message CH1 DISTURBANCE is reported and a

discrepancy is detected between Ug1ampUg2 The generation of the alarm ldquoUg2 Usyn2

FAILURErdquo is analogous to that for Channel 1 The ldquosecondary monitoringrdquo compares

Ug1 with Usyn1 amp Ug2 with Usyn2 This is enabled whenever the binary message of the

comparison channel reports a malfunction or whenever both binary messages report no

malfunction - but both secondary monitors report a malfunction As long as the secondary

monitoring is blocked the differences Ug1neUsyn1 or Ug2neUsyn2 trigger malfunction

signals for the measurement channel involved (suspicion that there is a corresponding

error in Usyn) The measurement channel malfunctions are enabled operationally

whenever after excitation has been switched on generator voltage Ug exceeds the value

set on F204 The voltages Ug1 and Ug2 are checked 16 seconds after the excitation is

switched off to see that they do not exceed the limit value F210 that applies to both of

them At the same time g1Ug2Usyn1Usyn2 are checked for extreme values (gt or lt

the operating range) Monitoring for extreme values is likewise enabled during normal

operation ( Excitation ON and Ug gt F204 ) Flag F754 is used to enable or block the

malfunction signals to the binary outputs Basically the monitoring of the actual values

for If1If2 functions like that of the Ug1Ug2 monitoring

62 Excitation Protection

621 General Information

The Excitation Protection switches off the excitation (and de-excites the machine

rapidly) whenever a danger arises that threatens the excitation transformer the converter

or the generator Generally limiter or monitoring functions precede the emergency trips

and these normally respond before the Excitation Protection must initiate a trip

Protective trip commands are issued directly to the field circuit-breaker from potential

free contacts of the board via the trip relays They are directed redundantly to the

operative field circuit-breaker ldquoOFFrdquo command

622 Protection against Excitation Transformer Overheating

This equipment monitors the excitation transformer for overheating in the

windings that could result from over current short-circuits or inadequate cooling The

monitoring uses temperature monitoring modules in conjunction with temperature

sensors built into the windings Normally the temperature is monitored in two stages the

first stage sets off an alarm the second causes a trip of the excitation

623 Rotor Over voltage Protection

Malfunctions in the generator circuit (eg terminal short-circuit failed

synchronization asynchronous operation) cause induced negative field currents that

produce high voltages in the field circuit These must be restricted to a level with a

sufficient safety margin below the insulation capacity of the field winding (test voltage)

and also below the peak blocking voltage of the converter thyristors The crow bar

employs spark gap elements to detect over voltages in the field circuit Whenever they

respond the associated thyristors are fired immediately switching the de-excitation

resistor parallel to the field The de-excitation current generated thereby initiates an

excitation trip via a supervision circuit causing an immediate opening of the field circuit-

breaker The malfunction isets off an alarm and an internal malfunction is indicated at the

cubicle

TEST VALUES OBTAINED WHEN EXCITATION IS RAISED

TEST

SNO PARTICULARS ACTUAL VALUE OBTAINED VALUE

1 VREF 996 100

2 VACT 997 1003

3 IFACT 735 765

4 IGACT 703 707

5 ACTIVE

POWER

703 705

6 REACTIVE

POWER

101 142

7 POWER

FACTOR

099 IND 098 IND

8 ACTIVE

CURRENT(IR)

705 703

9 REACTIVE

CURRENT(IX)

102 138

10 POWER

ANGLE

566 546

11 FIRING

ANGLE

640 632

VALUES OBTAINED WHEN EXCITATION IS LOWERED

SNO PARTICULARS ACTUAL

VALUE

OBTAINED VALUE

1 VREF 100 997

2 VACT 100 997

3 IFACT 787 761

4 IGACT 839 837

5 ACTIVE

POWER

839 830

6 REACTIVE

POWER

155 110

7 POWER

FACTOR

098 IND 099 IND

8 ACTIVE

CURRENT(IR)

835 830

9 REACTIVE

CURRENT(IX)

140 94

10 POWER

ANGLE

613 DEG 631 DEG

11 FIRING

ANGLE

601 DEG 599 DEG

CONCLUSION

For generating the EMF in stator winding excitation is required to the rotor of a

generator There are two types of excitation

1 Static excitation system

2 Brushless excitation system

A certain disadvantage in brushless excitation system is the slow response time of

the field in case of fast load changes specified No slip-rings and brushes direct

measurements of the field parameters not possible

To avoid all loses static excitation is used Since it does not have any rotating

parts mechanical loses and windage loses This system has fast response and speed

control While preferring this excitation system there are no limitations for the

redundancy of Thyristor bridge circuits

Static excitation has fast field discharge by resistor and inverter operation direct

measurement of field quantity is possible The meaning of excitation is nothing but

continuous supply of DC current (ie field current) to the rotor to buildup required

output voltage in the stator

Field current is changed with respect to the change of load so the digital

automatic voltage regulator (DAVR) is used to regulate the output voltage according to

the load variations

So we conclude that static excitation system with DAVR is preferred since it is

having excellent dynamic performance and better options for R amp M

b) Dependent on the time integral with -dUdt ENABLE

Flag programming F418 = inactive F41A = 1111

This variant likewise enables the transient value only when a collapse of network

voltage has been detected However the switch-back to the thermal limit is not made

dependent upon the time itself but on the calculated time integral intisup2dt of the The setting

on Parameter F414 in spu takes into account the time the rotor needs to cool down ie

the rate of temperature change in the case of intermittent operation The example below

shows how the timing of the switch-back to the thermal limit depends on the present

value for intisup2dt

The time integral is based on the formula

Example The setting of =isup2dt equivalent to Version a (F416) at a constant 16 times the

nominal field current for 10 seconds (with TH12 = 105) is

c) Dependent on the time integral without any preconditions

Flag programming F418 = 1111 F41A = 1111

In this variant the transient becomes available without any prior conditions

(without a -dUdt ENABLE) with the time integral intisup2dt

237 Inductive Stator Current Limiter

The Inductive Stator Current Limiter holds the stator current Ig within permissible

limits while the generator is in the ldquoover-excitedrdquo operating range by reducing the field

current accordingly The setting TH (thermal limit) provides the limit against stationary s

that might occur To take advantage of the generatorrsquos transient overload capacity a

switch-over is made to the higher setting MAX The principle of operation of this switch-

over to the value MAX permissible only transiently is identical to that employed for the

field current limiter (refer to the description above) When the drive output from the

turbine is very high stator current may exceed permissible limits even while inductive

loading of the generator is low In this case if the stator current limiter is not kept from

influencing the field current the control circuit will oscillate back and forth between the

Inductive Stator Current Limiter (de-

exciting) and the Capacitive Stator Current Limiter (exciting)The output signal of that

function then dominates the control variable of the Ig-dependent limiter via a maximum

value selection

238 Capacitive Stator Current Limiter

239 Load Angle Limiter

The Load Angle Limiter prevents the synchronous machine from slipping out of

phase due to slippage of the rotor The load angle δ the difference in phase between the

rotor and the stator rotating field results mainly from the driving torque (active power P)

acting on the generator and the level of rotor current (field current) If the driving torque

remains constant a increase in the field current reduces the load angle δ The current load

angle δ at any moment is obtained from the generator current and generator voltage based

on a simplified model of the generator Whenever this calculated load angle δ exceeds the

preset limit angle the limiter increases the field current until the load angle has dropped

back to its permissible value The quadrature reactance Xq of the generator and the

network reactance Xe during normal operation must be adjusted on the regulator in order

to obtain the load angle δ The graph below shows the Power Chart for a salient-pole

machine with typical limiter characteristics

The purpose of a Power System Stabilizer is to use the generator excitation to

damp electromechanical oscillations between the network and the generator Depending

on the design of the generator and the requirements imposed for network stability its

main function will be either to damp the oscillations originating in the machine or those

from the network A synchronous generator working in a combined power network is in

principle an oscillating structure In order to produce a torque the magnetic field of the

rotor and the stator must form a given angle (referred to as the rotor displacement or load

angle δ) The electrical torque ME increases as the angle δ increases just as with a

torsion spring Because the ME of the generator and the mechanical driving torque MA

from the turbine are in equilibrium during steady-state operation the angle δ remains in a

given position Whenever this state of equilibrium between MA and ME is disturbed the

load angle slips of this rest position and change thereby the electrical torque ME The

torque attempts to restore the load angle to a stationary position Due to the mass inertia

of the turbinegenerator rotor however this can only take place aperiodically It does so

in the form of more or less effectively damped oscillations (again similar to the effect of

mass inertia on a torsion spring) In order to damp the oscillations there must be a

damping torque produced depending not on the electrical torque ME associated with the

angle but on the difference in frequency (Df) between the rotor and the stator rotating

field ie on the slippage This torque is produced mainly by the so-called damper

winding in the rotor but the dimensioning of this is subject to limits imposed by

considerations of design and economy Some further action is therefore needed to

increase the damping effect The following drastically simplified formula shows the

parameters upon which the amount of active power PE supplied by the generator

depends

PE = active power

It can be seen from the above relationship that the active power that the generator

transfers depends not only on the load angle δ but also on the field current If That means

that a transient change can be made in the active power PE and with that in the effective

electrical torque ME by varying the field current The principle of operation of the DVR

Power System Stabilizer becomes clear from a consideration of the oscillations in power

output and frequency (ΔPE Δf) and the vector diagram If it is assumed that oscillations

in the network frequency generates load oscillations with the mass inertia of the rotor

then the active load of the generator (eg MW-measured) is influenced with a sinusoidal

value -ΔPE (ME-MA = -ΔPE) By inversion of -ΔPE one obtains the fluctuation in

power provided by the rotor +ΔPE As is known the slip signal Δf follows +ΔPE with a

phase delayed by 90deg The +ME produced by the periodic changes in the load angle δ is

in phase with +ΔPE A good damping is attained if ME is varied in phase with the slip

Δf However this signal must also be advanced somewhat to compensate for the time

constants in the excitation circuit and the generator

As mentioned above the electrical torque ME can be influenced by varying the

field current To accomplish this a suitable control signal referred to as variable

disturbance compensation must be imposed upon the voltage set-point or the converter

control variable Ucontr As can be seen from the vector diagram by applying proper

weighting factors (K1 K2) and then adding together the signals -ΔPE and Δf an overall

stabilization signal can be produced that rotates in advance of the Df signal by any angle

desired between 0deg and 90deg Because the amplitude of -DPE remains proportional to the

amplitude of Δf a constant angle in advance of Δf results for the compensation of the

time constants referred to above The optimum weighting factors K1 and K2 for a

synchronous generator working to a power network depend on its operating point at any

moment and the external reactance of the network Normally the selection of a

compromise setting is good enough to attain stability in all operating points and for all

external reactance For special demands these settings must be parameterized as a

function of the external reactance (which means optional equipment Xe-Identification)

The Power System Stabilizer PSS is a section of the AVR computer program and is

processed once per network cycle The voltage at the generator terminals and the

generator current are measured in order to define the signals ΔPE and Δf The calculated

signals for _P_ (=PE) and Δf are then sent across DC filters ldquoDrdquo (real differentiators) that

transmit only the dynamic portion of the signals The ΔPE and Δf signals obtained in this

way are then weighted (multiplied by) with the factors K1 and K2 and sent to the

summing point of the voltage regulator

The PSS stabilization signal is imposed on the automatic voltage regulator only if

the following prerequisites are met

bull Generator on line

bull Generator power output gt the value F338

bull Generator voltage in a range between F33C and F33A

The stabilization signal is limited at the output from the PSS to the lower and

upper limits Flag defines whether the stabilization signal is introduced before or after the

PID filter (usually before the filter) Because the PID filter as noted above already takes

the ceiling factor Vp1 into account the PSS signal needs to be multiplied by Vp1 if it is

added to the voltage regulator following the PID filter (divider at the input to the

minmax limiter) This precaution prevents the DC filter ldquoDrdquo in the P-channel from

producing an unnecessary ldquostabilizationrdquo effect in the case of rapid changes in turbine

load As an alternative for the AVRrsquos Power System Stabilizer a stabilization signal from

an outside system can be imposed by activating the binary input ldquoPSS-SIGNEXTrdquo Flag

F340 can be used to select between an analog and a 12-bit signal and F33E to select the

polarity desired for that signal

24 The MANUAL Channel

241 Summary

The MANUAL channel (Channel 2) has been built as a simple field current

regulator

without additional limiters Its main function is to maintain the excitation of the generator

even if the AUTOMATIC channel becomes non-operational The MANUAL channel

also performs valuable service for purposes of testing commissioning and preventive

maintenance Its measurements regulator generation of firing pulses and power supply

are physically separate from those on the AUTOMATIC channel

242 Principle of Operation

All the functions of the MANUAL channel including the generation of firing

pulses have been implemented in a single electronic module the Gate Control Unit The

control variable Ucontr of voltage regulator is used as the reference value for generating

firing pulses on the principle known as ldquoramp controlrdquo (Comparison of Ucontr with

Usynsynchronous sawtooth signal) For further processing in the UN 0096 Intermediate

Pulse Stage the Gate Control Unit supplies six firing pulses at its output whose phase

position with respect to the synchronous voltage Usyn is in accordance with control

variable Ucontr An internal linearization ensures that the field voltage produced via the

firing pulses remains proportional to the control variable Ucontr throughout the entire

range As a result the circuit amplification of the control remains constant over the entire

range Whenever excitation is switched ON the set-point for Generator Voltage is set

automatically at the preset - ref Value This provision ensures that the generator voltage

always attains approximately its nominal value after the field flashing The Gate Control

Unit can be refunctioned ( by pre-selection with a switch ) for purposes of testing to act

as a purely firing pulse control In this case the control variable Ucontr is adjusted

directly using the RAISELOWER push buttons on the front of the module In this way

for example the relationship between the phase position of the firing pulses and the

control variable Ucontr can be checked easily

CHAPTER 3

PULSE SECTION

31 Pulse Generation and Amplification

The Gate Control Units of both AUTOMATIC channel and MANUAL channel

each supply six firing pulses for operating the 6-pulse thyristor bridges The low-power

pulse signals from these Gate Control Units are then amplified in the Intermediate Pulse

Stage galvanically isolated and then sent to the common pulse bus On the output end

the Intermediate Pulse Stage of the non-active channel) is always blocked The Gate

Control Units generate the pulses based on microprocessor control The reference voltage

used for the firing pulse phase location is the output voltage from the excitation

transformer (Usyn1 Usyn2) The commutation spikes of the synchronous voltage caused

by the converter are calculated prior to use of the voltage as a reference value and are

deliberately filtered out The lower limit for the firing pulses (double pulses) which are

offset from one another by 60deg is defined by the limit rectifier position (αmin) and the

upper limit by the limit inverter position (αmax) for the firing angle αmin and αmax can

be adjusted on the Gate Control Units using BCD (Binary Coded Decimal) switches

αmin ensures that the firing pulses will not be issued (premature firing) until there is

sufficient positive phase voltage on the thyristor involved αmax prevents a dangerous

ldquotippingrdquo of the thyristor bridge into the rectifier mode if the firing angle α is too large

(ldquolate firingrdquo) The critical factors determining αmax are the overlap time uumlmax (max

commutation time) and the ldquorecovery timerdquo of the thyristors (αmax lt 180deg - uumlmax - γ )

An external control signal can force the firing pulses into their inverter limit position

Other binary inputs can block or direct the firing pulses of the Gate Control Units so as to

produce freewheeling on the thyristor bridge During freewheeling the firing pulses for

the thyristor pair R and S are blocked and the pulse signals T+T- are engaged with

chains of pulses Both Gate Control Units (for the MANUAL amp AUTOMATIC channels)

contain a field current monitor that blocks the firing pulses immediately whenever the

current exceeds a preset threshold level In this case the field circuit-breaker is also

tripped via an output contact The purpose of these provisions is to prevent damage to

thyristors and thyristor fuses in case of a slip-ring short-circuit or to keep any damage

that does occur to a minimum The pulse signals are galvanically separated at the outputs

from the Intermediate Pulse Stage (with pulse transmitters) and are then directed to the

common pulse bus This transmission of the pulse signals to the pulse bus via passive

transmitters ensures a high degree of active channel autonomy Practically no possible

malfunctions on the inactive channel (including for example sustained pulses) affect the

active channel

32 Pulse Monitoring

The ldquoPulse Busrdquo and the pulse signals of the AUTOMATIC channel are

monitored This monitoring device consists of potential isolating stages and the common

monitor If the pulse monitoring of the ldquoPulse Busrdquo responds a switch-over is made to

MANUAL channel The function of the potential isolating stages is to couple the pulse

monitoring device to the pulse circuits without any feedback effect The pulse monitoring

checks the six pulse lines for the following malfunctions continuous or periodic failure

of one or more pulses Periodic occurrence of synchronous or asynchronous false pulses

Continuous pulses the pulse monitoring device can be tested while the machine is in

operation

CHAPTER 4

CONVERTER

Thyristor

The term thyristor usually refers to a family of four layer solid state device having

turn on characteristics that can be externally controlled by either current or voltage They

are also referred to as breakdown device because their working depends on avalanche

breakdown Thyristors have only two stages OFF and ON Thyristors have a similar

function to Uni-junctions they act as switches Thyristors use current flow as a switch

Thyristors have three states

1 Reverse blocking mode mdash Voltage is applied in the direction that would be

blocked by a diode

2 Forward blocking mode mdash Voltage is applied in the direction that would cause

a diode to conduct but the thyristor has not yet been triggered into conduction

3 Forward conducting mode mdash The thyristor has been triggered into conduction

and will remain conducting until the forward current drops below a threshold value

known as the holding current Converter is a semiconductor device which converts ac

input voltage into a constant dc output voltage In present excitation system three phase

fully controlled thyristor converter is used

Because of the following advantages thyristor converters are used

a) Thyristors are used for high power applications ie up to 10Kv3500A1KHz

b) Having high reliability and low losses

c) Uni-directional device like diode

d) Itrsquos operation as a rectifier which are low resistance in forward conduction

mode and high resistance in reverse conduction mode

PROTECTION OF THYRISTORS

For reliable operation of a thyristor demands that its specified ratings are not

exceeded When Subjected to or over voltages During the turn - on of SCR didt

prohibitively large False triggering of SCR by high value of dvdt andSpurious signals

between gate and cathode may leads to unwanted turn ndash on

DIDT AND PROTECTION

When thyristor starts conducting in forward conduction mode and is turned on by

gate pulse The anode current increases rapidly whole area of the gate to Cathode

junction then hot spots will be formed near the gate connection this locality of heating

destroys the thyristor Thyristor thermal time is constant The causes due to faults and

short circuits or surge currents Electronic crowbar protection is used against the over

voltages The rate rise of anode current must be kept at the time of turn on below the

rated or specified limiting value The didt value maintained below limited value by using

a inductor also called ldquodidt inductorrdquo in series with anode circuit The locality of heating

is avoided by applying gate current but not greater the maximum gate current

DVDT AND OVER VOLTAGE PROTECTION

With forward voltage across the anode and cathode of a thyristor the two outer

junctions are forward biased but the inner junction is reverse biased This reverse biased

junction J2 has the characteristics of a capacitor due to charges existing across the

junction In other words space-charges exist in the depletion region around junction J2

and therefore junction J2 behaves like a capacitance If the entire anode to cathode

forward voltage Va appears across J2 junction and the charge is denoted by Q then a

charging current i given by Eq (46) follows

i = dQdt =d(Cj Va )dt

= Cj (d Va dt) + Va(d Cj dt) helliphelliphelliphellip(46 a)

As Cj the capacitance of junction J2 is almost constant the current is given by

i = Cj (d Va dt) helliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip(46 b)

If the rate of rise of forward voltage dVadt is high the charging current i will be

more This charging current plays the role of gate current and turns on the SCR even

when gate signal is zero Such phenomena of turning-on a thyristor called dvdt turn-on

must be avoided as it leads to false operation of the thyristor circuit

For controllable operation of the thyristor the rate of rise of forward anode to

cathode voltage dVadt must be kept below the specified rated limit Typical values of

dvdt are 20 ndash 500 Vμsec False turn-on of a thyristor by large dvdt can be prevented by

using a snubber circuit in parallel with the device thyristor are very sensitive for over

voltage than the semiconductor devices

Over voltage transients are perhaps the main cause of thyristor failure

In thyristor there are mainly two types

1 Internal over voltages

Due to the commutation of the thyristors large voltages are generated internally

Because of the series inductance of the SCR circuit the large transient voltages L didt

produced This voltage several times the break over voltage of the device then thyristor

destroys permanently

2 External over voltages

External over voltages are caused due to the interruptions of current flow in an

inductive circuit and also due to the lightening strokes on the lines feeding the thyristor

system For the reliable operation of thyristor the over voltages must be suppressed by

adopting suitable techniques

Suppression of over voltages

The RC circuit called snubber circuit is connected across the device to protect In

order to keep the protective components to a minimum the thyristors are chosen with

their peak voltages ratings are 25 to 3 times of the normal peak working voltage

ldquoselenium thyrector diodes metal oxide varistors or avalanche diode suppressers are

commonly employed for protecting the thyristor circuit against the over voltages

Gate protection

Gate circuit should also be protected against the over voltages and surges Over

voltage at gate circuit can cause false triggering of the SCR may rises the junction

temperature behind specified limit leading to its damage Protection against over

voltage can be achieved by connecting a ZD across the gate circuit and a resister is

connected in series with gate circuit to protect against the s A capacitor and resister are

connected across gate to cathode to by pass the noise

41 Final Pulse Stages

The Final Pulse Stages adapt the output pulses from the Intermediate Pulse Stage

(pulses on the pulse bus) to the gate currents needed for the thyristors Each thyristor

bridge is equipped with its own Final Pulse Stage Each Final Pulse Stages is provided

with a power supply module The amplified output pulses from the Final Pulse Stages

start as a short strong steep pulse with an amplitude approx 2frac12 times that of the main

pulse This initial pulse edge assures proper firing of the thyristors being triggered

Subsequently the weaker part of main pulse keeps firing conditions steady As already

mentioned the Final Pulse Stages and their associated thyristor bridges form single units

All six pulse outputs from a Final Pulse Stage can be blocked by an external control

signal so that all thyristors in the associated thyristor bridge will block the current A

blocking of the pulses is initiated whenever there is a malfunction in the associated

thyristor bridge

42 Converter Power Section

The thyristor converter consists of three independent parallel rectifier blocks TY1

to TY3 which are all in service Even if one block fails the remaining blocks take over

automatically the full design current of the excitation circuit During normal operation

(with ideal current share) and all three bridges in operation each of these blocks has to

carry only (n-2)n (ie33)of its design current If 2 thyristor bridges fail the excitation

is limited Only when all three bridges fail the excitation is switched off Each thyristor

bridge arm is equipped with current flow monitoring CTrsquos Failure of conduction in any

arm is identified by a Current flow monitoring module

43 Converter Cooling

A cooling system is needed to dissipate heat losses in the converter blocks and

electronics Each converter block has therefore been equipped with a fan supplied with

power from the converterrsquos primary voltage (via transformer ndashT8 in field flashing

cubicle) The fans are protected with motor protection circuit breakers An air flow

monitoring unit is provided for monitoring the air flow through the thyristor bridge If a

circuit breaker failure is detected or if the air flow monitor drops off at one of the

thyristor bridges the bridge involved is immediately set out of operation by blocking its

firing pulses

44 Thyristor Converter Monitoring

A thyristor bridge in which defects occur that could threaten the safety of

operation or cause secondary damage is switched off automatically ie its firing pulses

are blocked This happens whenever A thyristor fuse is blown The fuses are monitored

individually with micro switches The Final Pulse Stage fails which is detected by

internal monitors (supply voltage sustained pulse short-circuit on the output end) The

power supply to the fan fails fan air flow as monitored by the Air flow monitor fails or

is insufficient Isolator on ACDC side is open

CHAPTER 5

Field Current Circuit Excitation Transformer

51 Field Circuit Breaker

The circuit-breaker in the field circuit is used to isolate the field circuit from the

converter It is capable of switching off the synchronous machine from full load under the

maximum conditions of a 3-phase short-circuit In addition to its main contacts the field

circuit-breaker also has a de-excitation contact with which the field energy stored in the

field can be dissipated across the de-excitation resistor The de-excitation contact closes

shortly before the main contacts open so as to ensure proper commutation of the field

current from the main contacts to the de-excitation contact when the breaker is switched

off The field circuit-breaker is switched on by electromagnetic force and is kept switched

on by a mechanical latch When the latch is released by a trip coil the circuit-breaker

opens The circuit-breaker also has auxiliary contacts that report its status

52 Field Flashing

In shunt supplied excitation circuits (excitation transformer connected to the

generator terminals) the generator does not have enough remnant voltage for a generator

voltage build-up via the converter In this case a field flashing circuit is provided It

consists of the field flashing contactor the diode bridge and a transformer used to adapt

the auxiliary input voltage to the voltage needed for field flashing when power is

supplied from the auxiliaries network

Fig Field Flashing

Because the field flashing contactor is not able to switch off the energy stored in

the field the control ensures that the contactor can only reopen if the field circuit breaker

has already been opened (generating the TRIP order) or in a normal field flashing

sequence when the converter has taken over the field current Field flashing occurs in the

following stages

1048729The excitation is switched on closing the field flashing contactor ( Field

Circuit Breaker is already closed )

1048729The start-up excitation current flows through the rotor driving the generator

voltage up to approx 15 U

1048729After about 10 U the firing pulses to the converter are released and it begins

to excite the generator to its rated voltage

1048729After about 30 U the field flashing contactor opens (with no current since

the converter is now supplying the current)

The diode bridge at the input to the field flashing contactor prevents a feed-back

from the converter to the source of field flashing while the contactor is still closed

53 De-excitation

When malfunctions occur the stored field energy must be dissipated as quickly

and safely as possible to protect the generator This is done by the converter the field

circuit-breaker and the de-excitation (discharge) resistor

De-excitation (with opening of the field circuit-breaker) takes place in the following

stages

1048729The converter drives to its inverter limit position (negative ceiling voltage)

recovers a portion of the field energy into the network A trip command is given to the

field circuit breaker

1048729The de-excitation contact closes diverting the field voltage to the de-excitation

resistor

1048729Then immediately the main contacts open building voltage The field voltage

commutates to the de-excitation resistor

1048729The current diminishes at a given time constant TE

(With linear resistance TE = Lf (Rf + Re))

Due to the reversal of the field voltage by the converter the field current

commutates from the main contacts of the field circuit-breaker to the de-excitation

resistor in a very early phase This reversal of the field voltage prevents burn-off on the

main contacts and provides effective protection for the field circuit-breaker Depending

on the operating policy an operational shut-down of the excitation can also be effected

with the field circuit-breaker closed This method is useful mainly when the excitation is

switched on and off frequently In this case the converter is merely driven into the

inverter limit position so that the field energy is recovered into the network The

converter then blocks since it is supplying positive current only

54 Excitation Transformer

The excitation transformer matches the generator voltage to the field voltage

(required ceiling voltage) It also serves as a commutation reactance for the thyristor

converter and as a potential isolator between the network and the excitation circuit In

addition the transformer functions as a current limiter in that it makes it possible to keep

any short circuits in the excitation circuit under better control The excitation transformer

is equipped with temperature monitoring probes which set off an alarm when the

temperature exceeds a first max limit and then trips the excitation if the temperature

continues rising to a second (higher)limit

CHAPTER 6

Monitoring and Protection

61 Excitation Monitoring

611 General Information

The main goal of Excitation Monitoring is to make optimum use of the

redundancies provided in the excitation system and to give alarm whenever a malfunction

makes these redundancies unavailable The field current is monitored to see that it does

not exceed a maximum level and if necessary a switch-over to the MANUAL channel is

initiated In addition the criterion for switching off the field flashing is generated The

excitation Monitoring consists of an autonomous processor system

612 over current Alarms

In the Excitation Monitoring the limits for are set at higher levels than the

settings on the Field Current Maximum Limiter Whenever the current exceeds 110 of

the nominal field current contact R1 and the binary output associated with it are

activated immediately If field current remains gt 110 then after a preset inverse-time

has lapsed relay R2 and - after a further delay - relay R0 and the binary outputs

associated with them are activated Parameters match the measurements for If1 and If2 to

the nominal value for field current so that the internal values can be processed and read as

pu values It can be used to falsify the actual value of the field current If (to raise it) so

as to cause a response from the alarm limits for purposes of testing The processed If

signal is always taken from on the active channel (CHANNEL I OR CHANNEL II) As

long as the field current If is above the threshold value 11 Ifn its peak value is

measured This is stored (until RESET) and can be read at any time on the Micro-

Terminal Once the value of If exceeds 11 Ifn integration of this value starts Whenever

the integrated time-current value (intisup2dt) exceeds the preselected reference value the

alarm OVER CURRENT INVERSE-TIME is set off and a command is simultaneously

issued to switch over to the stand by AUTO channel Software switch F758 enables the

three over current alarm functions (R0 R1 R2) and selects one of three possible inverse-

time curves T1 T2 or T3 Within the characteristic curve (T1 T2 T3) selected the

desired limit curve for response is set using the factor F216

613 Switch-Off Criterion for Field Flashing

The Excitation Monitoring supplies the criterion for switching off the field

flashing Whether this criterion is activated based on the actual value for generator voltage

Ug or for field current If or both depends on the settings of the two threshold values

F200F202 (0 setting means that the output is always ldquological 1rdquo) The measurements

Ug12 and If12 are switched over depending on the present status of the channels

(Channel 1 or Channel 2 ON) Whenever Flag F750 is not activated the binary output is

fixed at ldquological 1rdquo

614 Storage of Alarm Status

The outputs of the over current alarms (R1 R2 R0) and the messages NO

FAILURE MONITORING PARAMETERS CHANGED are stored messages can be

erased by activating the input ldquoGENERAL RESETrdquo or by using the RESET button on the

front of the module Erasure with the input ldquoGENERAL RESETrdquo is effective only if the

situation causing the alarm or the malfunction is no longer present Whenever the self-

diagnosis equipment in the processor detects a malfunction the output NO FAILURE of

MONITORING is set at ldquological 0rdquo (= alarm) The alarm ldquoPARAMETERS CHANGEDrdquo

is activated whenever parameters or settings of software switches have been changed via

the Micro-Terminal

615 Actual Value Monitoring

The actual values for generator voltage Ug synchronous voltage Usyn and field

current If are monitored for malfunctions This monitoring is active regardless of whether

or not the generator is in operation Essentially when the generator is in operation the

measurements are monitored by comparing the signals (the smaller signal reading is

detected as incorrect) When the generator is not in operation the measured data are

monitored for extreme values The percentage of deviation permissible in the

measurement signals being compared

(Ug1ampUsyn1 Ug1ampUg2 Ug2ampUsyn2 If1ampIf2) is defined by parameters F208 and

F20AIf the excitation transformer is being supplied from an auxiliary power source (no

shunt operation) the values of Ug and Usyn will be different in some operational

conditions

In that case Flag F75A can be used to deactivate comparative measurements Ug1 Usyn1

and Ug2 Usyn2 The ldquoprimary monitoringrdquo compares Ug1 with Ug2 and generates the

messages that Channel 1 or Channel 2 has suffered a malfunction Whenever Ug1 lt Ug2

and the binary message from CH1 reports no malfunction a malfunction on

Measurement Channel 1 is reported (Ug1Usyn1 FAILURE) A similar malfunction is

also present whenever the binary message CH1 DISTURBANCE is reported and a

discrepancy is detected between Ug1ampUg2 The generation of the alarm ldquoUg2 Usyn2

FAILURErdquo is analogous to that for Channel 1 The ldquosecondary monitoringrdquo compares

Ug1 with Usyn1 amp Ug2 with Usyn2 This is enabled whenever the binary message of the

comparison channel reports a malfunction or whenever both binary messages report no

malfunction - but both secondary monitors report a malfunction As long as the secondary

monitoring is blocked the differences Ug1neUsyn1 or Ug2neUsyn2 trigger malfunction

signals for the measurement channel involved (suspicion that there is a corresponding

error in Usyn) The measurement channel malfunctions are enabled operationally

whenever after excitation has been switched on generator voltage Ug exceeds the value

set on F204 The voltages Ug1 and Ug2 are checked 16 seconds after the excitation is

switched off to see that they do not exceed the limit value F210 that applies to both of

them At the same time g1Ug2Usyn1Usyn2 are checked for extreme values (gt or lt

the operating range) Monitoring for extreme values is likewise enabled during normal

operation ( Excitation ON and Ug gt F204 ) Flag F754 is used to enable or block the

malfunction signals to the binary outputs Basically the monitoring of the actual values

for If1If2 functions like that of the Ug1Ug2 monitoring

62 Excitation Protection

621 General Information

The Excitation Protection switches off the excitation (and de-excites the machine

rapidly) whenever a danger arises that threatens the excitation transformer the converter

or the generator Generally limiter or monitoring functions precede the emergency trips

and these normally respond before the Excitation Protection must initiate a trip

Protective trip commands are issued directly to the field circuit-breaker from potential

free contacts of the board via the trip relays They are directed redundantly to the

operative field circuit-breaker ldquoOFFrdquo command

622 Protection against Excitation Transformer Overheating

This equipment monitors the excitation transformer for overheating in the

windings that could result from over current short-circuits or inadequate cooling The

monitoring uses temperature monitoring modules in conjunction with temperature

sensors built into the windings Normally the temperature is monitored in two stages the

first stage sets off an alarm the second causes a trip of the excitation

623 Rotor Over voltage Protection

Malfunctions in the generator circuit (eg terminal short-circuit failed

synchronization asynchronous operation) cause induced negative field currents that

produce high voltages in the field circuit These must be restricted to a level with a

sufficient safety margin below the insulation capacity of the field winding (test voltage)

and also below the peak blocking voltage of the converter thyristors The crow bar

employs spark gap elements to detect over voltages in the field circuit Whenever they

respond the associated thyristors are fired immediately switching the de-excitation

resistor parallel to the field The de-excitation current generated thereby initiates an

excitation trip via a supervision circuit causing an immediate opening of the field circuit-

breaker The malfunction isets off an alarm and an internal malfunction is indicated at the

cubicle

TEST VALUES OBTAINED WHEN EXCITATION IS RAISED

TEST

SNO PARTICULARS ACTUAL VALUE OBTAINED VALUE

1 VREF 996 100

2 VACT 997 1003

3 IFACT 735 765

4 IGACT 703 707

5 ACTIVE

POWER

703 705

6 REACTIVE

POWER

101 142

7 POWER

FACTOR

099 IND 098 IND

8 ACTIVE

CURRENT(IR)

705 703

9 REACTIVE

CURRENT(IX)

102 138

10 POWER

ANGLE

566 546

11 FIRING

ANGLE

640 632

VALUES OBTAINED WHEN EXCITATION IS LOWERED

SNO PARTICULARS ACTUAL

VALUE

OBTAINED VALUE

1 VREF 100 997

2 VACT 100 997

3 IFACT 787 761

4 IGACT 839 837

5 ACTIVE

POWER

839 830

6 REACTIVE

POWER

155 110

7 POWER

FACTOR

098 IND 099 IND

8 ACTIVE

CURRENT(IR)

835 830

9 REACTIVE

CURRENT(IX)

140 94

10 POWER

ANGLE

613 DEG 631 DEG

11 FIRING

ANGLE

601 DEG 599 DEG

CONCLUSION

For generating the EMF in stator winding excitation is required to the rotor of a

generator There are two types of excitation

1 Static excitation system

2 Brushless excitation system

A certain disadvantage in brushless excitation system is the slow response time of

the field in case of fast load changes specified No slip-rings and brushes direct

measurements of the field parameters not possible

To avoid all loses static excitation is used Since it does not have any rotating

parts mechanical loses and windage loses This system has fast response and speed

control While preferring this excitation system there are no limitations for the

redundancy of Thyristor bridge circuits

Static excitation has fast field discharge by resistor and inverter operation direct

measurement of field quantity is possible The meaning of excitation is nothing but

continuous supply of DC current (ie field current) to the rotor to buildup required

output voltage in the stator

Field current is changed with respect to the change of load so the digital

automatic voltage regulator (DAVR) is used to regulate the output voltage according to

the load variations

So we conclude that static excitation system with DAVR is preferred since it is

having excellent dynamic performance and better options for R amp M

loading of the generator is low In this case if the stator current limiter is not kept from

influencing the field current the control circuit will oscillate back and forth between the

Inductive Stator Current Limiter (de-

exciting) and the Capacitive Stator Current Limiter (exciting)The output signal of that

function then dominates the control variable of the Ig-dependent limiter via a maximum

value selection

238 Capacitive Stator Current Limiter

239 Load Angle Limiter

The Load Angle Limiter prevents the synchronous machine from slipping out of

phase due to slippage of the rotor The load angle δ the difference in phase between the

rotor and the stator rotating field results mainly from the driving torque (active power P)

acting on the generator and the level of rotor current (field current) If the driving torque

remains constant a increase in the field current reduces the load angle δ The current load

angle δ at any moment is obtained from the generator current and generator voltage based

on a simplified model of the generator Whenever this calculated load angle δ exceeds the

preset limit angle the limiter increases the field current until the load angle has dropped

back to its permissible value The quadrature reactance Xq of the generator and the

network reactance Xe during normal operation must be adjusted on the regulator in order

to obtain the load angle δ The graph below shows the Power Chart for a salient-pole

machine with typical limiter characteristics

The purpose of a Power System Stabilizer is to use the generator excitation to

damp electromechanical oscillations between the network and the generator Depending

on the design of the generator and the requirements imposed for network stability its

main function will be either to damp the oscillations originating in the machine or those

from the network A synchronous generator working in a combined power network is in

principle an oscillating structure In order to produce a torque the magnetic field of the

rotor and the stator must form a given angle (referred to as the rotor displacement or load

angle δ) The electrical torque ME increases as the angle δ increases just as with a

torsion spring Because the ME of the generator and the mechanical driving torque MA

from the turbine are in equilibrium during steady-state operation the angle δ remains in a

given position Whenever this state of equilibrium between MA and ME is disturbed the

load angle slips of this rest position and change thereby the electrical torque ME The

torque attempts to restore the load angle to a stationary position Due to the mass inertia

of the turbinegenerator rotor however this can only take place aperiodically It does so

in the form of more or less effectively damped oscillations (again similar to the effect of

mass inertia on a torsion spring) In order to damp the oscillations there must be a

damping torque produced depending not on the electrical torque ME associated with the

angle but on the difference in frequency (Df) between the rotor and the stator rotating

field ie on the slippage This torque is produced mainly by the so-called damper

winding in the rotor but the dimensioning of this is subject to limits imposed by

considerations of design and economy Some further action is therefore needed to

increase the damping effect The following drastically simplified formula shows the

parameters upon which the amount of active power PE supplied by the generator

depends

PE = active power

It can be seen from the above relationship that the active power that the generator

transfers depends not only on the load angle δ but also on the field current If That means

that a transient change can be made in the active power PE and with that in the effective

electrical torque ME by varying the field current The principle of operation of the DVR

Power System Stabilizer becomes clear from a consideration of the oscillations in power

output and frequency (ΔPE Δf) and the vector diagram If it is assumed that oscillations

in the network frequency generates load oscillations with the mass inertia of the rotor

then the active load of the generator (eg MW-measured) is influenced with a sinusoidal

value -ΔPE (ME-MA = -ΔPE) By inversion of -ΔPE one obtains the fluctuation in

power provided by the rotor +ΔPE As is known the slip signal Δf follows +ΔPE with a

phase delayed by 90deg The +ME produced by the periodic changes in the load angle δ is

in phase with +ΔPE A good damping is attained if ME is varied in phase with the slip

Δf However this signal must also be advanced somewhat to compensate for the time

constants in the excitation circuit and the generator

As mentioned above the electrical torque ME can be influenced by varying the

field current To accomplish this a suitable control signal referred to as variable

disturbance compensation must be imposed upon the voltage set-point or the converter

control variable Ucontr As can be seen from the vector diagram by applying proper

weighting factors (K1 K2) and then adding together the signals -ΔPE and Δf an overall

stabilization signal can be produced that rotates in advance of the Df signal by any angle

desired between 0deg and 90deg Because the amplitude of -DPE remains proportional to the

amplitude of Δf a constant angle in advance of Δf results for the compensation of the

time constants referred to above The optimum weighting factors K1 and K2 for a

synchronous generator working to a power network depend on its operating point at any

moment and the external reactance of the network Normally the selection of a

compromise setting is good enough to attain stability in all operating points and for all

external reactance For special demands these settings must be parameterized as a

function of the external reactance (which means optional equipment Xe-Identification)

The Power System Stabilizer PSS is a section of the AVR computer program and is

processed once per network cycle The voltage at the generator terminals and the

generator current are measured in order to define the signals ΔPE and Δf The calculated

signals for _P_ (=PE) and Δf are then sent across DC filters ldquoDrdquo (real differentiators) that

transmit only the dynamic portion of the signals The ΔPE and Δf signals obtained in this

way are then weighted (multiplied by) with the factors K1 and K2 and sent to the

summing point of the voltage regulator

The PSS stabilization signal is imposed on the automatic voltage regulator only if

the following prerequisites are met

bull Generator on line

bull Generator power output gt the value F338

bull Generator voltage in a range between F33C and F33A

The stabilization signal is limited at the output from the PSS to the lower and

upper limits Flag defines whether the stabilization signal is introduced before or after the

PID filter (usually before the filter) Because the PID filter as noted above already takes

the ceiling factor Vp1 into account the PSS signal needs to be multiplied by Vp1 if it is

added to the voltage regulator following the PID filter (divider at the input to the

minmax limiter) This precaution prevents the DC filter ldquoDrdquo in the P-channel from

producing an unnecessary ldquostabilizationrdquo effect in the case of rapid changes in turbine

load As an alternative for the AVRrsquos Power System Stabilizer a stabilization signal from

an outside system can be imposed by activating the binary input ldquoPSS-SIGNEXTrdquo Flag

F340 can be used to select between an analog and a 12-bit signal and F33E to select the

polarity desired for that signal

24 The MANUAL Channel

241 Summary

The MANUAL channel (Channel 2) has been built as a simple field current

regulator

without additional limiters Its main function is to maintain the excitation of the generator

even if the AUTOMATIC channel becomes non-operational The MANUAL channel

also performs valuable service for purposes of testing commissioning and preventive

maintenance Its measurements regulator generation of firing pulses and power supply

are physically separate from those on the AUTOMATIC channel

242 Principle of Operation

All the functions of the MANUAL channel including the generation of firing

pulses have been implemented in a single electronic module the Gate Control Unit The

control variable Ucontr of voltage regulator is used as the reference value for generating

firing pulses on the principle known as ldquoramp controlrdquo (Comparison of Ucontr with

Usynsynchronous sawtooth signal) For further processing in the UN 0096 Intermediate

Pulse Stage the Gate Control Unit supplies six firing pulses at its output whose phase

position with respect to the synchronous voltage Usyn is in accordance with control

variable Ucontr An internal linearization ensures that the field voltage produced via the

firing pulses remains proportional to the control variable Ucontr throughout the entire

range As a result the circuit amplification of the control remains constant over the entire

range Whenever excitation is switched ON the set-point for Generator Voltage is set

automatically at the preset - ref Value This provision ensures that the generator voltage

always attains approximately its nominal value after the field flashing The Gate Control

Unit can be refunctioned ( by pre-selection with a switch ) for purposes of testing to act

as a purely firing pulse control In this case the control variable Ucontr is adjusted

directly using the RAISELOWER push buttons on the front of the module In this way

for example the relationship between the phase position of the firing pulses and the

control variable Ucontr can be checked easily

CHAPTER 3

PULSE SECTION

31 Pulse Generation and Amplification

The Gate Control Units of both AUTOMATIC channel and MANUAL channel

each supply six firing pulses for operating the 6-pulse thyristor bridges The low-power

pulse signals from these Gate Control Units are then amplified in the Intermediate Pulse

Stage galvanically isolated and then sent to the common pulse bus On the output end

the Intermediate Pulse Stage of the non-active channel) is always blocked The Gate

Control Units generate the pulses based on microprocessor control The reference voltage

used for the firing pulse phase location is the output voltage from the excitation

transformer (Usyn1 Usyn2) The commutation spikes of the synchronous voltage caused

by the converter are calculated prior to use of the voltage as a reference value and are

deliberately filtered out The lower limit for the firing pulses (double pulses) which are

offset from one another by 60deg is defined by the limit rectifier position (αmin) and the

upper limit by the limit inverter position (αmax) for the firing angle αmin and αmax can

be adjusted on the Gate Control Units using BCD (Binary Coded Decimal) switches

αmin ensures that the firing pulses will not be issued (premature firing) until there is

sufficient positive phase voltage on the thyristor involved αmax prevents a dangerous

ldquotippingrdquo of the thyristor bridge into the rectifier mode if the firing angle α is too large

(ldquolate firingrdquo) The critical factors determining αmax are the overlap time uumlmax (max

commutation time) and the ldquorecovery timerdquo of the thyristors (αmax lt 180deg - uumlmax - γ )

An external control signal can force the firing pulses into their inverter limit position

Other binary inputs can block or direct the firing pulses of the Gate Control Units so as to

produce freewheeling on the thyristor bridge During freewheeling the firing pulses for

the thyristor pair R and S are blocked and the pulse signals T+T- are engaged with

chains of pulses Both Gate Control Units (for the MANUAL amp AUTOMATIC channels)

contain a field current monitor that blocks the firing pulses immediately whenever the

current exceeds a preset threshold level In this case the field circuit-breaker is also

tripped via an output contact The purpose of these provisions is to prevent damage to

thyristors and thyristor fuses in case of a slip-ring short-circuit or to keep any damage

that does occur to a minimum The pulse signals are galvanically separated at the outputs

from the Intermediate Pulse Stage (with pulse transmitters) and are then directed to the

common pulse bus This transmission of the pulse signals to the pulse bus via passive

transmitters ensures a high degree of active channel autonomy Practically no possible

malfunctions on the inactive channel (including for example sustained pulses) affect the

active channel

32 Pulse Monitoring

The ldquoPulse Busrdquo and the pulse signals of the AUTOMATIC channel are

monitored This monitoring device consists of potential isolating stages and the common

monitor If the pulse monitoring of the ldquoPulse Busrdquo responds a switch-over is made to

MANUAL channel The function of the potential isolating stages is to couple the pulse

monitoring device to the pulse circuits without any feedback effect The pulse monitoring

checks the six pulse lines for the following malfunctions continuous or periodic failure

of one or more pulses Periodic occurrence of synchronous or asynchronous false pulses

Continuous pulses the pulse monitoring device can be tested while the machine is in

operation

CHAPTER 4

CONVERTER

Thyristor

The term thyristor usually refers to a family of four layer solid state device having

turn on characteristics that can be externally controlled by either current or voltage They

are also referred to as breakdown device because their working depends on avalanche

breakdown Thyristors have only two stages OFF and ON Thyristors have a similar

function to Uni-junctions they act as switches Thyristors use current flow as a switch

Thyristors have three states

1 Reverse blocking mode mdash Voltage is applied in the direction that would be

blocked by a diode

2 Forward blocking mode mdash Voltage is applied in the direction that would cause

a diode to conduct but the thyristor has not yet been triggered into conduction

3 Forward conducting mode mdash The thyristor has been triggered into conduction

and will remain conducting until the forward current drops below a threshold value

known as the holding current Converter is a semiconductor device which converts ac

input voltage into a constant dc output voltage In present excitation system three phase

fully controlled thyristor converter is used

Because of the following advantages thyristor converters are used

a) Thyristors are used for high power applications ie up to 10Kv3500A1KHz

b) Having high reliability and low losses

c) Uni-directional device like diode

d) Itrsquos operation as a rectifier which are low resistance in forward conduction

mode and high resistance in reverse conduction mode

PROTECTION OF THYRISTORS

For reliable operation of a thyristor demands that its specified ratings are not

exceeded When Subjected to or over voltages During the turn - on of SCR didt

prohibitively large False triggering of SCR by high value of dvdt andSpurious signals

between gate and cathode may leads to unwanted turn ndash on

DIDT AND PROTECTION

When thyristor starts conducting in forward conduction mode and is turned on by

gate pulse The anode current increases rapidly whole area of the gate to Cathode

junction then hot spots will be formed near the gate connection this locality of heating

destroys the thyristor Thyristor thermal time is constant The causes due to faults and

short circuits or surge currents Electronic crowbar protection is used against the over

voltages The rate rise of anode current must be kept at the time of turn on below the

rated or specified limiting value The didt value maintained below limited value by using

a inductor also called ldquodidt inductorrdquo in series with anode circuit The locality of heating

is avoided by applying gate current but not greater the maximum gate current

DVDT AND OVER VOLTAGE PROTECTION

With forward voltage across the anode and cathode of a thyristor the two outer

junctions are forward biased but the inner junction is reverse biased This reverse biased

junction J2 has the characteristics of a capacitor due to charges existing across the

junction In other words space-charges exist in the depletion region around junction J2

and therefore junction J2 behaves like a capacitance If the entire anode to cathode

forward voltage Va appears across J2 junction and the charge is denoted by Q then a

charging current i given by Eq (46) follows

i = dQdt =d(Cj Va )dt

= Cj (d Va dt) + Va(d Cj dt) helliphelliphelliphellip(46 a)

As Cj the capacitance of junction J2 is almost constant the current is given by

i = Cj (d Va dt) helliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip(46 b)

If the rate of rise of forward voltage dVadt is high the charging current i will be

more This charging current plays the role of gate current and turns on the SCR even

when gate signal is zero Such phenomena of turning-on a thyristor called dvdt turn-on

must be avoided as it leads to false operation of the thyristor circuit

For controllable operation of the thyristor the rate of rise of forward anode to

cathode voltage dVadt must be kept below the specified rated limit Typical values of

dvdt are 20 ndash 500 Vμsec False turn-on of a thyristor by large dvdt can be prevented by

using a snubber circuit in parallel with the device thyristor are very sensitive for over

voltage than the semiconductor devices

Over voltage transients are perhaps the main cause of thyristor failure

In thyristor there are mainly two types

1 Internal over voltages

Due to the commutation of the thyristors large voltages are generated internally

Because of the series inductance of the SCR circuit the large transient voltages L didt

produced This voltage several times the break over voltage of the device then thyristor

destroys permanently

2 External over voltages

External over voltages are caused due to the interruptions of current flow in an

inductive circuit and also due to the lightening strokes on the lines feeding the thyristor

system For the reliable operation of thyristor the over voltages must be suppressed by

adopting suitable techniques

Suppression of over voltages

The RC circuit called snubber circuit is connected across the device to protect In

order to keep the protective components to a minimum the thyristors are chosen with

their peak voltages ratings are 25 to 3 times of the normal peak working voltage

ldquoselenium thyrector diodes metal oxide varistors or avalanche diode suppressers are

commonly employed for protecting the thyristor circuit against the over voltages

Gate protection

Gate circuit should also be protected against the over voltages and surges Over

voltage at gate circuit can cause false triggering of the SCR may rises the junction

temperature behind specified limit leading to its damage Protection against over

voltage can be achieved by connecting a ZD across the gate circuit and a resister is

connected in series with gate circuit to protect against the s A capacitor and resister are

connected across gate to cathode to by pass the noise

41 Final Pulse Stages

The Final Pulse Stages adapt the output pulses from the Intermediate Pulse Stage

(pulses on the pulse bus) to the gate currents needed for the thyristors Each thyristor

bridge is equipped with its own Final Pulse Stage Each Final Pulse Stages is provided

with a power supply module The amplified output pulses from the Final Pulse Stages

start as a short strong steep pulse with an amplitude approx 2frac12 times that of the main

pulse This initial pulse edge assures proper firing of the thyristors being triggered

Subsequently the weaker part of main pulse keeps firing conditions steady As already

mentioned the Final Pulse Stages and their associated thyristor bridges form single units

All six pulse outputs from a Final Pulse Stage can be blocked by an external control

signal so that all thyristors in the associated thyristor bridge will block the current A

blocking of the pulses is initiated whenever there is a malfunction in the associated

thyristor bridge

42 Converter Power Section

The thyristor converter consists of three independent parallel rectifier blocks TY1

to TY3 which are all in service Even if one block fails the remaining blocks take over

automatically the full design current of the excitation circuit During normal operation

(with ideal current share) and all three bridges in operation each of these blocks has to

carry only (n-2)n (ie33)of its design current If 2 thyristor bridges fail the excitation

is limited Only when all three bridges fail the excitation is switched off Each thyristor

bridge arm is equipped with current flow monitoring CTrsquos Failure of conduction in any

arm is identified by a Current flow monitoring module

43 Converter Cooling

A cooling system is needed to dissipate heat losses in the converter blocks and

electronics Each converter block has therefore been equipped with a fan supplied with

power from the converterrsquos primary voltage (via transformer ndashT8 in field flashing

cubicle) The fans are protected with motor protection circuit breakers An air flow

monitoring unit is provided for monitoring the air flow through the thyristor bridge If a

circuit breaker failure is detected or if the air flow monitor drops off at one of the

thyristor bridges the bridge involved is immediately set out of operation by blocking its

firing pulses

44 Thyristor Converter Monitoring

A thyristor bridge in which defects occur that could threaten the safety of

operation or cause secondary damage is switched off automatically ie its firing pulses

are blocked This happens whenever A thyristor fuse is blown The fuses are monitored

individually with micro switches The Final Pulse Stage fails which is detected by

internal monitors (supply voltage sustained pulse short-circuit on the output end) The

power supply to the fan fails fan air flow as monitored by the Air flow monitor fails or

is insufficient Isolator on ACDC side is open

CHAPTER 5

Field Current Circuit Excitation Transformer

51 Field Circuit Breaker

The circuit-breaker in the field circuit is used to isolate the field circuit from the

converter It is capable of switching off the synchronous machine from full load under the

maximum conditions of a 3-phase short-circuit In addition to its main contacts the field

circuit-breaker also has a de-excitation contact with which the field energy stored in the

field can be dissipated across the de-excitation resistor The de-excitation contact closes

shortly before the main contacts open so as to ensure proper commutation of the field

current from the main contacts to the de-excitation contact when the breaker is switched

off The field circuit-breaker is switched on by electromagnetic force and is kept switched

on by a mechanical latch When the latch is released by a trip coil the circuit-breaker

opens The circuit-breaker also has auxiliary contacts that report its status

52 Field Flashing

In shunt supplied excitation circuits (excitation transformer connected to the

generator terminals) the generator does not have enough remnant voltage for a generator

voltage build-up via the converter In this case a field flashing circuit is provided It

consists of the field flashing contactor the diode bridge and a transformer used to adapt

the auxiliary input voltage to the voltage needed for field flashing when power is

supplied from the auxiliaries network

Fig Field Flashing

Because the field flashing contactor is not able to switch off the energy stored in

the field the control ensures that the contactor can only reopen if the field circuit breaker

has already been opened (generating the TRIP order) or in a normal field flashing

sequence when the converter has taken over the field current Field flashing occurs in the

following stages

1048729The excitation is switched on closing the field flashing contactor ( Field

Circuit Breaker is already closed )

1048729The start-up excitation current flows through the rotor driving the generator

voltage up to approx 15 U

1048729After about 10 U the firing pulses to the converter are released and it begins

to excite the generator to its rated voltage

1048729After about 30 U the field flashing contactor opens (with no current since

the converter is now supplying the current)

The diode bridge at the input to the field flashing contactor prevents a feed-back

from the converter to the source of field flashing while the contactor is still closed

53 De-excitation

When malfunctions occur the stored field energy must be dissipated as quickly

and safely as possible to protect the generator This is done by the converter the field

circuit-breaker and the de-excitation (discharge) resistor

De-excitation (with opening of the field circuit-breaker) takes place in the following

stages

1048729The converter drives to its inverter limit position (negative ceiling voltage)

recovers a portion of the field energy into the network A trip command is given to the

field circuit breaker

1048729The de-excitation contact closes diverting the field voltage to the de-excitation

resistor

1048729Then immediately the main contacts open building voltage The field voltage

commutates to the de-excitation resistor

1048729The current diminishes at a given time constant TE

(With linear resistance TE = Lf (Rf + Re))

Due to the reversal of the field voltage by the converter the field current

commutates from the main contacts of the field circuit-breaker to the de-excitation

resistor in a very early phase This reversal of the field voltage prevents burn-off on the

main contacts and provides effective protection for the field circuit-breaker Depending

on the operating policy an operational shut-down of the excitation can also be effected

with the field circuit-breaker closed This method is useful mainly when the excitation is

switched on and off frequently In this case the converter is merely driven into the

inverter limit position so that the field energy is recovered into the network The

converter then blocks since it is supplying positive current only

54 Excitation Transformer

The excitation transformer matches the generator voltage to the field voltage

(required ceiling voltage) It also serves as a commutation reactance for the thyristor

converter and as a potential isolator between the network and the excitation circuit In

addition the transformer functions as a current limiter in that it makes it possible to keep

any short circuits in the excitation circuit under better control The excitation transformer

is equipped with temperature monitoring probes which set off an alarm when the

temperature exceeds a first max limit and then trips the excitation if the temperature

continues rising to a second (higher)limit

CHAPTER 6

Monitoring and Protection

61 Excitation Monitoring

611 General Information

The main goal of Excitation Monitoring is to make optimum use of the

redundancies provided in the excitation system and to give alarm whenever a malfunction

makes these redundancies unavailable The field current is monitored to see that it does

not exceed a maximum level and if necessary a switch-over to the MANUAL channel is

initiated In addition the criterion for switching off the field flashing is generated The

excitation Monitoring consists of an autonomous processor system

612 over current Alarms

In the Excitation Monitoring the limits for are set at higher levels than the

settings on the Field Current Maximum Limiter Whenever the current exceeds 110 of

the nominal field current contact R1 and the binary output associated with it are

activated immediately If field current remains gt 110 then after a preset inverse-time

has lapsed relay R2 and - after a further delay - relay R0 and the binary outputs

associated with them are activated Parameters match the measurements for If1 and If2 to

the nominal value for field current so that the internal values can be processed and read as

pu values It can be used to falsify the actual value of the field current If (to raise it) so

as to cause a response from the alarm limits for purposes of testing The processed If

signal is always taken from on the active channel (CHANNEL I OR CHANNEL II) As

long as the field current If is above the threshold value 11 Ifn its peak value is

measured This is stored (until RESET) and can be read at any time on the Micro-

Terminal Once the value of If exceeds 11 Ifn integration of this value starts Whenever

the integrated time-current value (intisup2dt) exceeds the preselected reference value the

alarm OVER CURRENT INVERSE-TIME is set off and a command is simultaneously

issued to switch over to the stand by AUTO channel Software switch F758 enables the

three over current alarm functions (R0 R1 R2) and selects one of three possible inverse-

time curves T1 T2 or T3 Within the characteristic curve (T1 T2 T3) selected the

desired limit curve for response is set using the factor F216

613 Switch-Off Criterion for Field Flashing

The Excitation Monitoring supplies the criterion for switching off the field

flashing Whether this criterion is activated based on the actual value for generator voltage

Ug or for field current If or both depends on the settings of the two threshold values

F200F202 (0 setting means that the output is always ldquological 1rdquo) The measurements

Ug12 and If12 are switched over depending on the present status of the channels

(Channel 1 or Channel 2 ON) Whenever Flag F750 is not activated the binary output is

fixed at ldquological 1rdquo

614 Storage of Alarm Status

The outputs of the over current alarms (R1 R2 R0) and the messages NO

FAILURE MONITORING PARAMETERS CHANGED are stored messages can be

erased by activating the input ldquoGENERAL RESETrdquo or by using the RESET button on the

front of the module Erasure with the input ldquoGENERAL RESETrdquo is effective only if the

situation causing the alarm or the malfunction is no longer present Whenever the self-

diagnosis equipment in the processor detects a malfunction the output NO FAILURE of

MONITORING is set at ldquological 0rdquo (= alarm) The alarm ldquoPARAMETERS CHANGEDrdquo

is activated whenever parameters or settings of software switches have been changed via

the Micro-Terminal

615 Actual Value Monitoring

The actual values for generator voltage Ug synchronous voltage Usyn and field

current If are monitored for malfunctions This monitoring is active regardless of whether

or not the generator is in operation Essentially when the generator is in operation the

measurements are monitored by comparing the signals (the smaller signal reading is

detected as incorrect) When the generator is not in operation the measured data are

monitored for extreme values The percentage of deviation permissible in the

measurement signals being compared

(Ug1ampUsyn1 Ug1ampUg2 Ug2ampUsyn2 If1ampIf2) is defined by parameters F208 and

F20AIf the excitation transformer is being supplied from an auxiliary power source (no

shunt operation) the values of Ug and Usyn will be different in some operational

conditions

In that case Flag F75A can be used to deactivate comparative measurements Ug1 Usyn1

and Ug2 Usyn2 The ldquoprimary monitoringrdquo compares Ug1 with Ug2 and generates the

messages that Channel 1 or Channel 2 has suffered a malfunction Whenever Ug1 lt Ug2

and the binary message from CH1 reports no malfunction a malfunction on

Measurement Channel 1 is reported (Ug1Usyn1 FAILURE) A similar malfunction is

also present whenever the binary message CH1 DISTURBANCE is reported and a

discrepancy is detected between Ug1ampUg2 The generation of the alarm ldquoUg2 Usyn2

FAILURErdquo is analogous to that for Channel 1 The ldquosecondary monitoringrdquo compares

Ug1 with Usyn1 amp Ug2 with Usyn2 This is enabled whenever the binary message of the

comparison channel reports a malfunction or whenever both binary messages report no

malfunction - but both secondary monitors report a malfunction As long as the secondary

monitoring is blocked the differences Ug1neUsyn1 or Ug2neUsyn2 trigger malfunction

signals for the measurement channel involved (suspicion that there is a corresponding

error in Usyn) The measurement channel malfunctions are enabled operationally

whenever after excitation has been switched on generator voltage Ug exceeds the value

set on F204 The voltages Ug1 and Ug2 are checked 16 seconds after the excitation is

switched off to see that they do not exceed the limit value F210 that applies to both of

them At the same time g1Ug2Usyn1Usyn2 are checked for extreme values (gt or lt

the operating range) Monitoring for extreme values is likewise enabled during normal

operation ( Excitation ON and Ug gt F204 ) Flag F754 is used to enable or block the

malfunction signals to the binary outputs Basically the monitoring of the actual values

for If1If2 functions like that of the Ug1Ug2 monitoring

62 Excitation Protection

621 General Information

The Excitation Protection switches off the excitation (and de-excites the machine

rapidly) whenever a danger arises that threatens the excitation transformer the converter

or the generator Generally limiter or monitoring functions precede the emergency trips

and these normally respond before the Excitation Protection must initiate a trip

Protective trip commands are issued directly to the field circuit-breaker from potential

free contacts of the board via the trip relays They are directed redundantly to the

operative field circuit-breaker ldquoOFFrdquo command

622 Protection against Excitation Transformer Overheating

This equipment monitors the excitation transformer for overheating in the

windings that could result from over current short-circuits or inadequate cooling The

monitoring uses temperature monitoring modules in conjunction with temperature

sensors built into the windings Normally the temperature is monitored in two stages the

first stage sets off an alarm the second causes a trip of the excitation

623 Rotor Over voltage Protection

Malfunctions in the generator circuit (eg terminal short-circuit failed

synchronization asynchronous operation) cause induced negative field currents that

produce high voltages in the field circuit These must be restricted to a level with a

sufficient safety margin below the insulation capacity of the field winding (test voltage)

and also below the peak blocking voltage of the converter thyristors The crow bar

employs spark gap elements to detect over voltages in the field circuit Whenever they

respond the associated thyristors are fired immediately switching the de-excitation

resistor parallel to the field The de-excitation current generated thereby initiates an

excitation trip via a supervision circuit causing an immediate opening of the field circuit-

breaker The malfunction isets off an alarm and an internal malfunction is indicated at the

cubicle

TEST VALUES OBTAINED WHEN EXCITATION IS RAISED

TEST

SNO PARTICULARS ACTUAL VALUE OBTAINED VALUE

1 VREF 996 100

2 VACT 997 1003

3 IFACT 735 765

4 IGACT 703 707

5 ACTIVE

POWER

703 705

6 REACTIVE

POWER

101 142

7 POWER

FACTOR

099 IND 098 IND

8 ACTIVE

CURRENT(IR)

705 703

9 REACTIVE

CURRENT(IX)

102 138

10 POWER

ANGLE

566 546

11 FIRING

ANGLE

640 632

VALUES OBTAINED WHEN EXCITATION IS LOWERED

SNO PARTICULARS ACTUAL

VALUE

OBTAINED VALUE

1 VREF 100 997

2 VACT 100 997

3 IFACT 787 761

4 IGACT 839 837

5 ACTIVE

POWER

839 830

6 REACTIVE

POWER

155 110

7 POWER

FACTOR

098 IND 099 IND

8 ACTIVE

CURRENT(IR)

835 830

9 REACTIVE

CURRENT(IX)

140 94

10 POWER

ANGLE

613 DEG 631 DEG

11 FIRING

ANGLE

601 DEG 599 DEG

CONCLUSION

For generating the EMF in stator winding excitation is required to the rotor of a

generator There are two types of excitation

1 Static excitation system

2 Brushless excitation system

A certain disadvantage in brushless excitation system is the slow response time of

the field in case of fast load changes specified No slip-rings and brushes direct

measurements of the field parameters not possible

To avoid all loses static excitation is used Since it does not have any rotating

parts mechanical loses and windage loses This system has fast response and speed

control While preferring this excitation system there are no limitations for the

redundancy of Thyristor bridge circuits

Static excitation has fast field discharge by resistor and inverter operation direct

measurement of field quantity is possible The meaning of excitation is nothing but

continuous supply of DC current (ie field current) to the rotor to buildup required

output voltage in the stator

Field current is changed with respect to the change of load so the digital

automatic voltage regulator (DAVR) is used to regulate the output voltage according to

the load variations

So we conclude that static excitation system with DAVR is preferred since it is

having excellent dynamic performance and better options for R amp M

The purpose of a Power System Stabilizer is to use the generator excitation to

damp electromechanical oscillations between the network and the generator Depending

on the design of the generator and the requirements imposed for network stability its

main function will be either to damp the oscillations originating in the machine or those

from the network A synchronous generator working in a combined power network is in

principle an oscillating structure In order to produce a torque the magnetic field of the

rotor and the stator must form a given angle (referred to as the rotor displacement or load

angle δ) The electrical torque ME increases as the angle δ increases just as with a

torsion spring Because the ME of the generator and the mechanical driving torque MA

from the turbine are in equilibrium during steady-state operation the angle δ remains in a

given position Whenever this state of equilibrium between MA and ME is disturbed the

load angle slips of this rest position and change thereby the electrical torque ME The

torque attempts to restore the load angle to a stationary position Due to the mass inertia

of the turbinegenerator rotor however this can only take place aperiodically It does so

in the form of more or less effectively damped oscillations (again similar to the effect of

mass inertia on a torsion spring) In order to damp the oscillations there must be a

damping torque produced depending not on the electrical torque ME associated with the

angle but on the difference in frequency (Df) between the rotor and the stator rotating

field ie on the slippage This torque is produced mainly by the so-called damper

winding in the rotor but the dimensioning of this is subject to limits imposed by

considerations of design and economy Some further action is therefore needed to

increase the damping effect The following drastically simplified formula shows the

parameters upon which the amount of active power PE supplied by the generator

depends

PE = active power

It can be seen from the above relationship that the active power that the generator

transfers depends not only on the load angle δ but also on the field current If That means

that a transient change can be made in the active power PE and with that in the effective

electrical torque ME by varying the field current The principle of operation of the DVR

Power System Stabilizer becomes clear from a consideration of the oscillations in power

output and frequency (ΔPE Δf) and the vector diagram If it is assumed that oscillations

in the network frequency generates load oscillations with the mass inertia of the rotor

then the active load of the generator (eg MW-measured) is influenced with a sinusoidal

value -ΔPE (ME-MA = -ΔPE) By inversion of -ΔPE one obtains the fluctuation in

power provided by the rotor +ΔPE As is known the slip signal Δf follows +ΔPE with a

phase delayed by 90deg The +ME produced by the periodic changes in the load angle δ is

in phase with +ΔPE A good damping is attained if ME is varied in phase with the slip

Δf However this signal must also be advanced somewhat to compensate for the time

constants in the excitation circuit and the generator

As mentioned above the electrical torque ME can be influenced by varying the

field current To accomplish this a suitable control signal referred to as variable

disturbance compensation must be imposed upon the voltage set-point or the converter

control variable Ucontr As can be seen from the vector diagram by applying proper

weighting factors (K1 K2) and then adding together the signals -ΔPE and Δf an overall

stabilization signal can be produced that rotates in advance of the Df signal by any angle

desired between 0deg and 90deg Because the amplitude of -DPE remains proportional to the

amplitude of Δf a constant angle in advance of Δf results for the compensation of the

time constants referred to above The optimum weighting factors K1 and K2 for a

synchronous generator working to a power network depend on its operating point at any

moment and the external reactance of the network Normally the selection of a

compromise setting is good enough to attain stability in all operating points and for all

external reactance For special demands these settings must be parameterized as a

function of the external reactance (which means optional equipment Xe-Identification)

The Power System Stabilizer PSS is a section of the AVR computer program and is

processed once per network cycle The voltage at the generator terminals and the

generator current are measured in order to define the signals ΔPE and Δf The calculated

signals for _P_ (=PE) and Δf are then sent across DC filters ldquoDrdquo (real differentiators) that

transmit only the dynamic portion of the signals The ΔPE and Δf signals obtained in this

way are then weighted (multiplied by) with the factors K1 and K2 and sent to the

summing point of the voltage regulator

The PSS stabilization signal is imposed on the automatic voltage regulator only if

the following prerequisites are met

bull Generator on line

bull Generator power output gt the value F338

bull Generator voltage in a range between F33C and F33A

The stabilization signal is limited at the output from the PSS to the lower and

upper limits Flag defines whether the stabilization signal is introduced before or after the

PID filter (usually before the filter) Because the PID filter as noted above already takes

the ceiling factor Vp1 into account the PSS signal needs to be multiplied by Vp1 if it is

added to the voltage regulator following the PID filter (divider at the input to the

minmax limiter) This precaution prevents the DC filter ldquoDrdquo in the P-channel from

producing an unnecessary ldquostabilizationrdquo effect in the case of rapid changes in turbine

load As an alternative for the AVRrsquos Power System Stabilizer a stabilization signal from

an outside system can be imposed by activating the binary input ldquoPSS-SIGNEXTrdquo Flag

F340 can be used to select between an analog and a 12-bit signal and F33E to select the

polarity desired for that signal

24 The MANUAL Channel

241 Summary

The MANUAL channel (Channel 2) has been built as a simple field current

regulator

without additional limiters Its main function is to maintain the excitation of the generator

even if the AUTOMATIC channel becomes non-operational The MANUAL channel

also performs valuable service for purposes of testing commissioning and preventive

maintenance Its measurements regulator generation of firing pulses and power supply

are physically separate from those on the AUTOMATIC channel

242 Principle of Operation

All the functions of the MANUAL channel including the generation of firing

pulses have been implemented in a single electronic module the Gate Control Unit The

control variable Ucontr of voltage regulator is used as the reference value for generating

firing pulses on the principle known as ldquoramp controlrdquo (Comparison of Ucontr with

Usynsynchronous sawtooth signal) For further processing in the UN 0096 Intermediate

Pulse Stage the Gate Control Unit supplies six firing pulses at its output whose phase

position with respect to the synchronous voltage Usyn is in accordance with control

variable Ucontr An internal linearization ensures that the field voltage produced via the

firing pulses remains proportional to the control variable Ucontr throughout the entire

range As a result the circuit amplification of the control remains constant over the entire

range Whenever excitation is switched ON the set-point for Generator Voltage is set

automatically at the preset - ref Value This provision ensures that the generator voltage

always attains approximately its nominal value after the field flashing The Gate Control

Unit can be refunctioned ( by pre-selection with a switch ) for purposes of testing to act

as a purely firing pulse control In this case the control variable Ucontr is adjusted

directly using the RAISELOWER push buttons on the front of the module In this way

for example the relationship between the phase position of the firing pulses and the

control variable Ucontr can be checked easily

CHAPTER 3

PULSE SECTION

31 Pulse Generation and Amplification

The Gate Control Units of both AUTOMATIC channel and MANUAL channel

each supply six firing pulses for operating the 6-pulse thyristor bridges The low-power

pulse signals from these Gate Control Units are then amplified in the Intermediate Pulse

Stage galvanically isolated and then sent to the common pulse bus On the output end

the Intermediate Pulse Stage of the non-active channel) is always blocked The Gate

Control Units generate the pulses based on microprocessor control The reference voltage

used for the firing pulse phase location is the output voltage from the excitation

transformer (Usyn1 Usyn2) The commutation spikes of the synchronous voltage caused

by the converter are calculated prior to use of the voltage as a reference value and are

deliberately filtered out The lower limit for the firing pulses (double pulses) which are

offset from one another by 60deg is defined by the limit rectifier position (αmin) and the

upper limit by the limit inverter position (αmax) for the firing angle αmin and αmax can

be adjusted on the Gate Control Units using BCD (Binary Coded Decimal) switches

αmin ensures that the firing pulses will not be issued (premature firing) until there is

sufficient positive phase voltage on the thyristor involved αmax prevents a dangerous

ldquotippingrdquo of the thyristor bridge into the rectifier mode if the firing angle α is too large

(ldquolate firingrdquo) The critical factors determining αmax are the overlap time uumlmax (max

commutation time) and the ldquorecovery timerdquo of the thyristors (αmax lt 180deg - uumlmax - γ )

An external control signal can force the firing pulses into their inverter limit position

Other binary inputs can block or direct the firing pulses of the Gate Control Units so as to

produce freewheeling on the thyristor bridge During freewheeling the firing pulses for

the thyristor pair R and S are blocked and the pulse signals T+T- are engaged with

chains of pulses Both Gate Control Units (for the MANUAL amp AUTOMATIC channels)

contain a field current monitor that blocks the firing pulses immediately whenever the

current exceeds a preset threshold level In this case the field circuit-breaker is also

tripped via an output contact The purpose of these provisions is to prevent damage to

thyristors and thyristor fuses in case of a slip-ring short-circuit or to keep any damage

that does occur to a minimum The pulse signals are galvanically separated at the outputs

from the Intermediate Pulse Stage (with pulse transmitters) and are then directed to the

common pulse bus This transmission of the pulse signals to the pulse bus via passive

transmitters ensures a high degree of active channel autonomy Practically no possible

malfunctions on the inactive channel (including for example sustained pulses) affect the

active channel

32 Pulse Monitoring

The ldquoPulse Busrdquo and the pulse signals of the AUTOMATIC channel are

monitored This monitoring device consists of potential isolating stages and the common

monitor If the pulse monitoring of the ldquoPulse Busrdquo responds a switch-over is made to

MANUAL channel The function of the potential isolating stages is to couple the pulse

monitoring device to the pulse circuits without any feedback effect The pulse monitoring

checks the six pulse lines for the following malfunctions continuous or periodic failure

of one or more pulses Periodic occurrence of synchronous or asynchronous false pulses

Continuous pulses the pulse monitoring device can be tested while the machine is in

operation

CHAPTER 4

CONVERTER

Thyristor

The term thyristor usually refers to a family of four layer solid state device having

turn on characteristics that can be externally controlled by either current or voltage They

are also referred to as breakdown device because their working depends on avalanche

breakdown Thyristors have only two stages OFF and ON Thyristors have a similar

function to Uni-junctions they act as switches Thyristors use current flow as a switch

Thyristors have three states

1 Reverse blocking mode mdash Voltage is applied in the direction that would be

blocked by a diode

2 Forward blocking mode mdash Voltage is applied in the direction that would cause

a diode to conduct but the thyristor has not yet been triggered into conduction

3 Forward conducting mode mdash The thyristor has been triggered into conduction

and will remain conducting until the forward current drops below a threshold value

known as the holding current Converter is a semiconductor device which converts ac

input voltage into a constant dc output voltage In present excitation system three phase

fully controlled thyristor converter is used

Because of the following advantages thyristor converters are used

a) Thyristors are used for high power applications ie up to 10Kv3500A1KHz

b) Having high reliability and low losses

c) Uni-directional device like diode

d) Itrsquos operation as a rectifier which are low resistance in forward conduction

mode and high resistance in reverse conduction mode

PROTECTION OF THYRISTORS

For reliable operation of a thyristor demands that its specified ratings are not

exceeded When Subjected to or over voltages During the turn - on of SCR didt

prohibitively large False triggering of SCR by high value of dvdt andSpurious signals

between gate and cathode may leads to unwanted turn ndash on

DIDT AND PROTECTION

When thyristor starts conducting in forward conduction mode and is turned on by

gate pulse The anode current increases rapidly whole area of the gate to Cathode

junction then hot spots will be formed near the gate connection this locality of heating

destroys the thyristor Thyristor thermal time is constant The causes due to faults and

short circuits or surge currents Electronic crowbar protection is used against the over

voltages The rate rise of anode current must be kept at the time of turn on below the

rated or specified limiting value The didt value maintained below limited value by using

a inductor also called ldquodidt inductorrdquo in series with anode circuit The locality of heating

is avoided by applying gate current but not greater the maximum gate current

DVDT AND OVER VOLTAGE PROTECTION

With forward voltage across the anode and cathode of a thyristor the two outer

junctions are forward biased but the inner junction is reverse biased This reverse biased

junction J2 has the characteristics of a capacitor due to charges existing across the

junction In other words space-charges exist in the depletion region around junction J2

and therefore junction J2 behaves like a capacitance If the entire anode to cathode

forward voltage Va appears across J2 junction and the charge is denoted by Q then a

charging current i given by Eq (46) follows

i = dQdt =d(Cj Va )dt

= Cj (d Va dt) + Va(d Cj dt) helliphelliphelliphellip(46 a)

As Cj the capacitance of junction J2 is almost constant the current is given by

i = Cj (d Va dt) helliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip(46 b)

If the rate of rise of forward voltage dVadt is high the charging current i will be

more This charging current plays the role of gate current and turns on the SCR even

when gate signal is zero Such phenomena of turning-on a thyristor called dvdt turn-on

must be avoided as it leads to false operation of the thyristor circuit

For controllable operation of the thyristor the rate of rise of forward anode to

cathode voltage dVadt must be kept below the specified rated limit Typical values of

dvdt are 20 ndash 500 Vμsec False turn-on of a thyristor by large dvdt can be prevented by

using a snubber circuit in parallel with the device thyristor are very sensitive for over

voltage than the semiconductor devices

Over voltage transients are perhaps the main cause of thyristor failure

In thyristor there are mainly two types

1 Internal over voltages

Due to the commutation of the thyristors large voltages are generated internally

Because of the series inductance of the SCR circuit the large transient voltages L didt

produced This voltage several times the break over voltage of the device then thyristor

destroys permanently

2 External over voltages

External over voltages are caused due to the interruptions of current flow in an

inductive circuit and also due to the lightening strokes on the lines feeding the thyristor

system For the reliable operation of thyristor the over voltages must be suppressed by

adopting suitable techniques

Suppression of over voltages

The RC circuit called snubber circuit is connected across the device to protect In

order to keep the protective components to a minimum the thyristors are chosen with

their peak voltages ratings are 25 to 3 times of the normal peak working voltage

ldquoselenium thyrector diodes metal oxide varistors or avalanche diode suppressers are

commonly employed for protecting the thyristor circuit against the over voltages

Gate protection

Gate circuit should also be protected against the over voltages and surges Over

voltage at gate circuit can cause false triggering of the SCR may rises the junction

temperature behind specified limit leading to its damage Protection against over

voltage can be achieved by connecting a ZD across the gate circuit and a resister is

connected in series with gate circuit to protect against the s A capacitor and resister are

connected across gate to cathode to by pass the noise

41 Final Pulse Stages

The Final Pulse Stages adapt the output pulses from the Intermediate Pulse Stage

(pulses on the pulse bus) to the gate currents needed for the thyristors Each thyristor

bridge is equipped with its own Final Pulse Stage Each Final Pulse Stages is provided

with a power supply module The amplified output pulses from the Final Pulse Stages

start as a short strong steep pulse with an amplitude approx 2frac12 times that of the main

pulse This initial pulse edge assures proper firing of the thyristors being triggered

Subsequently the weaker part of main pulse keeps firing conditions steady As already

mentioned the Final Pulse Stages and their associated thyristor bridges form single units

All six pulse outputs from a Final Pulse Stage can be blocked by an external control

signal so that all thyristors in the associated thyristor bridge will block the current A

blocking of the pulses is initiated whenever there is a malfunction in the associated

thyristor bridge

42 Converter Power Section

The thyristor converter consists of three independent parallel rectifier blocks TY1

to TY3 which are all in service Even if one block fails the remaining blocks take over

automatically the full design current of the excitation circuit During normal operation

(with ideal current share) and all three bridges in operation each of these blocks has to

carry only (n-2)n (ie33)of its design current If 2 thyristor bridges fail the excitation

is limited Only when all three bridges fail the excitation is switched off Each thyristor

bridge arm is equipped with current flow monitoring CTrsquos Failure of conduction in any

arm is identified by a Current flow monitoring module

43 Converter Cooling

A cooling system is needed to dissipate heat losses in the converter blocks and

electronics Each converter block has therefore been equipped with a fan supplied with

power from the converterrsquos primary voltage (via transformer ndashT8 in field flashing

cubicle) The fans are protected with motor protection circuit breakers An air flow

monitoring unit is provided for monitoring the air flow through the thyristor bridge If a

circuit breaker failure is detected or if the air flow monitor drops off at one of the

thyristor bridges the bridge involved is immediately set out of operation by blocking its

firing pulses

44 Thyristor Converter Monitoring

A thyristor bridge in which defects occur that could threaten the safety of

operation or cause secondary damage is switched off automatically ie its firing pulses

are blocked This happens whenever A thyristor fuse is blown The fuses are monitored

individually with micro switches The Final Pulse Stage fails which is detected by

internal monitors (supply voltage sustained pulse short-circuit on the output end) The

power supply to the fan fails fan air flow as monitored by the Air flow monitor fails or

is insufficient Isolator on ACDC side is open

CHAPTER 5

Field Current Circuit Excitation Transformer

51 Field Circuit Breaker

The circuit-breaker in the field circuit is used to isolate the field circuit from the

converter It is capable of switching off the synchronous machine from full load under the

maximum conditions of a 3-phase short-circuit In addition to its main contacts the field

circuit-breaker also has a de-excitation contact with which the field energy stored in the

field can be dissipated across the de-excitation resistor The de-excitation contact closes

shortly before the main contacts open so as to ensure proper commutation of the field

current from the main contacts to the de-excitation contact when the breaker is switched

off The field circuit-breaker is switched on by electromagnetic force and is kept switched

on by a mechanical latch When the latch is released by a trip coil the circuit-breaker

opens The circuit-breaker also has auxiliary contacts that report its status

52 Field Flashing

In shunt supplied excitation circuits (excitation transformer connected to the

generator terminals) the generator does not have enough remnant voltage for a generator

voltage build-up via the converter In this case a field flashing circuit is provided It

consists of the field flashing contactor the diode bridge and a transformer used to adapt

the auxiliary input voltage to the voltage needed for field flashing when power is

supplied from the auxiliaries network

Fig Field Flashing

Because the field flashing contactor is not able to switch off the energy stored in

the field the control ensures that the contactor can only reopen if the field circuit breaker

has already been opened (generating the TRIP order) or in a normal field flashing

sequence when the converter has taken over the field current Field flashing occurs in the

following stages

1048729The excitation is switched on closing the field flashing contactor ( Field

Circuit Breaker is already closed )

1048729The start-up excitation current flows through the rotor driving the generator

voltage up to approx 15 U

1048729After about 10 U the firing pulses to the converter are released and it begins

to excite the generator to its rated voltage

1048729After about 30 U the field flashing contactor opens (with no current since

the converter is now supplying the current)

The diode bridge at the input to the field flashing contactor prevents a feed-back

from the converter to the source of field flashing while the contactor is still closed

53 De-excitation

When malfunctions occur the stored field energy must be dissipated as quickly

and safely as possible to protect the generator This is done by the converter the field

circuit-breaker and the de-excitation (discharge) resistor

De-excitation (with opening of the field circuit-breaker) takes place in the following

stages

1048729The converter drives to its inverter limit position (negative ceiling voltage)

recovers a portion of the field energy into the network A trip command is given to the

field circuit breaker

1048729The de-excitation contact closes diverting the field voltage to the de-excitation

resistor

1048729Then immediately the main contacts open building voltage The field voltage

commutates to the de-excitation resistor

1048729The current diminishes at a given time constant TE

(With linear resistance TE = Lf (Rf + Re))

Due to the reversal of the field voltage by the converter the field current

commutates from the main contacts of the field circuit-breaker to the de-excitation

resistor in a very early phase This reversal of the field voltage prevents burn-off on the

main contacts and provides effective protection for the field circuit-breaker Depending

on the operating policy an operational shut-down of the excitation can also be effected

with the field circuit-breaker closed This method is useful mainly when the excitation is

switched on and off frequently In this case the converter is merely driven into the

inverter limit position so that the field energy is recovered into the network The

converter then blocks since it is supplying positive current only

54 Excitation Transformer

The excitation transformer matches the generator voltage to the field voltage

(required ceiling voltage) It also serves as a commutation reactance for the thyristor

converter and as a potential isolator between the network and the excitation circuit In

addition the transformer functions as a current limiter in that it makes it possible to keep

any short circuits in the excitation circuit under better control The excitation transformer

is equipped with temperature monitoring probes which set off an alarm when the

temperature exceeds a first max limit and then trips the excitation if the temperature

continues rising to a second (higher)limit

CHAPTER 6

Monitoring and Protection

61 Excitation Monitoring

611 General Information

The main goal of Excitation Monitoring is to make optimum use of the

redundancies provided in the excitation system and to give alarm whenever a malfunction

makes these redundancies unavailable The field current is monitored to see that it does

not exceed a maximum level and if necessary a switch-over to the MANUAL channel is

initiated In addition the criterion for switching off the field flashing is generated The

excitation Monitoring consists of an autonomous processor system

612 over current Alarms

In the Excitation Monitoring the limits for are set at higher levels than the

settings on the Field Current Maximum Limiter Whenever the current exceeds 110 of

the nominal field current contact R1 and the binary output associated with it are

activated immediately If field current remains gt 110 then after a preset inverse-time

has lapsed relay R2 and - after a further delay - relay R0 and the binary outputs

associated with them are activated Parameters match the measurements for If1 and If2 to

the nominal value for field current so that the internal values can be processed and read as

pu values It can be used to falsify the actual value of the field current If (to raise it) so

as to cause a response from the alarm limits for purposes of testing The processed If

signal is always taken from on the active channel (CHANNEL I OR CHANNEL II) As

long as the field current If is above the threshold value 11 Ifn its peak value is

measured This is stored (until RESET) and can be read at any time on the Micro-

Terminal Once the value of If exceeds 11 Ifn integration of this value starts Whenever

the integrated time-current value (intisup2dt) exceeds the preselected reference value the

alarm OVER CURRENT INVERSE-TIME is set off and a command is simultaneously

issued to switch over to the stand by AUTO channel Software switch F758 enables the

three over current alarm functions (R0 R1 R2) and selects one of three possible inverse-

time curves T1 T2 or T3 Within the characteristic curve (T1 T2 T3) selected the

desired limit curve for response is set using the factor F216

613 Switch-Off Criterion for Field Flashing

The Excitation Monitoring supplies the criterion for switching off the field

flashing Whether this criterion is activated based on the actual value for generator voltage

Ug or for field current If or both depends on the settings of the two threshold values

F200F202 (0 setting means that the output is always ldquological 1rdquo) The measurements

Ug12 and If12 are switched over depending on the present status of the channels

(Channel 1 or Channel 2 ON) Whenever Flag F750 is not activated the binary output is

fixed at ldquological 1rdquo

614 Storage of Alarm Status

The outputs of the over current alarms (R1 R2 R0) and the messages NO

FAILURE MONITORING PARAMETERS CHANGED are stored messages can be

erased by activating the input ldquoGENERAL RESETrdquo or by using the RESET button on the

front of the module Erasure with the input ldquoGENERAL RESETrdquo is effective only if the

situation causing the alarm or the malfunction is no longer present Whenever the self-

diagnosis equipment in the processor detects a malfunction the output NO FAILURE of

MONITORING is set at ldquological 0rdquo (= alarm) The alarm ldquoPARAMETERS CHANGEDrdquo

is activated whenever parameters or settings of software switches have been changed via

the Micro-Terminal

615 Actual Value Monitoring

The actual values for generator voltage Ug synchronous voltage Usyn and field

current If are monitored for malfunctions This monitoring is active regardless of whether

or not the generator is in operation Essentially when the generator is in operation the

measurements are monitored by comparing the signals (the smaller signal reading is

detected as incorrect) When the generator is not in operation the measured data are

monitored for extreme values The percentage of deviation permissible in the

measurement signals being compared

(Ug1ampUsyn1 Ug1ampUg2 Ug2ampUsyn2 If1ampIf2) is defined by parameters F208 and

F20AIf the excitation transformer is being supplied from an auxiliary power source (no

shunt operation) the values of Ug and Usyn will be different in some operational

conditions

In that case Flag F75A can be used to deactivate comparative measurements Ug1 Usyn1

and Ug2 Usyn2 The ldquoprimary monitoringrdquo compares Ug1 with Ug2 and generates the

messages that Channel 1 or Channel 2 has suffered a malfunction Whenever Ug1 lt Ug2

and the binary message from CH1 reports no malfunction a malfunction on

Measurement Channel 1 is reported (Ug1Usyn1 FAILURE) A similar malfunction is

also present whenever the binary message CH1 DISTURBANCE is reported and a

discrepancy is detected between Ug1ampUg2 The generation of the alarm ldquoUg2 Usyn2

FAILURErdquo is analogous to that for Channel 1 The ldquosecondary monitoringrdquo compares

Ug1 with Usyn1 amp Ug2 with Usyn2 This is enabled whenever the binary message of the

comparison channel reports a malfunction or whenever both binary messages report no

malfunction - but both secondary monitors report a malfunction As long as the secondary

monitoring is blocked the differences Ug1neUsyn1 or Ug2neUsyn2 trigger malfunction

signals for the measurement channel involved (suspicion that there is a corresponding

error in Usyn) The measurement channel malfunctions are enabled operationally

whenever after excitation has been switched on generator voltage Ug exceeds the value

set on F204 The voltages Ug1 and Ug2 are checked 16 seconds after the excitation is

switched off to see that they do not exceed the limit value F210 that applies to both of

them At the same time g1Ug2Usyn1Usyn2 are checked for extreme values (gt or lt

the operating range) Monitoring for extreme values is likewise enabled during normal

operation ( Excitation ON and Ug gt F204 ) Flag F754 is used to enable or block the

malfunction signals to the binary outputs Basically the monitoring of the actual values

for If1If2 functions like that of the Ug1Ug2 monitoring

62 Excitation Protection

621 General Information

The Excitation Protection switches off the excitation (and de-excites the machine

rapidly) whenever a danger arises that threatens the excitation transformer the converter

or the generator Generally limiter or monitoring functions precede the emergency trips

and these normally respond before the Excitation Protection must initiate a trip

Protective trip commands are issued directly to the field circuit-breaker from potential

free contacts of the board via the trip relays They are directed redundantly to the

operative field circuit-breaker ldquoOFFrdquo command

622 Protection against Excitation Transformer Overheating

This equipment monitors the excitation transformer for overheating in the

windings that could result from over current short-circuits or inadequate cooling The

monitoring uses temperature monitoring modules in conjunction with temperature

sensors built into the windings Normally the temperature is monitored in two stages the

first stage sets off an alarm the second causes a trip of the excitation

623 Rotor Over voltage Protection

Malfunctions in the generator circuit (eg terminal short-circuit failed

synchronization asynchronous operation) cause induced negative field currents that

produce high voltages in the field circuit These must be restricted to a level with a

sufficient safety margin below the insulation capacity of the field winding (test voltage)

and also below the peak blocking voltage of the converter thyristors The crow bar

employs spark gap elements to detect over voltages in the field circuit Whenever they

respond the associated thyristors are fired immediately switching the de-excitation

resistor parallel to the field The de-excitation current generated thereby initiates an

excitation trip via a supervision circuit causing an immediate opening of the field circuit-

breaker The malfunction isets off an alarm and an internal malfunction is indicated at the

cubicle

TEST VALUES OBTAINED WHEN EXCITATION IS RAISED

TEST

SNO PARTICULARS ACTUAL VALUE OBTAINED VALUE

1 VREF 996 100

2 VACT 997 1003

3 IFACT 735 765

4 IGACT 703 707

5 ACTIVE

POWER

703 705

6 REACTIVE

POWER

101 142

7 POWER

FACTOR

099 IND 098 IND

8 ACTIVE

CURRENT(IR)

705 703

9 REACTIVE

CURRENT(IX)

102 138

10 POWER

ANGLE

566 546

11 FIRING

ANGLE

640 632

VALUES OBTAINED WHEN EXCITATION IS LOWERED

SNO PARTICULARS ACTUAL

VALUE

OBTAINED VALUE

1 VREF 100 997

2 VACT 100 997

3 IFACT 787 761

4 IGACT 839 837

5 ACTIVE

POWER

839 830

6 REACTIVE

POWER

155 110

7 POWER

FACTOR

098 IND 099 IND

8 ACTIVE

CURRENT(IR)

835 830

9 REACTIVE

CURRENT(IX)

140 94

10 POWER

ANGLE

613 DEG 631 DEG

11 FIRING

ANGLE

601 DEG 599 DEG

CONCLUSION

For generating the EMF in stator winding excitation is required to the rotor of a

generator There are two types of excitation

1 Static excitation system

2 Brushless excitation system

A certain disadvantage in brushless excitation system is the slow response time of

the field in case of fast load changes specified No slip-rings and brushes direct

measurements of the field parameters not possible

To avoid all loses static excitation is used Since it does not have any rotating

parts mechanical loses and windage loses This system has fast response and speed

control While preferring this excitation system there are no limitations for the

redundancy of Thyristor bridge circuits

Static excitation has fast field discharge by resistor and inverter operation direct

measurement of field quantity is possible The meaning of excitation is nothing but

continuous supply of DC current (ie field current) to the rotor to buildup required

output voltage in the stator

Field current is changed with respect to the change of load so the digital

automatic voltage regulator (DAVR) is used to regulate the output voltage according to

the load variations

So we conclude that static excitation system with DAVR is preferred since it is

having excellent dynamic performance and better options for R amp M

that a transient change can be made in the active power PE and with that in the effective

electrical torque ME by varying the field current The principle of operation of the DVR

Power System Stabilizer becomes clear from a consideration of the oscillations in power

output and frequency (ΔPE Δf) and the vector diagram If it is assumed that oscillations

in the network frequency generates load oscillations with the mass inertia of the rotor

then the active load of the generator (eg MW-measured) is influenced with a sinusoidal

value -ΔPE (ME-MA = -ΔPE) By inversion of -ΔPE one obtains the fluctuation in

power provided by the rotor +ΔPE As is known the slip signal Δf follows +ΔPE with a

phase delayed by 90deg The +ME produced by the periodic changes in the load angle δ is

in phase with +ΔPE A good damping is attained if ME is varied in phase with the slip

Δf However this signal must also be advanced somewhat to compensate for the time

constants in the excitation circuit and the generator

As mentioned above the electrical torque ME can be influenced by varying the

field current To accomplish this a suitable control signal referred to as variable

disturbance compensation must be imposed upon the voltage set-point or the converter

control variable Ucontr As can be seen from the vector diagram by applying proper

weighting factors (K1 K2) and then adding together the signals -ΔPE and Δf an overall

stabilization signal can be produced that rotates in advance of the Df signal by any angle

desired between 0deg and 90deg Because the amplitude of -DPE remains proportional to the

amplitude of Δf a constant angle in advance of Δf results for the compensation of the

time constants referred to above The optimum weighting factors K1 and K2 for a

synchronous generator working to a power network depend on its operating point at any

moment and the external reactance of the network Normally the selection of a

compromise setting is good enough to attain stability in all operating points and for all

external reactance For special demands these settings must be parameterized as a

function of the external reactance (which means optional equipment Xe-Identification)

The Power System Stabilizer PSS is a section of the AVR computer program and is

processed once per network cycle The voltage at the generator terminals and the

generator current are measured in order to define the signals ΔPE and Δf The calculated

signals for _P_ (=PE) and Δf are then sent across DC filters ldquoDrdquo (real differentiators) that

transmit only the dynamic portion of the signals The ΔPE and Δf signals obtained in this

way are then weighted (multiplied by) with the factors K1 and K2 and sent to the

summing point of the voltage regulator

The PSS stabilization signal is imposed on the automatic voltage regulator only if

the following prerequisites are met

bull Generator on line

bull Generator power output gt the value F338

bull Generator voltage in a range between F33C and F33A

The stabilization signal is limited at the output from the PSS to the lower and

upper limits Flag defines whether the stabilization signal is introduced before or after the

PID filter (usually before the filter) Because the PID filter as noted above already takes

the ceiling factor Vp1 into account the PSS signal needs to be multiplied by Vp1 if it is

added to the voltage regulator following the PID filter (divider at the input to the

minmax limiter) This precaution prevents the DC filter ldquoDrdquo in the P-channel from

producing an unnecessary ldquostabilizationrdquo effect in the case of rapid changes in turbine

load As an alternative for the AVRrsquos Power System Stabilizer a stabilization signal from

an outside system can be imposed by activating the binary input ldquoPSS-SIGNEXTrdquo Flag

F340 can be used to select between an analog and a 12-bit signal and F33E to select the

polarity desired for that signal

24 The MANUAL Channel

241 Summary

The MANUAL channel (Channel 2) has been built as a simple field current

regulator

without additional limiters Its main function is to maintain the excitation of the generator

even if the AUTOMATIC channel becomes non-operational The MANUAL channel

also performs valuable service for purposes of testing commissioning and preventive

maintenance Its measurements regulator generation of firing pulses and power supply

are physically separate from those on the AUTOMATIC channel

242 Principle of Operation

All the functions of the MANUAL channel including the generation of firing

pulses have been implemented in a single electronic module the Gate Control Unit The

control variable Ucontr of voltage regulator is used as the reference value for generating

firing pulses on the principle known as ldquoramp controlrdquo (Comparison of Ucontr with

Usynsynchronous sawtooth signal) For further processing in the UN 0096 Intermediate

Pulse Stage the Gate Control Unit supplies six firing pulses at its output whose phase

position with respect to the synchronous voltage Usyn is in accordance with control

variable Ucontr An internal linearization ensures that the field voltage produced via the

firing pulses remains proportional to the control variable Ucontr throughout the entire

range As a result the circuit amplification of the control remains constant over the entire

range Whenever excitation is switched ON the set-point for Generator Voltage is set

automatically at the preset - ref Value This provision ensures that the generator voltage

always attains approximately its nominal value after the field flashing The Gate Control

Unit can be refunctioned ( by pre-selection with a switch ) for purposes of testing to act

as a purely firing pulse control In this case the control variable Ucontr is adjusted

directly using the RAISELOWER push buttons on the front of the module In this way

for example the relationship between the phase position of the firing pulses and the

control variable Ucontr can be checked easily

CHAPTER 3

PULSE SECTION

31 Pulse Generation and Amplification

The Gate Control Units of both AUTOMATIC channel and MANUAL channel

each supply six firing pulses for operating the 6-pulse thyristor bridges The low-power

pulse signals from these Gate Control Units are then amplified in the Intermediate Pulse

Stage galvanically isolated and then sent to the common pulse bus On the output end

the Intermediate Pulse Stage of the non-active channel) is always blocked The Gate

Control Units generate the pulses based on microprocessor control The reference voltage

used for the firing pulse phase location is the output voltage from the excitation

transformer (Usyn1 Usyn2) The commutation spikes of the synchronous voltage caused

by the converter are calculated prior to use of the voltage as a reference value and are

deliberately filtered out The lower limit for the firing pulses (double pulses) which are

offset from one another by 60deg is defined by the limit rectifier position (αmin) and the

upper limit by the limit inverter position (αmax) for the firing angle αmin and αmax can

be adjusted on the Gate Control Units using BCD (Binary Coded Decimal) switches

αmin ensures that the firing pulses will not be issued (premature firing) until there is

sufficient positive phase voltage on the thyristor involved αmax prevents a dangerous

ldquotippingrdquo of the thyristor bridge into the rectifier mode if the firing angle α is too large

(ldquolate firingrdquo) The critical factors determining αmax are the overlap time uumlmax (max

commutation time) and the ldquorecovery timerdquo of the thyristors (αmax lt 180deg - uumlmax - γ )

An external control signal can force the firing pulses into their inverter limit position

Other binary inputs can block or direct the firing pulses of the Gate Control Units so as to

produce freewheeling on the thyristor bridge During freewheeling the firing pulses for

the thyristor pair R and S are blocked and the pulse signals T+T- are engaged with

chains of pulses Both Gate Control Units (for the MANUAL amp AUTOMATIC channels)

contain a field current monitor that blocks the firing pulses immediately whenever the

current exceeds a preset threshold level In this case the field circuit-breaker is also

tripped via an output contact The purpose of these provisions is to prevent damage to

thyristors and thyristor fuses in case of a slip-ring short-circuit or to keep any damage

that does occur to a minimum The pulse signals are galvanically separated at the outputs

from the Intermediate Pulse Stage (with pulse transmitters) and are then directed to the

common pulse bus This transmission of the pulse signals to the pulse bus via passive

transmitters ensures a high degree of active channel autonomy Practically no possible

malfunctions on the inactive channel (including for example sustained pulses) affect the

active channel

32 Pulse Monitoring

The ldquoPulse Busrdquo and the pulse signals of the AUTOMATIC channel are

monitored This monitoring device consists of potential isolating stages and the common

monitor If the pulse monitoring of the ldquoPulse Busrdquo responds a switch-over is made to

MANUAL channel The function of the potential isolating stages is to couple the pulse

monitoring device to the pulse circuits without any feedback effect The pulse monitoring

checks the six pulse lines for the following malfunctions continuous or periodic failure

of one or more pulses Periodic occurrence of synchronous or asynchronous false pulses

Continuous pulses the pulse monitoring device can be tested while the machine is in

operation

CHAPTER 4

CONVERTER

Thyristor

The term thyristor usually refers to a family of four layer solid state device having

turn on characteristics that can be externally controlled by either current or voltage They

are also referred to as breakdown device because their working depends on avalanche

breakdown Thyristors have only two stages OFF and ON Thyristors have a similar

function to Uni-junctions they act as switches Thyristors use current flow as a switch

Thyristors have three states

1 Reverse blocking mode mdash Voltage is applied in the direction that would be

blocked by a diode

2 Forward blocking mode mdash Voltage is applied in the direction that would cause

a diode to conduct but the thyristor has not yet been triggered into conduction

3 Forward conducting mode mdash The thyristor has been triggered into conduction

and will remain conducting until the forward current drops below a threshold value

known as the holding current Converter is a semiconductor device which converts ac

input voltage into a constant dc output voltage In present excitation system three phase

fully controlled thyristor converter is used

Because of the following advantages thyristor converters are used

a) Thyristors are used for high power applications ie up to 10Kv3500A1KHz

b) Having high reliability and low losses

c) Uni-directional device like diode

d) Itrsquos operation as a rectifier which are low resistance in forward conduction

mode and high resistance in reverse conduction mode

PROTECTION OF THYRISTORS

For reliable operation of a thyristor demands that its specified ratings are not

exceeded When Subjected to or over voltages During the turn - on of SCR didt

prohibitively large False triggering of SCR by high value of dvdt andSpurious signals

between gate and cathode may leads to unwanted turn ndash on

DIDT AND PROTECTION

When thyristor starts conducting in forward conduction mode and is turned on by

gate pulse The anode current increases rapidly whole area of the gate to Cathode

junction then hot spots will be formed near the gate connection this locality of heating

destroys the thyristor Thyristor thermal time is constant The causes due to faults and

short circuits or surge currents Electronic crowbar protection is used against the over

voltages The rate rise of anode current must be kept at the time of turn on below the

rated or specified limiting value The didt value maintained below limited value by using

a inductor also called ldquodidt inductorrdquo in series with anode circuit The locality of heating

is avoided by applying gate current but not greater the maximum gate current

DVDT AND OVER VOLTAGE PROTECTION

With forward voltage across the anode and cathode of a thyristor the two outer

junctions are forward biased but the inner junction is reverse biased This reverse biased

junction J2 has the characteristics of a capacitor due to charges existing across the

junction In other words space-charges exist in the depletion region around junction J2

and therefore junction J2 behaves like a capacitance If the entire anode to cathode

forward voltage Va appears across J2 junction and the charge is denoted by Q then a

charging current i given by Eq (46) follows

i = dQdt =d(Cj Va )dt

= Cj (d Va dt) + Va(d Cj dt) helliphelliphelliphellip(46 a)

As Cj the capacitance of junction J2 is almost constant the current is given by

i = Cj (d Va dt) helliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip(46 b)

If the rate of rise of forward voltage dVadt is high the charging current i will be

more This charging current plays the role of gate current and turns on the SCR even

when gate signal is zero Such phenomena of turning-on a thyristor called dvdt turn-on

must be avoided as it leads to false operation of the thyristor circuit

For controllable operation of the thyristor the rate of rise of forward anode to

cathode voltage dVadt must be kept below the specified rated limit Typical values of

dvdt are 20 ndash 500 Vμsec False turn-on of a thyristor by large dvdt can be prevented by

using a snubber circuit in parallel with the device thyristor are very sensitive for over

voltage than the semiconductor devices

Over voltage transients are perhaps the main cause of thyristor failure

In thyristor there are mainly two types

1 Internal over voltages

Due to the commutation of the thyristors large voltages are generated internally

Because of the series inductance of the SCR circuit the large transient voltages L didt

produced This voltage several times the break over voltage of the device then thyristor

destroys permanently

2 External over voltages

External over voltages are caused due to the interruptions of current flow in an

inductive circuit and also due to the lightening strokes on the lines feeding the thyristor

system For the reliable operation of thyristor the over voltages must be suppressed by

adopting suitable techniques

Suppression of over voltages

The RC circuit called snubber circuit is connected across the device to protect In

order to keep the protective components to a minimum the thyristors are chosen with

their peak voltages ratings are 25 to 3 times of the normal peak working voltage

ldquoselenium thyrector diodes metal oxide varistors or avalanche diode suppressers are

commonly employed for protecting the thyristor circuit against the over voltages

Gate protection

Gate circuit should also be protected against the over voltages and surges Over

voltage at gate circuit can cause false triggering of the SCR may rises the junction

temperature behind specified limit leading to its damage Protection against over

voltage can be achieved by connecting a ZD across the gate circuit and a resister is

connected in series with gate circuit to protect against the s A capacitor and resister are

connected across gate to cathode to by pass the noise

41 Final Pulse Stages

The Final Pulse Stages adapt the output pulses from the Intermediate Pulse Stage

(pulses on the pulse bus) to the gate currents needed for the thyristors Each thyristor

bridge is equipped with its own Final Pulse Stage Each Final Pulse Stages is provided

with a power supply module The amplified output pulses from the Final Pulse Stages

start as a short strong steep pulse with an amplitude approx 2frac12 times that of the main

pulse This initial pulse edge assures proper firing of the thyristors being triggered

Subsequently the weaker part of main pulse keeps firing conditions steady As already

mentioned the Final Pulse Stages and their associated thyristor bridges form single units

All six pulse outputs from a Final Pulse Stage can be blocked by an external control

signal so that all thyristors in the associated thyristor bridge will block the current A

blocking of the pulses is initiated whenever there is a malfunction in the associated

thyristor bridge

42 Converter Power Section

The thyristor converter consists of three independent parallel rectifier blocks TY1

to TY3 which are all in service Even if one block fails the remaining blocks take over

automatically the full design current of the excitation circuit During normal operation

(with ideal current share) and all three bridges in operation each of these blocks has to

carry only (n-2)n (ie33)of its design current If 2 thyristor bridges fail the excitation

is limited Only when all three bridges fail the excitation is switched off Each thyristor

bridge arm is equipped with current flow monitoring CTrsquos Failure of conduction in any

arm is identified by a Current flow monitoring module

43 Converter Cooling

A cooling system is needed to dissipate heat losses in the converter blocks and

electronics Each converter block has therefore been equipped with a fan supplied with

power from the converterrsquos primary voltage (via transformer ndashT8 in field flashing

cubicle) The fans are protected with motor protection circuit breakers An air flow

monitoring unit is provided for monitoring the air flow through the thyristor bridge If a

circuit breaker failure is detected or if the air flow monitor drops off at one of the

thyristor bridges the bridge involved is immediately set out of operation by blocking its

firing pulses

44 Thyristor Converter Monitoring

A thyristor bridge in which defects occur that could threaten the safety of

operation or cause secondary damage is switched off automatically ie its firing pulses

are blocked This happens whenever A thyristor fuse is blown The fuses are monitored

individually with micro switches The Final Pulse Stage fails which is detected by

internal monitors (supply voltage sustained pulse short-circuit on the output end) The

power supply to the fan fails fan air flow as monitored by the Air flow monitor fails or

is insufficient Isolator on ACDC side is open

CHAPTER 5

Field Current Circuit Excitation Transformer

51 Field Circuit Breaker

The circuit-breaker in the field circuit is used to isolate the field circuit from the

converter It is capable of switching off the synchronous machine from full load under the

maximum conditions of a 3-phase short-circuit In addition to its main contacts the field

circuit-breaker also has a de-excitation contact with which the field energy stored in the

field can be dissipated across the de-excitation resistor The de-excitation contact closes

shortly before the main contacts open so as to ensure proper commutation of the field

current from the main contacts to the de-excitation contact when the breaker is switched

off The field circuit-breaker is switched on by electromagnetic force and is kept switched

on by a mechanical latch When the latch is released by a trip coil the circuit-breaker

opens The circuit-breaker also has auxiliary contacts that report its status

52 Field Flashing

In shunt supplied excitation circuits (excitation transformer connected to the

generator terminals) the generator does not have enough remnant voltage for a generator

voltage build-up via the converter In this case a field flashing circuit is provided It

consists of the field flashing contactor the diode bridge and a transformer used to adapt

the auxiliary input voltage to the voltage needed for field flashing when power is

supplied from the auxiliaries network

Fig Field Flashing

Because the field flashing contactor is not able to switch off the energy stored in

the field the control ensures that the contactor can only reopen if the field circuit breaker

has already been opened (generating the TRIP order) or in a normal field flashing

sequence when the converter has taken over the field current Field flashing occurs in the

following stages

1048729The excitation is switched on closing the field flashing contactor ( Field

Circuit Breaker is already closed )

1048729The start-up excitation current flows through the rotor driving the generator

voltage up to approx 15 U

1048729After about 10 U the firing pulses to the converter are released and it begins

to excite the generator to its rated voltage

1048729After about 30 U the field flashing contactor opens (with no current since

the converter is now supplying the current)

The diode bridge at the input to the field flashing contactor prevents a feed-back

from the converter to the source of field flashing while the contactor is still closed

53 De-excitation

When malfunctions occur the stored field energy must be dissipated as quickly

and safely as possible to protect the generator This is done by the converter the field

circuit-breaker and the de-excitation (discharge) resistor

De-excitation (with opening of the field circuit-breaker) takes place in the following

stages

1048729The converter drives to its inverter limit position (negative ceiling voltage)

recovers a portion of the field energy into the network A trip command is given to the

field circuit breaker

1048729The de-excitation contact closes diverting the field voltage to the de-excitation

resistor

1048729Then immediately the main contacts open building voltage The field voltage

commutates to the de-excitation resistor

1048729The current diminishes at a given time constant TE

(With linear resistance TE = Lf (Rf + Re))

Due to the reversal of the field voltage by the converter the field current

commutates from the main contacts of the field circuit-breaker to the de-excitation

resistor in a very early phase This reversal of the field voltage prevents burn-off on the

main contacts and provides effective protection for the field circuit-breaker Depending

on the operating policy an operational shut-down of the excitation can also be effected

with the field circuit-breaker closed This method is useful mainly when the excitation is

switched on and off frequently In this case the converter is merely driven into the

inverter limit position so that the field energy is recovered into the network The

converter then blocks since it is supplying positive current only

54 Excitation Transformer

The excitation transformer matches the generator voltage to the field voltage

(required ceiling voltage) It also serves as a commutation reactance for the thyristor

converter and as a potential isolator between the network and the excitation circuit In

addition the transformer functions as a current limiter in that it makes it possible to keep

any short circuits in the excitation circuit under better control The excitation transformer

is equipped with temperature monitoring probes which set off an alarm when the

temperature exceeds a first max limit and then trips the excitation if the temperature

continues rising to a second (higher)limit

CHAPTER 6

Monitoring and Protection

61 Excitation Monitoring

611 General Information

The main goal of Excitation Monitoring is to make optimum use of the

redundancies provided in the excitation system and to give alarm whenever a malfunction

makes these redundancies unavailable The field current is monitored to see that it does

not exceed a maximum level and if necessary a switch-over to the MANUAL channel is

initiated In addition the criterion for switching off the field flashing is generated The

excitation Monitoring consists of an autonomous processor system

612 over current Alarms

In the Excitation Monitoring the limits for are set at higher levels than the

settings on the Field Current Maximum Limiter Whenever the current exceeds 110 of

the nominal field current contact R1 and the binary output associated with it are

activated immediately If field current remains gt 110 then after a preset inverse-time

has lapsed relay R2 and - after a further delay - relay R0 and the binary outputs

associated with them are activated Parameters match the measurements for If1 and If2 to

the nominal value for field current so that the internal values can be processed and read as

pu values It can be used to falsify the actual value of the field current If (to raise it) so

as to cause a response from the alarm limits for purposes of testing The processed If

signal is always taken from on the active channel (CHANNEL I OR CHANNEL II) As

long as the field current If is above the threshold value 11 Ifn its peak value is

measured This is stored (until RESET) and can be read at any time on the Micro-

Terminal Once the value of If exceeds 11 Ifn integration of this value starts Whenever

the integrated time-current value (intisup2dt) exceeds the preselected reference value the

alarm OVER CURRENT INVERSE-TIME is set off and a command is simultaneously

issued to switch over to the stand by AUTO channel Software switch F758 enables the

three over current alarm functions (R0 R1 R2) and selects one of three possible inverse-

time curves T1 T2 or T3 Within the characteristic curve (T1 T2 T3) selected the

desired limit curve for response is set using the factor F216

613 Switch-Off Criterion for Field Flashing

The Excitation Monitoring supplies the criterion for switching off the field

flashing Whether this criterion is activated based on the actual value for generator voltage

Ug or for field current If or both depends on the settings of the two threshold values

F200F202 (0 setting means that the output is always ldquological 1rdquo) The measurements

Ug12 and If12 are switched over depending on the present status of the channels

(Channel 1 or Channel 2 ON) Whenever Flag F750 is not activated the binary output is

fixed at ldquological 1rdquo

614 Storage of Alarm Status

The outputs of the over current alarms (R1 R2 R0) and the messages NO

FAILURE MONITORING PARAMETERS CHANGED are stored messages can be

erased by activating the input ldquoGENERAL RESETrdquo or by using the RESET button on the

front of the module Erasure with the input ldquoGENERAL RESETrdquo is effective only if the

situation causing the alarm or the malfunction is no longer present Whenever the self-

diagnosis equipment in the processor detects a malfunction the output NO FAILURE of

MONITORING is set at ldquological 0rdquo (= alarm) The alarm ldquoPARAMETERS CHANGEDrdquo

is activated whenever parameters or settings of software switches have been changed via

the Micro-Terminal

615 Actual Value Monitoring

The actual values for generator voltage Ug synchronous voltage Usyn and field

current If are monitored for malfunctions This monitoring is active regardless of whether

or not the generator is in operation Essentially when the generator is in operation the

measurements are monitored by comparing the signals (the smaller signal reading is

detected as incorrect) When the generator is not in operation the measured data are

monitored for extreme values The percentage of deviation permissible in the

measurement signals being compared

(Ug1ampUsyn1 Ug1ampUg2 Ug2ampUsyn2 If1ampIf2) is defined by parameters F208 and

F20AIf the excitation transformer is being supplied from an auxiliary power source (no

shunt operation) the values of Ug and Usyn will be different in some operational

conditions

In that case Flag F75A can be used to deactivate comparative measurements Ug1 Usyn1

and Ug2 Usyn2 The ldquoprimary monitoringrdquo compares Ug1 with Ug2 and generates the

messages that Channel 1 or Channel 2 has suffered a malfunction Whenever Ug1 lt Ug2

and the binary message from CH1 reports no malfunction a malfunction on

Measurement Channel 1 is reported (Ug1Usyn1 FAILURE) A similar malfunction is

also present whenever the binary message CH1 DISTURBANCE is reported and a

discrepancy is detected between Ug1ampUg2 The generation of the alarm ldquoUg2 Usyn2

FAILURErdquo is analogous to that for Channel 1 The ldquosecondary monitoringrdquo compares

Ug1 with Usyn1 amp Ug2 with Usyn2 This is enabled whenever the binary message of the

comparison channel reports a malfunction or whenever both binary messages report no

malfunction - but both secondary monitors report a malfunction As long as the secondary

monitoring is blocked the differences Ug1neUsyn1 or Ug2neUsyn2 trigger malfunction

signals for the measurement channel involved (suspicion that there is a corresponding

error in Usyn) The measurement channel malfunctions are enabled operationally

whenever after excitation has been switched on generator voltage Ug exceeds the value

set on F204 The voltages Ug1 and Ug2 are checked 16 seconds after the excitation is

switched off to see that they do not exceed the limit value F210 that applies to both of

them At the same time g1Ug2Usyn1Usyn2 are checked for extreme values (gt or lt

the operating range) Monitoring for extreme values is likewise enabled during normal

operation ( Excitation ON and Ug gt F204 ) Flag F754 is used to enable or block the

malfunction signals to the binary outputs Basically the monitoring of the actual values

for If1If2 functions like that of the Ug1Ug2 monitoring

62 Excitation Protection

621 General Information

The Excitation Protection switches off the excitation (and de-excites the machine

rapidly) whenever a danger arises that threatens the excitation transformer the converter

or the generator Generally limiter or monitoring functions precede the emergency trips

and these normally respond before the Excitation Protection must initiate a trip

Protective trip commands are issued directly to the field circuit-breaker from potential

free contacts of the board via the trip relays They are directed redundantly to the

operative field circuit-breaker ldquoOFFrdquo command

622 Protection against Excitation Transformer Overheating

This equipment monitors the excitation transformer for overheating in the

windings that could result from over current short-circuits or inadequate cooling The

monitoring uses temperature monitoring modules in conjunction with temperature

sensors built into the windings Normally the temperature is monitored in two stages the

first stage sets off an alarm the second causes a trip of the excitation

623 Rotor Over voltage Protection

Malfunctions in the generator circuit (eg terminal short-circuit failed

synchronization asynchronous operation) cause induced negative field currents that

produce high voltages in the field circuit These must be restricted to a level with a

sufficient safety margin below the insulation capacity of the field winding (test voltage)

and also below the peak blocking voltage of the converter thyristors The crow bar

employs spark gap elements to detect over voltages in the field circuit Whenever they

respond the associated thyristors are fired immediately switching the de-excitation

resistor parallel to the field The de-excitation current generated thereby initiates an

excitation trip via a supervision circuit causing an immediate opening of the field circuit-

breaker The malfunction isets off an alarm and an internal malfunction is indicated at the

cubicle

TEST VALUES OBTAINED WHEN EXCITATION IS RAISED

TEST

SNO PARTICULARS ACTUAL VALUE OBTAINED VALUE

1 VREF 996 100

2 VACT 997 1003

3 IFACT 735 765

4 IGACT 703 707

5 ACTIVE

POWER

703 705

6 REACTIVE

POWER

101 142

7 POWER

FACTOR

099 IND 098 IND

8 ACTIVE

CURRENT(IR)

705 703

9 REACTIVE

CURRENT(IX)

102 138

10 POWER

ANGLE

566 546

11 FIRING

ANGLE

640 632

VALUES OBTAINED WHEN EXCITATION IS LOWERED

SNO PARTICULARS ACTUAL

VALUE

OBTAINED VALUE

1 VREF 100 997

2 VACT 100 997

3 IFACT 787 761

4 IGACT 839 837

5 ACTIVE

POWER

839 830

6 REACTIVE

POWER

155 110

7 POWER

FACTOR

098 IND 099 IND

8 ACTIVE

CURRENT(IR)

835 830

9 REACTIVE

CURRENT(IX)

140 94

10 POWER

ANGLE

613 DEG 631 DEG

11 FIRING

ANGLE

601 DEG 599 DEG

CONCLUSION

For generating the EMF in stator winding excitation is required to the rotor of a

generator There are two types of excitation

1 Static excitation system

2 Brushless excitation system

A certain disadvantage in brushless excitation system is the slow response time of

the field in case of fast load changes specified No slip-rings and brushes direct

measurements of the field parameters not possible

To avoid all loses static excitation is used Since it does not have any rotating

parts mechanical loses and windage loses This system has fast response and speed

control While preferring this excitation system there are no limitations for the

redundancy of Thyristor bridge circuits

Static excitation has fast field discharge by resistor and inverter operation direct

measurement of field quantity is possible The meaning of excitation is nothing but

continuous supply of DC current (ie field current) to the rotor to buildup required

output voltage in the stator

Field current is changed with respect to the change of load so the digital

automatic voltage regulator (DAVR) is used to regulate the output voltage according to

the load variations

So we conclude that static excitation system with DAVR is preferred since it is

having excellent dynamic performance and better options for R amp M

way are then weighted (multiplied by) with the factors K1 and K2 and sent to the

summing point of the voltage regulator

The PSS stabilization signal is imposed on the automatic voltage regulator only if

the following prerequisites are met

bull Generator on line

bull Generator power output gt the value F338

bull Generator voltage in a range between F33C and F33A

The stabilization signal is limited at the output from the PSS to the lower and

upper limits Flag defines whether the stabilization signal is introduced before or after the

PID filter (usually before the filter) Because the PID filter as noted above already takes

the ceiling factor Vp1 into account the PSS signal needs to be multiplied by Vp1 if it is

added to the voltage regulator following the PID filter (divider at the input to the

minmax limiter) This precaution prevents the DC filter ldquoDrdquo in the P-channel from

producing an unnecessary ldquostabilizationrdquo effect in the case of rapid changes in turbine

load As an alternative for the AVRrsquos Power System Stabilizer a stabilization signal from

an outside system can be imposed by activating the binary input ldquoPSS-SIGNEXTrdquo Flag

F340 can be used to select between an analog and a 12-bit signal and F33E to select the

polarity desired for that signal

24 The MANUAL Channel

241 Summary

The MANUAL channel (Channel 2) has been built as a simple field current

regulator

without additional limiters Its main function is to maintain the excitation of the generator

even if the AUTOMATIC channel becomes non-operational The MANUAL channel

also performs valuable service for purposes of testing commissioning and preventive

maintenance Its measurements regulator generation of firing pulses and power supply

are physically separate from those on the AUTOMATIC channel

242 Principle of Operation

All the functions of the MANUAL channel including the generation of firing

pulses have been implemented in a single electronic module the Gate Control Unit The

control variable Ucontr of voltage regulator is used as the reference value for generating

firing pulses on the principle known as ldquoramp controlrdquo (Comparison of Ucontr with

Usynsynchronous sawtooth signal) For further processing in the UN 0096 Intermediate

Pulse Stage the Gate Control Unit supplies six firing pulses at its output whose phase

position with respect to the synchronous voltage Usyn is in accordance with control

variable Ucontr An internal linearization ensures that the field voltage produced via the

firing pulses remains proportional to the control variable Ucontr throughout the entire

range As a result the circuit amplification of the control remains constant over the entire

range Whenever excitation is switched ON the set-point for Generator Voltage is set

automatically at the preset - ref Value This provision ensures that the generator voltage

always attains approximately its nominal value after the field flashing The Gate Control

Unit can be refunctioned ( by pre-selection with a switch ) for purposes of testing to act

as a purely firing pulse control In this case the control variable Ucontr is adjusted

directly using the RAISELOWER push buttons on the front of the module In this way

for example the relationship between the phase position of the firing pulses and the

control variable Ucontr can be checked easily

CHAPTER 3

PULSE SECTION

31 Pulse Generation and Amplification

The Gate Control Units of both AUTOMATIC channel and MANUAL channel

each supply six firing pulses for operating the 6-pulse thyristor bridges The low-power

pulse signals from these Gate Control Units are then amplified in the Intermediate Pulse

Stage galvanically isolated and then sent to the common pulse bus On the output end

the Intermediate Pulse Stage of the non-active channel) is always blocked The Gate

Control Units generate the pulses based on microprocessor control The reference voltage

used for the firing pulse phase location is the output voltage from the excitation

transformer (Usyn1 Usyn2) The commutation spikes of the synchronous voltage caused

by the converter are calculated prior to use of the voltage as a reference value and are

deliberately filtered out The lower limit for the firing pulses (double pulses) which are

offset from one another by 60deg is defined by the limit rectifier position (αmin) and the

upper limit by the limit inverter position (αmax) for the firing angle αmin and αmax can

be adjusted on the Gate Control Units using BCD (Binary Coded Decimal) switches

αmin ensures that the firing pulses will not be issued (premature firing) until there is

sufficient positive phase voltage on the thyristor involved αmax prevents a dangerous

ldquotippingrdquo of the thyristor bridge into the rectifier mode if the firing angle α is too large

(ldquolate firingrdquo) The critical factors determining αmax are the overlap time uumlmax (max

commutation time) and the ldquorecovery timerdquo of the thyristors (αmax lt 180deg - uumlmax - γ )

An external control signal can force the firing pulses into their inverter limit position

Other binary inputs can block or direct the firing pulses of the Gate Control Units so as to

produce freewheeling on the thyristor bridge During freewheeling the firing pulses for

the thyristor pair R and S are blocked and the pulse signals T+T- are engaged with

chains of pulses Both Gate Control Units (for the MANUAL amp AUTOMATIC channels)

contain a field current monitor that blocks the firing pulses immediately whenever the

current exceeds a preset threshold level In this case the field circuit-breaker is also

tripped via an output contact The purpose of these provisions is to prevent damage to

thyristors and thyristor fuses in case of a slip-ring short-circuit or to keep any damage

that does occur to a minimum The pulse signals are galvanically separated at the outputs

from the Intermediate Pulse Stage (with pulse transmitters) and are then directed to the

common pulse bus This transmission of the pulse signals to the pulse bus via passive

transmitters ensures a high degree of active channel autonomy Practically no possible

malfunctions on the inactive channel (including for example sustained pulses) affect the

active channel

32 Pulse Monitoring

The ldquoPulse Busrdquo and the pulse signals of the AUTOMATIC channel are

monitored This monitoring device consists of potential isolating stages and the common

monitor If the pulse monitoring of the ldquoPulse Busrdquo responds a switch-over is made to

MANUAL channel The function of the potential isolating stages is to couple the pulse

monitoring device to the pulse circuits without any feedback effect The pulse monitoring

checks the six pulse lines for the following malfunctions continuous or periodic failure

of one or more pulses Periodic occurrence of synchronous or asynchronous false pulses

Continuous pulses the pulse monitoring device can be tested while the machine is in

operation

CHAPTER 4

CONVERTER

Thyristor

The term thyristor usually refers to a family of four layer solid state device having

turn on characteristics that can be externally controlled by either current or voltage They

are also referred to as breakdown device because their working depends on avalanche

breakdown Thyristors have only two stages OFF and ON Thyristors have a similar

function to Uni-junctions they act as switches Thyristors use current flow as a switch

Thyristors have three states

1 Reverse blocking mode mdash Voltage is applied in the direction that would be

blocked by a diode

2 Forward blocking mode mdash Voltage is applied in the direction that would cause

a diode to conduct but the thyristor has not yet been triggered into conduction

3 Forward conducting mode mdash The thyristor has been triggered into conduction

and will remain conducting until the forward current drops below a threshold value

known as the holding current Converter is a semiconductor device which converts ac

input voltage into a constant dc output voltage In present excitation system three phase

fully controlled thyristor converter is used

Because of the following advantages thyristor converters are used

a) Thyristors are used for high power applications ie up to 10Kv3500A1KHz

b) Having high reliability and low losses

c) Uni-directional device like diode

d) Itrsquos operation as a rectifier which are low resistance in forward conduction

mode and high resistance in reverse conduction mode

PROTECTION OF THYRISTORS

For reliable operation of a thyristor demands that its specified ratings are not

exceeded When Subjected to or over voltages During the turn - on of SCR didt

prohibitively large False triggering of SCR by high value of dvdt andSpurious signals

between gate and cathode may leads to unwanted turn ndash on

DIDT AND PROTECTION

When thyristor starts conducting in forward conduction mode and is turned on by

gate pulse The anode current increases rapidly whole area of the gate to Cathode

junction then hot spots will be formed near the gate connection this locality of heating

destroys the thyristor Thyristor thermal time is constant The causes due to faults and

short circuits or surge currents Electronic crowbar protection is used against the over

voltages The rate rise of anode current must be kept at the time of turn on below the

rated or specified limiting value The didt value maintained below limited value by using

a inductor also called ldquodidt inductorrdquo in series with anode circuit The locality of heating

is avoided by applying gate current but not greater the maximum gate current

DVDT AND OVER VOLTAGE PROTECTION

With forward voltage across the anode and cathode of a thyristor the two outer

junctions are forward biased but the inner junction is reverse biased This reverse biased

junction J2 has the characteristics of a capacitor due to charges existing across the

junction In other words space-charges exist in the depletion region around junction J2

and therefore junction J2 behaves like a capacitance If the entire anode to cathode

forward voltage Va appears across J2 junction and the charge is denoted by Q then a

charging current i given by Eq (46) follows

i = dQdt =d(Cj Va )dt

= Cj (d Va dt) + Va(d Cj dt) helliphelliphelliphellip(46 a)

As Cj the capacitance of junction J2 is almost constant the current is given by

i = Cj (d Va dt) helliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip(46 b)

If the rate of rise of forward voltage dVadt is high the charging current i will be

more This charging current plays the role of gate current and turns on the SCR even

when gate signal is zero Such phenomena of turning-on a thyristor called dvdt turn-on

must be avoided as it leads to false operation of the thyristor circuit

For controllable operation of the thyristor the rate of rise of forward anode to

cathode voltage dVadt must be kept below the specified rated limit Typical values of

dvdt are 20 ndash 500 Vμsec False turn-on of a thyristor by large dvdt can be prevented by

using a snubber circuit in parallel with the device thyristor are very sensitive for over

voltage than the semiconductor devices

Over voltage transients are perhaps the main cause of thyristor failure

In thyristor there are mainly two types

1 Internal over voltages

Due to the commutation of the thyristors large voltages are generated internally

Because of the series inductance of the SCR circuit the large transient voltages L didt

produced This voltage several times the break over voltage of the device then thyristor

destroys permanently

2 External over voltages

External over voltages are caused due to the interruptions of current flow in an

inductive circuit and also due to the lightening strokes on the lines feeding the thyristor

system For the reliable operation of thyristor the over voltages must be suppressed by

adopting suitable techniques

Suppression of over voltages

The RC circuit called snubber circuit is connected across the device to protect In

order to keep the protective components to a minimum the thyristors are chosen with

their peak voltages ratings are 25 to 3 times of the normal peak working voltage

ldquoselenium thyrector diodes metal oxide varistors or avalanche diode suppressers are

commonly employed for protecting the thyristor circuit against the over voltages

Gate protection

Gate circuit should also be protected against the over voltages and surges Over

voltage at gate circuit can cause false triggering of the SCR may rises the junction

temperature behind specified limit leading to its damage Protection against over

voltage can be achieved by connecting a ZD across the gate circuit and a resister is

connected in series with gate circuit to protect against the s A capacitor and resister are

connected across gate to cathode to by pass the noise

41 Final Pulse Stages

The Final Pulse Stages adapt the output pulses from the Intermediate Pulse Stage

(pulses on the pulse bus) to the gate currents needed for the thyristors Each thyristor

bridge is equipped with its own Final Pulse Stage Each Final Pulse Stages is provided

with a power supply module The amplified output pulses from the Final Pulse Stages

start as a short strong steep pulse with an amplitude approx 2frac12 times that of the main

pulse This initial pulse edge assures proper firing of the thyristors being triggered

Subsequently the weaker part of main pulse keeps firing conditions steady As already

mentioned the Final Pulse Stages and their associated thyristor bridges form single units

All six pulse outputs from a Final Pulse Stage can be blocked by an external control

signal so that all thyristors in the associated thyristor bridge will block the current A

blocking of the pulses is initiated whenever there is a malfunction in the associated

thyristor bridge

42 Converter Power Section

The thyristor converter consists of three independent parallel rectifier blocks TY1

to TY3 which are all in service Even if one block fails the remaining blocks take over

automatically the full design current of the excitation circuit During normal operation

(with ideal current share) and all three bridges in operation each of these blocks has to

carry only (n-2)n (ie33)of its design current If 2 thyristor bridges fail the excitation

is limited Only when all three bridges fail the excitation is switched off Each thyristor

bridge arm is equipped with current flow monitoring CTrsquos Failure of conduction in any

arm is identified by a Current flow monitoring module

43 Converter Cooling

A cooling system is needed to dissipate heat losses in the converter blocks and

electronics Each converter block has therefore been equipped with a fan supplied with

power from the converterrsquos primary voltage (via transformer ndashT8 in field flashing

cubicle) The fans are protected with motor protection circuit breakers An air flow

monitoring unit is provided for monitoring the air flow through the thyristor bridge If a

circuit breaker failure is detected or if the air flow monitor drops off at one of the

thyristor bridges the bridge involved is immediately set out of operation by blocking its

firing pulses

44 Thyristor Converter Monitoring

A thyristor bridge in which defects occur that could threaten the safety of

operation or cause secondary damage is switched off automatically ie its firing pulses

are blocked This happens whenever A thyristor fuse is blown The fuses are monitored

individually with micro switches The Final Pulse Stage fails which is detected by

internal monitors (supply voltage sustained pulse short-circuit on the output end) The

power supply to the fan fails fan air flow as monitored by the Air flow monitor fails or

is insufficient Isolator on ACDC side is open

CHAPTER 5

Field Current Circuit Excitation Transformer

51 Field Circuit Breaker

The circuit-breaker in the field circuit is used to isolate the field circuit from the

converter It is capable of switching off the synchronous machine from full load under the

maximum conditions of a 3-phase short-circuit In addition to its main contacts the field

circuit-breaker also has a de-excitation contact with which the field energy stored in the

field can be dissipated across the de-excitation resistor The de-excitation contact closes

shortly before the main contacts open so as to ensure proper commutation of the field

current from the main contacts to the de-excitation contact when the breaker is switched

off The field circuit-breaker is switched on by electromagnetic force and is kept switched

on by a mechanical latch When the latch is released by a trip coil the circuit-breaker

opens The circuit-breaker also has auxiliary contacts that report its status

52 Field Flashing

In shunt supplied excitation circuits (excitation transformer connected to the

generator terminals) the generator does not have enough remnant voltage for a generator

voltage build-up via the converter In this case a field flashing circuit is provided It

consists of the field flashing contactor the diode bridge and a transformer used to adapt

the auxiliary input voltage to the voltage needed for field flashing when power is

supplied from the auxiliaries network

Fig Field Flashing

Because the field flashing contactor is not able to switch off the energy stored in

the field the control ensures that the contactor can only reopen if the field circuit breaker

has already been opened (generating the TRIP order) or in a normal field flashing

sequence when the converter has taken over the field current Field flashing occurs in the

following stages

1048729The excitation is switched on closing the field flashing contactor ( Field

Circuit Breaker is already closed )

1048729The start-up excitation current flows through the rotor driving the generator

voltage up to approx 15 U

1048729After about 10 U the firing pulses to the converter are released and it begins

to excite the generator to its rated voltage

1048729After about 30 U the field flashing contactor opens (with no current since

the converter is now supplying the current)

The diode bridge at the input to the field flashing contactor prevents a feed-back

from the converter to the source of field flashing while the contactor is still closed

53 De-excitation

When malfunctions occur the stored field energy must be dissipated as quickly

and safely as possible to protect the generator This is done by the converter the field

circuit-breaker and the de-excitation (discharge) resistor

De-excitation (with opening of the field circuit-breaker) takes place in the following

stages

1048729The converter drives to its inverter limit position (negative ceiling voltage)

recovers a portion of the field energy into the network A trip command is given to the

field circuit breaker

1048729The de-excitation contact closes diverting the field voltage to the de-excitation

resistor

1048729Then immediately the main contacts open building voltage The field voltage

commutates to the de-excitation resistor

1048729The current diminishes at a given time constant TE

(With linear resistance TE = Lf (Rf + Re))

Due to the reversal of the field voltage by the converter the field current

commutates from the main contacts of the field circuit-breaker to the de-excitation

resistor in a very early phase This reversal of the field voltage prevents burn-off on the

main contacts and provides effective protection for the field circuit-breaker Depending

on the operating policy an operational shut-down of the excitation can also be effected

with the field circuit-breaker closed This method is useful mainly when the excitation is

switched on and off frequently In this case the converter is merely driven into the

inverter limit position so that the field energy is recovered into the network The

converter then blocks since it is supplying positive current only

54 Excitation Transformer

The excitation transformer matches the generator voltage to the field voltage

(required ceiling voltage) It also serves as a commutation reactance for the thyristor

converter and as a potential isolator between the network and the excitation circuit In

addition the transformer functions as a current limiter in that it makes it possible to keep

any short circuits in the excitation circuit under better control The excitation transformer

is equipped with temperature monitoring probes which set off an alarm when the

temperature exceeds a first max limit and then trips the excitation if the temperature

continues rising to a second (higher)limit

CHAPTER 6

Monitoring and Protection

61 Excitation Monitoring

611 General Information

The main goal of Excitation Monitoring is to make optimum use of the

redundancies provided in the excitation system and to give alarm whenever a malfunction

makes these redundancies unavailable The field current is monitored to see that it does

not exceed a maximum level and if necessary a switch-over to the MANUAL channel is

initiated In addition the criterion for switching off the field flashing is generated The

excitation Monitoring consists of an autonomous processor system

612 over current Alarms

In the Excitation Monitoring the limits for are set at higher levels than the

settings on the Field Current Maximum Limiter Whenever the current exceeds 110 of

the nominal field current contact R1 and the binary output associated with it are

activated immediately If field current remains gt 110 then after a preset inverse-time

has lapsed relay R2 and - after a further delay - relay R0 and the binary outputs

associated with them are activated Parameters match the measurements for If1 and If2 to

the nominal value for field current so that the internal values can be processed and read as

pu values It can be used to falsify the actual value of the field current If (to raise it) so

as to cause a response from the alarm limits for purposes of testing The processed If

signal is always taken from on the active channel (CHANNEL I OR CHANNEL II) As

long as the field current If is above the threshold value 11 Ifn its peak value is

measured This is stored (until RESET) and can be read at any time on the Micro-

Terminal Once the value of If exceeds 11 Ifn integration of this value starts Whenever

the integrated time-current value (intisup2dt) exceeds the preselected reference value the

alarm OVER CURRENT INVERSE-TIME is set off and a command is simultaneously

issued to switch over to the stand by AUTO channel Software switch F758 enables the

three over current alarm functions (R0 R1 R2) and selects one of three possible inverse-

time curves T1 T2 or T3 Within the characteristic curve (T1 T2 T3) selected the

desired limit curve for response is set using the factor F216

613 Switch-Off Criterion for Field Flashing

The Excitation Monitoring supplies the criterion for switching off the field

flashing Whether this criterion is activated based on the actual value for generator voltage

Ug or for field current If or both depends on the settings of the two threshold values

F200F202 (0 setting means that the output is always ldquological 1rdquo) The measurements

Ug12 and If12 are switched over depending on the present status of the channels

(Channel 1 or Channel 2 ON) Whenever Flag F750 is not activated the binary output is

fixed at ldquological 1rdquo

614 Storage of Alarm Status

The outputs of the over current alarms (R1 R2 R0) and the messages NO

FAILURE MONITORING PARAMETERS CHANGED are stored messages can be

erased by activating the input ldquoGENERAL RESETrdquo or by using the RESET button on the

front of the module Erasure with the input ldquoGENERAL RESETrdquo is effective only if the

situation causing the alarm or the malfunction is no longer present Whenever the self-

diagnosis equipment in the processor detects a malfunction the output NO FAILURE of

MONITORING is set at ldquological 0rdquo (= alarm) The alarm ldquoPARAMETERS CHANGEDrdquo

is activated whenever parameters or settings of software switches have been changed via

the Micro-Terminal

615 Actual Value Monitoring

The actual values for generator voltage Ug synchronous voltage Usyn and field

current If are monitored for malfunctions This monitoring is active regardless of whether

or not the generator is in operation Essentially when the generator is in operation the

measurements are monitored by comparing the signals (the smaller signal reading is

detected as incorrect) When the generator is not in operation the measured data are

monitored for extreme values The percentage of deviation permissible in the

measurement signals being compared

(Ug1ampUsyn1 Ug1ampUg2 Ug2ampUsyn2 If1ampIf2) is defined by parameters F208 and

F20AIf the excitation transformer is being supplied from an auxiliary power source (no

shunt operation) the values of Ug and Usyn will be different in some operational

conditions

In that case Flag F75A can be used to deactivate comparative measurements Ug1 Usyn1

and Ug2 Usyn2 The ldquoprimary monitoringrdquo compares Ug1 with Ug2 and generates the

messages that Channel 1 or Channel 2 has suffered a malfunction Whenever Ug1 lt Ug2

and the binary message from CH1 reports no malfunction a malfunction on

Measurement Channel 1 is reported (Ug1Usyn1 FAILURE) A similar malfunction is

also present whenever the binary message CH1 DISTURBANCE is reported and a

discrepancy is detected between Ug1ampUg2 The generation of the alarm ldquoUg2 Usyn2

FAILURErdquo is analogous to that for Channel 1 The ldquosecondary monitoringrdquo compares

Ug1 with Usyn1 amp Ug2 with Usyn2 This is enabled whenever the binary message of the

comparison channel reports a malfunction or whenever both binary messages report no

malfunction - but both secondary monitors report a malfunction As long as the secondary

monitoring is blocked the differences Ug1neUsyn1 or Ug2neUsyn2 trigger malfunction

signals for the measurement channel involved (suspicion that there is a corresponding

error in Usyn) The measurement channel malfunctions are enabled operationally

whenever after excitation has been switched on generator voltage Ug exceeds the value

set on F204 The voltages Ug1 and Ug2 are checked 16 seconds after the excitation is

switched off to see that they do not exceed the limit value F210 that applies to both of

them At the same time g1Ug2Usyn1Usyn2 are checked for extreme values (gt or lt

the operating range) Monitoring for extreme values is likewise enabled during normal

operation ( Excitation ON and Ug gt F204 ) Flag F754 is used to enable or block the

malfunction signals to the binary outputs Basically the monitoring of the actual values

for If1If2 functions like that of the Ug1Ug2 monitoring

62 Excitation Protection

621 General Information

The Excitation Protection switches off the excitation (and de-excites the machine

rapidly) whenever a danger arises that threatens the excitation transformer the converter

or the generator Generally limiter or monitoring functions precede the emergency trips

and these normally respond before the Excitation Protection must initiate a trip

Protective trip commands are issued directly to the field circuit-breaker from potential

free contacts of the board via the trip relays They are directed redundantly to the

operative field circuit-breaker ldquoOFFrdquo command

622 Protection against Excitation Transformer Overheating

This equipment monitors the excitation transformer for overheating in the

windings that could result from over current short-circuits or inadequate cooling The

monitoring uses temperature monitoring modules in conjunction with temperature

sensors built into the windings Normally the temperature is monitored in two stages the

first stage sets off an alarm the second causes a trip of the excitation

623 Rotor Over voltage Protection

Malfunctions in the generator circuit (eg terminal short-circuit failed

synchronization asynchronous operation) cause induced negative field currents that

produce high voltages in the field circuit These must be restricted to a level with a

sufficient safety margin below the insulation capacity of the field winding (test voltage)

and also below the peak blocking voltage of the converter thyristors The crow bar

employs spark gap elements to detect over voltages in the field circuit Whenever they

respond the associated thyristors are fired immediately switching the de-excitation

resistor parallel to the field The de-excitation current generated thereby initiates an

excitation trip via a supervision circuit causing an immediate opening of the field circuit-

breaker The malfunction isets off an alarm and an internal malfunction is indicated at the

cubicle

TEST VALUES OBTAINED WHEN EXCITATION IS RAISED

TEST

SNO PARTICULARS ACTUAL VALUE OBTAINED VALUE

1 VREF 996 100

2 VACT 997 1003

3 IFACT 735 765

4 IGACT 703 707

5 ACTIVE

POWER

703 705

6 REACTIVE

POWER

101 142

7 POWER

FACTOR

099 IND 098 IND

8 ACTIVE

CURRENT(IR)

705 703

9 REACTIVE

CURRENT(IX)

102 138

10 POWER

ANGLE

566 546

11 FIRING

ANGLE

640 632

VALUES OBTAINED WHEN EXCITATION IS LOWERED

SNO PARTICULARS ACTUAL

VALUE

OBTAINED VALUE

1 VREF 100 997

2 VACT 100 997

3 IFACT 787 761

4 IGACT 839 837

5 ACTIVE

POWER

839 830

6 REACTIVE

POWER

155 110

7 POWER

FACTOR

098 IND 099 IND

8 ACTIVE

CURRENT(IR)

835 830

9 REACTIVE

CURRENT(IX)

140 94

10 POWER

ANGLE

613 DEG 631 DEG

11 FIRING

ANGLE

601 DEG 599 DEG

CONCLUSION

For generating the EMF in stator winding excitation is required to the rotor of a

generator There are two types of excitation

1 Static excitation system

2 Brushless excitation system

A certain disadvantage in brushless excitation system is the slow response time of

the field in case of fast load changes specified No slip-rings and brushes direct

measurements of the field parameters not possible

To avoid all loses static excitation is used Since it does not have any rotating

parts mechanical loses and windage loses This system has fast response and speed

control While preferring this excitation system there are no limitations for the

redundancy of Thyristor bridge circuits

Static excitation has fast field discharge by resistor and inverter operation direct

measurement of field quantity is possible The meaning of excitation is nothing but

continuous supply of DC current (ie field current) to the rotor to buildup required

output voltage in the stator

Field current is changed with respect to the change of load so the digital

automatic voltage regulator (DAVR) is used to regulate the output voltage according to

the load variations

So we conclude that static excitation system with DAVR is preferred since it is

having excellent dynamic performance and better options for R amp M

without additional limiters Its main function is to maintain the excitation of the generator

even if the AUTOMATIC channel becomes non-operational The MANUAL channel

also performs valuable service for purposes of testing commissioning and preventive

maintenance Its measurements regulator generation of firing pulses and power supply

are physically separate from those on the AUTOMATIC channel

242 Principle of Operation

All the functions of the MANUAL channel including the generation of firing

pulses have been implemented in a single electronic module the Gate Control Unit The

control variable Ucontr of voltage regulator is used as the reference value for generating

firing pulses on the principle known as ldquoramp controlrdquo (Comparison of Ucontr with

Usynsynchronous sawtooth signal) For further processing in the UN 0096 Intermediate

Pulse Stage the Gate Control Unit supplies six firing pulses at its output whose phase

position with respect to the synchronous voltage Usyn is in accordance with control

variable Ucontr An internal linearization ensures that the field voltage produced via the

firing pulses remains proportional to the control variable Ucontr throughout the entire

range As a result the circuit amplification of the control remains constant over the entire

range Whenever excitation is switched ON the set-point for Generator Voltage is set

automatically at the preset - ref Value This provision ensures that the generator voltage

always attains approximately its nominal value after the field flashing The Gate Control

Unit can be refunctioned ( by pre-selection with a switch ) for purposes of testing to act

as a purely firing pulse control In this case the control variable Ucontr is adjusted

directly using the RAISELOWER push buttons on the front of the module In this way

for example the relationship between the phase position of the firing pulses and the

control variable Ucontr can be checked easily

CHAPTER 3

PULSE SECTION

31 Pulse Generation and Amplification

The Gate Control Units of both AUTOMATIC channel and MANUAL channel

each supply six firing pulses for operating the 6-pulse thyristor bridges The low-power

pulse signals from these Gate Control Units are then amplified in the Intermediate Pulse

Stage galvanically isolated and then sent to the common pulse bus On the output end

the Intermediate Pulse Stage of the non-active channel) is always blocked The Gate

Control Units generate the pulses based on microprocessor control The reference voltage

used for the firing pulse phase location is the output voltage from the excitation

transformer (Usyn1 Usyn2) The commutation spikes of the synchronous voltage caused

by the converter are calculated prior to use of the voltage as a reference value and are

deliberately filtered out The lower limit for the firing pulses (double pulses) which are

offset from one another by 60deg is defined by the limit rectifier position (αmin) and the

upper limit by the limit inverter position (αmax) for the firing angle αmin and αmax can

be adjusted on the Gate Control Units using BCD (Binary Coded Decimal) switches

αmin ensures that the firing pulses will not be issued (premature firing) until there is

sufficient positive phase voltage on the thyristor involved αmax prevents a dangerous

ldquotippingrdquo of the thyristor bridge into the rectifier mode if the firing angle α is too large

(ldquolate firingrdquo) The critical factors determining αmax are the overlap time uumlmax (max

commutation time) and the ldquorecovery timerdquo of the thyristors (αmax lt 180deg - uumlmax - γ )

An external control signal can force the firing pulses into their inverter limit position

Other binary inputs can block or direct the firing pulses of the Gate Control Units so as to

produce freewheeling on the thyristor bridge During freewheeling the firing pulses for

the thyristor pair R and S are blocked and the pulse signals T+T- are engaged with

chains of pulses Both Gate Control Units (for the MANUAL amp AUTOMATIC channels)

contain a field current monitor that blocks the firing pulses immediately whenever the

current exceeds a preset threshold level In this case the field circuit-breaker is also

tripped via an output contact The purpose of these provisions is to prevent damage to

thyristors and thyristor fuses in case of a slip-ring short-circuit or to keep any damage

that does occur to a minimum The pulse signals are galvanically separated at the outputs

from the Intermediate Pulse Stage (with pulse transmitters) and are then directed to the

common pulse bus This transmission of the pulse signals to the pulse bus via passive

transmitters ensures a high degree of active channel autonomy Practically no possible

malfunctions on the inactive channel (including for example sustained pulses) affect the

active channel

32 Pulse Monitoring

The ldquoPulse Busrdquo and the pulse signals of the AUTOMATIC channel are

monitored This monitoring device consists of potential isolating stages and the common

monitor If the pulse monitoring of the ldquoPulse Busrdquo responds a switch-over is made to

MANUAL channel The function of the potential isolating stages is to couple the pulse

monitoring device to the pulse circuits without any feedback effect The pulse monitoring

checks the six pulse lines for the following malfunctions continuous or periodic failure

of one or more pulses Periodic occurrence of synchronous or asynchronous false pulses

Continuous pulses the pulse monitoring device can be tested while the machine is in

operation

CHAPTER 4

CONVERTER

Thyristor

The term thyristor usually refers to a family of four layer solid state device having

turn on characteristics that can be externally controlled by either current or voltage They

are also referred to as breakdown device because their working depends on avalanche

breakdown Thyristors have only two stages OFF and ON Thyristors have a similar

function to Uni-junctions they act as switches Thyristors use current flow as a switch

Thyristors have three states

1 Reverse blocking mode mdash Voltage is applied in the direction that would be

blocked by a diode

2 Forward blocking mode mdash Voltage is applied in the direction that would cause

a diode to conduct but the thyristor has not yet been triggered into conduction

3 Forward conducting mode mdash The thyristor has been triggered into conduction

and will remain conducting until the forward current drops below a threshold value

known as the holding current Converter is a semiconductor device which converts ac

input voltage into a constant dc output voltage In present excitation system three phase

fully controlled thyristor converter is used

Because of the following advantages thyristor converters are used

a) Thyristors are used for high power applications ie up to 10Kv3500A1KHz

b) Having high reliability and low losses

c) Uni-directional device like diode

d) Itrsquos operation as a rectifier which are low resistance in forward conduction

mode and high resistance in reverse conduction mode

PROTECTION OF THYRISTORS

For reliable operation of a thyristor demands that its specified ratings are not

exceeded When Subjected to or over voltages During the turn - on of SCR didt

prohibitively large False triggering of SCR by high value of dvdt andSpurious signals

between gate and cathode may leads to unwanted turn ndash on

DIDT AND PROTECTION

When thyristor starts conducting in forward conduction mode and is turned on by

gate pulse The anode current increases rapidly whole area of the gate to Cathode

junction then hot spots will be formed near the gate connection this locality of heating

destroys the thyristor Thyristor thermal time is constant The causes due to faults and

short circuits or surge currents Electronic crowbar protection is used against the over

voltages The rate rise of anode current must be kept at the time of turn on below the

rated or specified limiting value The didt value maintained below limited value by using

a inductor also called ldquodidt inductorrdquo in series with anode circuit The locality of heating

is avoided by applying gate current but not greater the maximum gate current

DVDT AND OVER VOLTAGE PROTECTION

With forward voltage across the anode and cathode of a thyristor the two outer

junctions are forward biased but the inner junction is reverse biased This reverse biased

junction J2 has the characteristics of a capacitor due to charges existing across the

junction In other words space-charges exist in the depletion region around junction J2

and therefore junction J2 behaves like a capacitance If the entire anode to cathode

forward voltage Va appears across J2 junction and the charge is denoted by Q then a

charging current i given by Eq (46) follows

i = dQdt =d(Cj Va )dt

= Cj (d Va dt) + Va(d Cj dt) helliphelliphelliphellip(46 a)

As Cj the capacitance of junction J2 is almost constant the current is given by

i = Cj (d Va dt) helliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip(46 b)

If the rate of rise of forward voltage dVadt is high the charging current i will be

more This charging current plays the role of gate current and turns on the SCR even

when gate signal is zero Such phenomena of turning-on a thyristor called dvdt turn-on

must be avoided as it leads to false operation of the thyristor circuit

For controllable operation of the thyristor the rate of rise of forward anode to

cathode voltage dVadt must be kept below the specified rated limit Typical values of

dvdt are 20 ndash 500 Vμsec False turn-on of a thyristor by large dvdt can be prevented by

using a snubber circuit in parallel with the device thyristor are very sensitive for over

voltage than the semiconductor devices

Over voltage transients are perhaps the main cause of thyristor failure

In thyristor there are mainly two types

1 Internal over voltages

Due to the commutation of the thyristors large voltages are generated internally

Because of the series inductance of the SCR circuit the large transient voltages L didt

produced This voltage several times the break over voltage of the device then thyristor

destroys permanently

2 External over voltages

External over voltages are caused due to the interruptions of current flow in an

inductive circuit and also due to the lightening strokes on the lines feeding the thyristor

system For the reliable operation of thyristor the over voltages must be suppressed by

adopting suitable techniques

Suppression of over voltages

The RC circuit called snubber circuit is connected across the device to protect In

order to keep the protective components to a minimum the thyristors are chosen with

their peak voltages ratings are 25 to 3 times of the normal peak working voltage

ldquoselenium thyrector diodes metal oxide varistors or avalanche diode suppressers are

commonly employed for protecting the thyristor circuit against the over voltages

Gate protection

Gate circuit should also be protected against the over voltages and surges Over

voltage at gate circuit can cause false triggering of the SCR may rises the junction

temperature behind specified limit leading to its damage Protection against over

voltage can be achieved by connecting a ZD across the gate circuit and a resister is

connected in series with gate circuit to protect against the s A capacitor and resister are

connected across gate to cathode to by pass the noise

41 Final Pulse Stages

The Final Pulse Stages adapt the output pulses from the Intermediate Pulse Stage

(pulses on the pulse bus) to the gate currents needed for the thyristors Each thyristor

bridge is equipped with its own Final Pulse Stage Each Final Pulse Stages is provided

with a power supply module The amplified output pulses from the Final Pulse Stages

start as a short strong steep pulse with an amplitude approx 2frac12 times that of the main

pulse This initial pulse edge assures proper firing of the thyristors being triggered

Subsequently the weaker part of main pulse keeps firing conditions steady As already

mentioned the Final Pulse Stages and their associated thyristor bridges form single units

All six pulse outputs from a Final Pulse Stage can be blocked by an external control

signal so that all thyristors in the associated thyristor bridge will block the current A

blocking of the pulses is initiated whenever there is a malfunction in the associated

thyristor bridge

42 Converter Power Section

The thyristor converter consists of three independent parallel rectifier blocks TY1

to TY3 which are all in service Even if one block fails the remaining blocks take over

automatically the full design current of the excitation circuit During normal operation

(with ideal current share) and all three bridges in operation each of these blocks has to

carry only (n-2)n (ie33)of its design current If 2 thyristor bridges fail the excitation

is limited Only when all three bridges fail the excitation is switched off Each thyristor

bridge arm is equipped with current flow monitoring CTrsquos Failure of conduction in any

arm is identified by a Current flow monitoring module

43 Converter Cooling

A cooling system is needed to dissipate heat losses in the converter blocks and

electronics Each converter block has therefore been equipped with a fan supplied with

power from the converterrsquos primary voltage (via transformer ndashT8 in field flashing

cubicle) The fans are protected with motor protection circuit breakers An air flow

monitoring unit is provided for monitoring the air flow through the thyristor bridge If a

circuit breaker failure is detected or if the air flow monitor drops off at one of the

thyristor bridges the bridge involved is immediately set out of operation by blocking its

firing pulses

44 Thyristor Converter Monitoring

A thyristor bridge in which defects occur that could threaten the safety of

operation or cause secondary damage is switched off automatically ie its firing pulses

are blocked This happens whenever A thyristor fuse is blown The fuses are monitored

individually with micro switches The Final Pulse Stage fails which is detected by

internal monitors (supply voltage sustained pulse short-circuit on the output end) The

power supply to the fan fails fan air flow as monitored by the Air flow monitor fails or

is insufficient Isolator on ACDC side is open

CHAPTER 5

Field Current Circuit Excitation Transformer

51 Field Circuit Breaker

The circuit-breaker in the field circuit is used to isolate the field circuit from the

converter It is capable of switching off the synchronous machine from full load under the

maximum conditions of a 3-phase short-circuit In addition to its main contacts the field

circuit-breaker also has a de-excitation contact with which the field energy stored in the

field can be dissipated across the de-excitation resistor The de-excitation contact closes

shortly before the main contacts open so as to ensure proper commutation of the field

current from the main contacts to the de-excitation contact when the breaker is switched

off The field circuit-breaker is switched on by electromagnetic force and is kept switched

on by a mechanical latch When the latch is released by a trip coil the circuit-breaker

opens The circuit-breaker also has auxiliary contacts that report its status

52 Field Flashing

In shunt supplied excitation circuits (excitation transformer connected to the

generator terminals) the generator does not have enough remnant voltage for a generator

voltage build-up via the converter In this case a field flashing circuit is provided It

consists of the field flashing contactor the diode bridge and a transformer used to adapt

the auxiliary input voltage to the voltage needed for field flashing when power is

supplied from the auxiliaries network

Fig Field Flashing

Because the field flashing contactor is not able to switch off the energy stored in

the field the control ensures that the contactor can only reopen if the field circuit breaker

has already been opened (generating the TRIP order) or in a normal field flashing

sequence when the converter has taken over the field current Field flashing occurs in the

following stages

1048729The excitation is switched on closing the field flashing contactor ( Field

Circuit Breaker is already closed )

1048729The start-up excitation current flows through the rotor driving the generator

voltage up to approx 15 U

1048729After about 10 U the firing pulses to the converter are released and it begins

to excite the generator to its rated voltage

1048729After about 30 U the field flashing contactor opens (with no current since

the converter is now supplying the current)

The diode bridge at the input to the field flashing contactor prevents a feed-back

from the converter to the source of field flashing while the contactor is still closed

53 De-excitation

When malfunctions occur the stored field energy must be dissipated as quickly

and safely as possible to protect the generator This is done by the converter the field

circuit-breaker and the de-excitation (discharge) resistor

De-excitation (with opening of the field circuit-breaker) takes place in the following

stages

1048729The converter drives to its inverter limit position (negative ceiling voltage)

recovers a portion of the field energy into the network A trip command is given to the

field circuit breaker

1048729The de-excitation contact closes diverting the field voltage to the de-excitation

resistor

1048729Then immediately the main contacts open building voltage The field voltage

commutates to the de-excitation resistor

1048729The current diminishes at a given time constant TE

(With linear resistance TE = Lf (Rf + Re))

Due to the reversal of the field voltage by the converter the field current

commutates from the main contacts of the field circuit-breaker to the de-excitation

resistor in a very early phase This reversal of the field voltage prevents burn-off on the

main contacts and provides effective protection for the field circuit-breaker Depending

on the operating policy an operational shut-down of the excitation can also be effected

with the field circuit-breaker closed This method is useful mainly when the excitation is

switched on and off frequently In this case the converter is merely driven into the

inverter limit position so that the field energy is recovered into the network The

converter then blocks since it is supplying positive current only

54 Excitation Transformer

The excitation transformer matches the generator voltage to the field voltage

(required ceiling voltage) It also serves as a commutation reactance for the thyristor

converter and as a potential isolator between the network and the excitation circuit In

addition the transformer functions as a current limiter in that it makes it possible to keep

any short circuits in the excitation circuit under better control The excitation transformer

is equipped with temperature monitoring probes which set off an alarm when the

temperature exceeds a first max limit and then trips the excitation if the temperature

continues rising to a second (higher)limit

CHAPTER 6

Monitoring and Protection

61 Excitation Monitoring

611 General Information

The main goal of Excitation Monitoring is to make optimum use of the

redundancies provided in the excitation system and to give alarm whenever a malfunction

makes these redundancies unavailable The field current is monitored to see that it does

not exceed a maximum level and if necessary a switch-over to the MANUAL channel is

initiated In addition the criterion for switching off the field flashing is generated The

excitation Monitoring consists of an autonomous processor system

612 over current Alarms

In the Excitation Monitoring the limits for are set at higher levels than the

settings on the Field Current Maximum Limiter Whenever the current exceeds 110 of

the nominal field current contact R1 and the binary output associated with it are

activated immediately If field current remains gt 110 then after a preset inverse-time

has lapsed relay R2 and - after a further delay - relay R0 and the binary outputs

associated with them are activated Parameters match the measurements for If1 and If2 to

the nominal value for field current so that the internal values can be processed and read as

pu values It can be used to falsify the actual value of the field current If (to raise it) so

as to cause a response from the alarm limits for purposes of testing The processed If

signal is always taken from on the active channel (CHANNEL I OR CHANNEL II) As

long as the field current If is above the threshold value 11 Ifn its peak value is

measured This is stored (until RESET) and can be read at any time on the Micro-

Terminal Once the value of If exceeds 11 Ifn integration of this value starts Whenever

the integrated time-current value (intisup2dt) exceeds the preselected reference value the

alarm OVER CURRENT INVERSE-TIME is set off and a command is simultaneously

issued to switch over to the stand by AUTO channel Software switch F758 enables the

three over current alarm functions (R0 R1 R2) and selects one of three possible inverse-

time curves T1 T2 or T3 Within the characteristic curve (T1 T2 T3) selected the

desired limit curve for response is set using the factor F216

613 Switch-Off Criterion for Field Flashing

The Excitation Monitoring supplies the criterion for switching off the field

flashing Whether this criterion is activated based on the actual value for generator voltage

Ug or for field current If or both depends on the settings of the two threshold values

F200F202 (0 setting means that the output is always ldquological 1rdquo) The measurements

Ug12 and If12 are switched over depending on the present status of the channels

(Channel 1 or Channel 2 ON) Whenever Flag F750 is not activated the binary output is

fixed at ldquological 1rdquo

614 Storage of Alarm Status

The outputs of the over current alarms (R1 R2 R0) and the messages NO

FAILURE MONITORING PARAMETERS CHANGED are stored messages can be

erased by activating the input ldquoGENERAL RESETrdquo or by using the RESET button on the

front of the module Erasure with the input ldquoGENERAL RESETrdquo is effective only if the

situation causing the alarm or the malfunction is no longer present Whenever the self-

diagnosis equipment in the processor detects a malfunction the output NO FAILURE of

MONITORING is set at ldquological 0rdquo (= alarm) The alarm ldquoPARAMETERS CHANGEDrdquo

is activated whenever parameters or settings of software switches have been changed via

the Micro-Terminal

615 Actual Value Monitoring

The actual values for generator voltage Ug synchronous voltage Usyn and field

current If are monitored for malfunctions This monitoring is active regardless of whether

or not the generator is in operation Essentially when the generator is in operation the

measurements are monitored by comparing the signals (the smaller signal reading is

detected as incorrect) When the generator is not in operation the measured data are

monitored for extreme values The percentage of deviation permissible in the

measurement signals being compared

(Ug1ampUsyn1 Ug1ampUg2 Ug2ampUsyn2 If1ampIf2) is defined by parameters F208 and

F20AIf the excitation transformer is being supplied from an auxiliary power source (no

shunt operation) the values of Ug and Usyn will be different in some operational

conditions

In that case Flag F75A can be used to deactivate comparative measurements Ug1 Usyn1

and Ug2 Usyn2 The ldquoprimary monitoringrdquo compares Ug1 with Ug2 and generates the

messages that Channel 1 or Channel 2 has suffered a malfunction Whenever Ug1 lt Ug2

and the binary message from CH1 reports no malfunction a malfunction on

Measurement Channel 1 is reported (Ug1Usyn1 FAILURE) A similar malfunction is

also present whenever the binary message CH1 DISTURBANCE is reported and a

discrepancy is detected between Ug1ampUg2 The generation of the alarm ldquoUg2 Usyn2

FAILURErdquo is analogous to that for Channel 1 The ldquosecondary monitoringrdquo compares

Ug1 with Usyn1 amp Ug2 with Usyn2 This is enabled whenever the binary message of the

comparison channel reports a malfunction or whenever both binary messages report no

malfunction - but both secondary monitors report a malfunction As long as the secondary

monitoring is blocked the differences Ug1neUsyn1 or Ug2neUsyn2 trigger malfunction

signals for the measurement channel involved (suspicion that there is a corresponding

error in Usyn) The measurement channel malfunctions are enabled operationally

whenever after excitation has been switched on generator voltage Ug exceeds the value

set on F204 The voltages Ug1 and Ug2 are checked 16 seconds after the excitation is

switched off to see that they do not exceed the limit value F210 that applies to both of

them At the same time g1Ug2Usyn1Usyn2 are checked for extreme values (gt or lt

the operating range) Monitoring for extreme values is likewise enabled during normal

operation ( Excitation ON and Ug gt F204 ) Flag F754 is used to enable or block the

malfunction signals to the binary outputs Basically the monitoring of the actual values

for If1If2 functions like that of the Ug1Ug2 monitoring

62 Excitation Protection

621 General Information

The Excitation Protection switches off the excitation (and de-excites the machine

rapidly) whenever a danger arises that threatens the excitation transformer the converter

or the generator Generally limiter or monitoring functions precede the emergency trips

and these normally respond before the Excitation Protection must initiate a trip

Protective trip commands are issued directly to the field circuit-breaker from potential

free contacts of the board via the trip relays They are directed redundantly to the

operative field circuit-breaker ldquoOFFrdquo command

622 Protection against Excitation Transformer Overheating

This equipment monitors the excitation transformer for overheating in the

windings that could result from over current short-circuits or inadequate cooling The

monitoring uses temperature monitoring modules in conjunction with temperature

sensors built into the windings Normally the temperature is monitored in two stages the

first stage sets off an alarm the second causes a trip of the excitation

623 Rotor Over voltage Protection

Malfunctions in the generator circuit (eg terminal short-circuit failed

synchronization asynchronous operation) cause induced negative field currents that

produce high voltages in the field circuit These must be restricted to a level with a

sufficient safety margin below the insulation capacity of the field winding (test voltage)

and also below the peak blocking voltage of the converter thyristors The crow bar

employs spark gap elements to detect over voltages in the field circuit Whenever they

respond the associated thyristors are fired immediately switching the de-excitation

resistor parallel to the field The de-excitation current generated thereby initiates an

excitation trip via a supervision circuit causing an immediate opening of the field circuit-

breaker The malfunction isets off an alarm and an internal malfunction is indicated at the

cubicle

TEST VALUES OBTAINED WHEN EXCITATION IS RAISED

TEST

SNO PARTICULARS ACTUAL VALUE OBTAINED VALUE

1 VREF 996 100

2 VACT 997 1003

3 IFACT 735 765

4 IGACT 703 707

5 ACTIVE

POWER

703 705

6 REACTIVE

POWER

101 142

7 POWER

FACTOR

099 IND 098 IND

8 ACTIVE

CURRENT(IR)

705 703

9 REACTIVE

CURRENT(IX)

102 138

10 POWER

ANGLE

566 546

11 FIRING

ANGLE

640 632

VALUES OBTAINED WHEN EXCITATION IS LOWERED

SNO PARTICULARS ACTUAL

VALUE

OBTAINED VALUE

1 VREF 100 997

2 VACT 100 997

3 IFACT 787 761

4 IGACT 839 837

5 ACTIVE

POWER

839 830

6 REACTIVE

POWER

155 110

7 POWER

FACTOR

098 IND 099 IND

8 ACTIVE

CURRENT(IR)

835 830

9 REACTIVE

CURRENT(IX)

140 94

10 POWER

ANGLE

613 DEG 631 DEG

11 FIRING

ANGLE

601 DEG 599 DEG

CONCLUSION

For generating the EMF in stator winding excitation is required to the rotor of a

generator There are two types of excitation

1 Static excitation system

2 Brushless excitation system

A certain disadvantage in brushless excitation system is the slow response time of

the field in case of fast load changes specified No slip-rings and brushes direct

measurements of the field parameters not possible

To avoid all loses static excitation is used Since it does not have any rotating

parts mechanical loses and windage loses This system has fast response and speed

control While preferring this excitation system there are no limitations for the

redundancy of Thyristor bridge circuits

Static excitation has fast field discharge by resistor and inverter operation direct

measurement of field quantity is possible The meaning of excitation is nothing but

continuous supply of DC current (ie field current) to the rotor to buildup required

output voltage in the stator

Field current is changed with respect to the change of load so the digital

automatic voltage regulator (DAVR) is used to regulate the output voltage according to

the load variations

So we conclude that static excitation system with DAVR is preferred since it is

having excellent dynamic performance and better options for R amp M

CHAPTER 3

PULSE SECTION

31 Pulse Generation and Amplification

The Gate Control Units of both AUTOMATIC channel and MANUAL channel

each supply six firing pulses for operating the 6-pulse thyristor bridges The low-power

pulse signals from these Gate Control Units are then amplified in the Intermediate Pulse

Stage galvanically isolated and then sent to the common pulse bus On the output end

the Intermediate Pulse Stage of the non-active channel) is always blocked The Gate

Control Units generate the pulses based on microprocessor control The reference voltage

used for the firing pulse phase location is the output voltage from the excitation

transformer (Usyn1 Usyn2) The commutation spikes of the synchronous voltage caused

by the converter are calculated prior to use of the voltage as a reference value and are

deliberately filtered out The lower limit for the firing pulses (double pulses) which are

offset from one another by 60deg is defined by the limit rectifier position (αmin) and the

upper limit by the limit inverter position (αmax) for the firing angle αmin and αmax can

be adjusted on the Gate Control Units using BCD (Binary Coded Decimal) switches

αmin ensures that the firing pulses will not be issued (premature firing) until there is

sufficient positive phase voltage on the thyristor involved αmax prevents a dangerous

ldquotippingrdquo of the thyristor bridge into the rectifier mode if the firing angle α is too large

(ldquolate firingrdquo) The critical factors determining αmax are the overlap time uumlmax (max

commutation time) and the ldquorecovery timerdquo of the thyristors (αmax lt 180deg - uumlmax - γ )

An external control signal can force the firing pulses into their inverter limit position

Other binary inputs can block or direct the firing pulses of the Gate Control Units so as to

produce freewheeling on the thyristor bridge During freewheeling the firing pulses for

the thyristor pair R and S are blocked and the pulse signals T+T- are engaged with

chains of pulses Both Gate Control Units (for the MANUAL amp AUTOMATIC channels)

contain a field current monitor that blocks the firing pulses immediately whenever the

current exceeds a preset threshold level In this case the field circuit-breaker is also

tripped via an output contact The purpose of these provisions is to prevent damage to

thyristors and thyristor fuses in case of a slip-ring short-circuit or to keep any damage

that does occur to a minimum The pulse signals are galvanically separated at the outputs

from the Intermediate Pulse Stage (with pulse transmitters) and are then directed to the

common pulse bus This transmission of the pulse signals to the pulse bus via passive

transmitters ensures a high degree of active channel autonomy Practically no possible

malfunctions on the inactive channel (including for example sustained pulses) affect the

active channel

32 Pulse Monitoring

The ldquoPulse Busrdquo and the pulse signals of the AUTOMATIC channel are

monitored This monitoring device consists of potential isolating stages and the common

monitor If the pulse monitoring of the ldquoPulse Busrdquo responds a switch-over is made to

MANUAL channel The function of the potential isolating stages is to couple the pulse

monitoring device to the pulse circuits without any feedback effect The pulse monitoring

checks the six pulse lines for the following malfunctions continuous or periodic failure

of one or more pulses Periodic occurrence of synchronous or asynchronous false pulses

Continuous pulses the pulse monitoring device can be tested while the machine is in

operation

CHAPTER 4

CONVERTER

Thyristor

The term thyristor usually refers to a family of four layer solid state device having

turn on characteristics that can be externally controlled by either current or voltage They

are also referred to as breakdown device because their working depends on avalanche

breakdown Thyristors have only two stages OFF and ON Thyristors have a similar

function to Uni-junctions they act as switches Thyristors use current flow as a switch

Thyristors have three states

1 Reverse blocking mode mdash Voltage is applied in the direction that would be

blocked by a diode

2 Forward blocking mode mdash Voltage is applied in the direction that would cause

a diode to conduct but the thyristor has not yet been triggered into conduction

3 Forward conducting mode mdash The thyristor has been triggered into conduction

and will remain conducting until the forward current drops below a threshold value

known as the holding current Converter is a semiconductor device which converts ac

input voltage into a constant dc output voltage In present excitation system three phase

fully controlled thyristor converter is used

Because of the following advantages thyristor converters are used

a) Thyristors are used for high power applications ie up to 10Kv3500A1KHz

b) Having high reliability and low losses

c) Uni-directional device like diode

d) Itrsquos operation as a rectifier which are low resistance in forward conduction

mode and high resistance in reverse conduction mode

PROTECTION OF THYRISTORS

For reliable operation of a thyristor demands that its specified ratings are not

exceeded When Subjected to or over voltages During the turn - on of SCR didt

prohibitively large False triggering of SCR by high value of dvdt andSpurious signals

between gate and cathode may leads to unwanted turn ndash on

DIDT AND PROTECTION

When thyristor starts conducting in forward conduction mode and is turned on by

gate pulse The anode current increases rapidly whole area of the gate to Cathode

junction then hot spots will be formed near the gate connection this locality of heating

destroys the thyristor Thyristor thermal time is constant The causes due to faults and

short circuits or surge currents Electronic crowbar protection is used against the over

voltages The rate rise of anode current must be kept at the time of turn on below the

rated or specified limiting value The didt value maintained below limited value by using

a inductor also called ldquodidt inductorrdquo in series with anode circuit The locality of heating

is avoided by applying gate current but not greater the maximum gate current

DVDT AND OVER VOLTAGE PROTECTION

With forward voltage across the anode and cathode of a thyristor the two outer

junctions are forward biased but the inner junction is reverse biased This reverse biased

junction J2 has the characteristics of a capacitor due to charges existing across the

junction In other words space-charges exist in the depletion region around junction J2

and therefore junction J2 behaves like a capacitance If the entire anode to cathode

forward voltage Va appears across J2 junction and the charge is denoted by Q then a

charging current i given by Eq (46) follows

i = dQdt =d(Cj Va )dt

= Cj (d Va dt) + Va(d Cj dt) helliphelliphelliphellip(46 a)

As Cj the capacitance of junction J2 is almost constant the current is given by

i = Cj (d Va dt) helliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip(46 b)

If the rate of rise of forward voltage dVadt is high the charging current i will be

more This charging current plays the role of gate current and turns on the SCR even

when gate signal is zero Such phenomena of turning-on a thyristor called dvdt turn-on

must be avoided as it leads to false operation of the thyristor circuit

For controllable operation of the thyristor the rate of rise of forward anode to

cathode voltage dVadt must be kept below the specified rated limit Typical values of

dvdt are 20 ndash 500 Vμsec False turn-on of a thyristor by large dvdt can be prevented by

using a snubber circuit in parallel with the device thyristor are very sensitive for over

voltage than the semiconductor devices

Over voltage transients are perhaps the main cause of thyristor failure

In thyristor there are mainly two types

1 Internal over voltages

Due to the commutation of the thyristors large voltages are generated internally

Because of the series inductance of the SCR circuit the large transient voltages L didt

produced This voltage several times the break over voltage of the device then thyristor

destroys permanently

2 External over voltages

External over voltages are caused due to the interruptions of current flow in an

inductive circuit and also due to the lightening strokes on the lines feeding the thyristor

system For the reliable operation of thyristor the over voltages must be suppressed by

adopting suitable techniques

Suppression of over voltages

The RC circuit called snubber circuit is connected across the device to protect In

order to keep the protective components to a minimum the thyristors are chosen with

their peak voltages ratings are 25 to 3 times of the normal peak working voltage

ldquoselenium thyrector diodes metal oxide varistors or avalanche diode suppressers are

commonly employed for protecting the thyristor circuit against the over voltages

Gate protection

Gate circuit should also be protected against the over voltages and surges Over

voltage at gate circuit can cause false triggering of the SCR may rises the junction

temperature behind specified limit leading to its damage Protection against over

voltage can be achieved by connecting a ZD across the gate circuit and a resister is

connected in series with gate circuit to protect against the s A capacitor and resister are

connected across gate to cathode to by pass the noise

41 Final Pulse Stages

The Final Pulse Stages adapt the output pulses from the Intermediate Pulse Stage

(pulses on the pulse bus) to the gate currents needed for the thyristors Each thyristor

bridge is equipped with its own Final Pulse Stage Each Final Pulse Stages is provided

with a power supply module The amplified output pulses from the Final Pulse Stages

start as a short strong steep pulse with an amplitude approx 2frac12 times that of the main

pulse This initial pulse edge assures proper firing of the thyristors being triggered

Subsequently the weaker part of main pulse keeps firing conditions steady As already

mentioned the Final Pulse Stages and their associated thyristor bridges form single units

All six pulse outputs from a Final Pulse Stage can be blocked by an external control

signal so that all thyristors in the associated thyristor bridge will block the current A

blocking of the pulses is initiated whenever there is a malfunction in the associated

thyristor bridge

42 Converter Power Section

The thyristor converter consists of three independent parallel rectifier blocks TY1

to TY3 which are all in service Even if one block fails the remaining blocks take over

automatically the full design current of the excitation circuit During normal operation

(with ideal current share) and all three bridges in operation each of these blocks has to

carry only (n-2)n (ie33)of its design current If 2 thyristor bridges fail the excitation

is limited Only when all three bridges fail the excitation is switched off Each thyristor

bridge arm is equipped with current flow monitoring CTrsquos Failure of conduction in any

arm is identified by a Current flow monitoring module

43 Converter Cooling

A cooling system is needed to dissipate heat losses in the converter blocks and

electronics Each converter block has therefore been equipped with a fan supplied with

power from the converterrsquos primary voltage (via transformer ndashT8 in field flashing

cubicle) The fans are protected with motor protection circuit breakers An air flow

monitoring unit is provided for monitoring the air flow through the thyristor bridge If a

circuit breaker failure is detected or if the air flow monitor drops off at one of the

thyristor bridges the bridge involved is immediately set out of operation by blocking its

firing pulses

44 Thyristor Converter Monitoring

A thyristor bridge in which defects occur that could threaten the safety of

operation or cause secondary damage is switched off automatically ie its firing pulses

are blocked This happens whenever A thyristor fuse is blown The fuses are monitored

individually with micro switches The Final Pulse Stage fails which is detected by

internal monitors (supply voltage sustained pulse short-circuit on the output end) The

power supply to the fan fails fan air flow as monitored by the Air flow monitor fails or

is insufficient Isolator on ACDC side is open

CHAPTER 5

Field Current Circuit Excitation Transformer

51 Field Circuit Breaker

The circuit-breaker in the field circuit is used to isolate the field circuit from the

converter It is capable of switching off the synchronous machine from full load under the

maximum conditions of a 3-phase short-circuit In addition to its main contacts the field

circuit-breaker also has a de-excitation contact with which the field energy stored in the

field can be dissipated across the de-excitation resistor The de-excitation contact closes

shortly before the main contacts open so as to ensure proper commutation of the field

current from the main contacts to the de-excitation contact when the breaker is switched

off The field circuit-breaker is switched on by electromagnetic force and is kept switched

on by a mechanical latch When the latch is released by a trip coil the circuit-breaker

opens The circuit-breaker also has auxiliary contacts that report its status

52 Field Flashing

In shunt supplied excitation circuits (excitation transformer connected to the

generator terminals) the generator does not have enough remnant voltage for a generator

voltage build-up via the converter In this case a field flashing circuit is provided It

consists of the field flashing contactor the diode bridge and a transformer used to adapt

the auxiliary input voltage to the voltage needed for field flashing when power is

supplied from the auxiliaries network

Fig Field Flashing

Because the field flashing contactor is not able to switch off the energy stored in

the field the control ensures that the contactor can only reopen if the field circuit breaker

has already been opened (generating the TRIP order) or in a normal field flashing

sequence when the converter has taken over the field current Field flashing occurs in the

following stages

1048729The excitation is switched on closing the field flashing contactor ( Field

Circuit Breaker is already closed )

1048729The start-up excitation current flows through the rotor driving the generator

voltage up to approx 15 U

1048729After about 10 U the firing pulses to the converter are released and it begins

to excite the generator to its rated voltage

1048729After about 30 U the field flashing contactor opens (with no current since

the converter is now supplying the current)

The diode bridge at the input to the field flashing contactor prevents a feed-back

from the converter to the source of field flashing while the contactor is still closed

53 De-excitation

When malfunctions occur the stored field energy must be dissipated as quickly

and safely as possible to protect the generator This is done by the converter the field

circuit-breaker and the de-excitation (discharge) resistor

De-excitation (with opening of the field circuit-breaker) takes place in the following

stages

1048729The converter drives to its inverter limit position (negative ceiling voltage)

recovers a portion of the field energy into the network A trip command is given to the

field circuit breaker

1048729The de-excitation contact closes diverting the field voltage to the de-excitation

resistor

1048729Then immediately the main contacts open building voltage The field voltage

commutates to the de-excitation resistor

1048729The current diminishes at a given time constant TE

(With linear resistance TE = Lf (Rf + Re))

Due to the reversal of the field voltage by the converter the field current

commutates from the main contacts of the field circuit-breaker to the de-excitation

resistor in a very early phase This reversal of the field voltage prevents burn-off on the

main contacts and provides effective protection for the field circuit-breaker Depending

on the operating policy an operational shut-down of the excitation can also be effected

with the field circuit-breaker closed This method is useful mainly when the excitation is

switched on and off frequently In this case the converter is merely driven into the

inverter limit position so that the field energy is recovered into the network The

converter then blocks since it is supplying positive current only

54 Excitation Transformer

The excitation transformer matches the generator voltage to the field voltage

(required ceiling voltage) It also serves as a commutation reactance for the thyristor

converter and as a potential isolator between the network and the excitation circuit In

addition the transformer functions as a current limiter in that it makes it possible to keep

any short circuits in the excitation circuit under better control The excitation transformer

is equipped with temperature monitoring probes which set off an alarm when the

temperature exceeds a first max limit and then trips the excitation if the temperature

continues rising to a second (higher)limit

CHAPTER 6

Monitoring and Protection

61 Excitation Monitoring

611 General Information

The main goal of Excitation Monitoring is to make optimum use of the

redundancies provided in the excitation system and to give alarm whenever a malfunction

makes these redundancies unavailable The field current is monitored to see that it does

not exceed a maximum level and if necessary a switch-over to the MANUAL channel is

initiated In addition the criterion for switching off the field flashing is generated The

excitation Monitoring consists of an autonomous processor system

612 over current Alarms

In the Excitation Monitoring the limits for are set at higher levels than the

settings on the Field Current Maximum Limiter Whenever the current exceeds 110 of

the nominal field current contact R1 and the binary output associated with it are

activated immediately If field current remains gt 110 then after a preset inverse-time

has lapsed relay R2 and - after a further delay - relay R0 and the binary outputs

associated with them are activated Parameters match the measurements for If1 and If2 to

the nominal value for field current so that the internal values can be processed and read as

pu values It can be used to falsify the actual value of the field current If (to raise it) so

as to cause a response from the alarm limits for purposes of testing The processed If

signal is always taken from on the active channel (CHANNEL I OR CHANNEL II) As

long as the field current If is above the threshold value 11 Ifn its peak value is

measured This is stored (until RESET) and can be read at any time on the Micro-

Terminal Once the value of If exceeds 11 Ifn integration of this value starts Whenever

the integrated time-current value (intisup2dt) exceeds the preselected reference value the

alarm OVER CURRENT INVERSE-TIME is set off and a command is simultaneously

issued to switch over to the stand by AUTO channel Software switch F758 enables the

three over current alarm functions (R0 R1 R2) and selects one of three possible inverse-

time curves T1 T2 or T3 Within the characteristic curve (T1 T2 T3) selected the

desired limit curve for response is set using the factor F216

613 Switch-Off Criterion for Field Flashing

The Excitation Monitoring supplies the criterion for switching off the field

flashing Whether this criterion is activated based on the actual value for generator voltage

Ug or for field current If or both depends on the settings of the two threshold values

F200F202 (0 setting means that the output is always ldquological 1rdquo) The measurements

Ug12 and If12 are switched over depending on the present status of the channels

(Channel 1 or Channel 2 ON) Whenever Flag F750 is not activated the binary output is

fixed at ldquological 1rdquo

614 Storage of Alarm Status

The outputs of the over current alarms (R1 R2 R0) and the messages NO

FAILURE MONITORING PARAMETERS CHANGED are stored messages can be

erased by activating the input ldquoGENERAL RESETrdquo or by using the RESET button on the

front of the module Erasure with the input ldquoGENERAL RESETrdquo is effective only if the

situation causing the alarm or the malfunction is no longer present Whenever the self-

diagnosis equipment in the processor detects a malfunction the output NO FAILURE of

MONITORING is set at ldquological 0rdquo (= alarm) The alarm ldquoPARAMETERS CHANGEDrdquo

is activated whenever parameters or settings of software switches have been changed via

the Micro-Terminal

615 Actual Value Monitoring

The actual values for generator voltage Ug synchronous voltage Usyn and field

current If are monitored for malfunctions This monitoring is active regardless of whether

or not the generator is in operation Essentially when the generator is in operation the

measurements are monitored by comparing the signals (the smaller signal reading is

detected as incorrect) When the generator is not in operation the measured data are

monitored for extreme values The percentage of deviation permissible in the

measurement signals being compared

(Ug1ampUsyn1 Ug1ampUg2 Ug2ampUsyn2 If1ampIf2) is defined by parameters F208 and

F20AIf the excitation transformer is being supplied from an auxiliary power source (no

shunt operation) the values of Ug and Usyn will be different in some operational

conditions

In that case Flag F75A can be used to deactivate comparative measurements Ug1 Usyn1

and Ug2 Usyn2 The ldquoprimary monitoringrdquo compares Ug1 with Ug2 and generates the

messages that Channel 1 or Channel 2 has suffered a malfunction Whenever Ug1 lt Ug2

and the binary message from CH1 reports no malfunction a malfunction on

Measurement Channel 1 is reported (Ug1Usyn1 FAILURE) A similar malfunction is

also present whenever the binary message CH1 DISTURBANCE is reported and a

discrepancy is detected between Ug1ampUg2 The generation of the alarm ldquoUg2 Usyn2

FAILURErdquo is analogous to that for Channel 1 The ldquosecondary monitoringrdquo compares

Ug1 with Usyn1 amp Ug2 with Usyn2 This is enabled whenever the binary message of the

comparison channel reports a malfunction or whenever both binary messages report no

malfunction - but both secondary monitors report a malfunction As long as the secondary

monitoring is blocked the differences Ug1neUsyn1 or Ug2neUsyn2 trigger malfunction

signals for the measurement channel involved (suspicion that there is a corresponding

error in Usyn) The measurement channel malfunctions are enabled operationally

whenever after excitation has been switched on generator voltage Ug exceeds the value

set on F204 The voltages Ug1 and Ug2 are checked 16 seconds after the excitation is

switched off to see that they do not exceed the limit value F210 that applies to both of

them At the same time g1Ug2Usyn1Usyn2 are checked for extreme values (gt or lt

the operating range) Monitoring for extreme values is likewise enabled during normal

operation ( Excitation ON and Ug gt F204 ) Flag F754 is used to enable or block the

malfunction signals to the binary outputs Basically the monitoring of the actual values

for If1If2 functions like that of the Ug1Ug2 monitoring

62 Excitation Protection

621 General Information

The Excitation Protection switches off the excitation (and de-excites the machine

rapidly) whenever a danger arises that threatens the excitation transformer the converter

or the generator Generally limiter or monitoring functions precede the emergency trips

and these normally respond before the Excitation Protection must initiate a trip

Protective trip commands are issued directly to the field circuit-breaker from potential

free contacts of the board via the trip relays They are directed redundantly to the

operative field circuit-breaker ldquoOFFrdquo command

622 Protection against Excitation Transformer Overheating

This equipment monitors the excitation transformer for overheating in the

windings that could result from over current short-circuits or inadequate cooling The

monitoring uses temperature monitoring modules in conjunction with temperature

sensors built into the windings Normally the temperature is monitored in two stages the

first stage sets off an alarm the second causes a trip of the excitation

623 Rotor Over voltage Protection

Malfunctions in the generator circuit (eg terminal short-circuit failed

synchronization asynchronous operation) cause induced negative field currents that

produce high voltages in the field circuit These must be restricted to a level with a

sufficient safety margin below the insulation capacity of the field winding (test voltage)

and also below the peak blocking voltage of the converter thyristors The crow bar

employs spark gap elements to detect over voltages in the field circuit Whenever they

respond the associated thyristors are fired immediately switching the de-excitation

resistor parallel to the field The de-excitation current generated thereby initiates an

excitation trip via a supervision circuit causing an immediate opening of the field circuit-

breaker The malfunction isets off an alarm and an internal malfunction is indicated at the

cubicle

TEST VALUES OBTAINED WHEN EXCITATION IS RAISED

TEST

SNO PARTICULARS ACTUAL VALUE OBTAINED VALUE

1 VREF 996 100

2 VACT 997 1003

3 IFACT 735 765

4 IGACT 703 707

5 ACTIVE

POWER

703 705

6 REACTIVE

POWER

101 142

7 POWER

FACTOR

099 IND 098 IND

8 ACTIVE

CURRENT(IR)

705 703

9 REACTIVE

CURRENT(IX)

102 138

10 POWER

ANGLE

566 546

11 FIRING

ANGLE

640 632

VALUES OBTAINED WHEN EXCITATION IS LOWERED

SNO PARTICULARS ACTUAL

VALUE

OBTAINED VALUE

1 VREF 100 997

2 VACT 100 997

3 IFACT 787 761

4 IGACT 839 837

5 ACTIVE

POWER

839 830

6 REACTIVE

POWER

155 110

7 POWER

FACTOR

098 IND 099 IND

8 ACTIVE

CURRENT(IR)

835 830

9 REACTIVE

CURRENT(IX)

140 94

10 POWER

ANGLE

613 DEG 631 DEG

11 FIRING

ANGLE

601 DEG 599 DEG

CONCLUSION

For generating the EMF in stator winding excitation is required to the rotor of a

generator There are two types of excitation

1 Static excitation system

2 Brushless excitation system

A certain disadvantage in brushless excitation system is the slow response time of

the field in case of fast load changes specified No slip-rings and brushes direct

measurements of the field parameters not possible

To avoid all loses static excitation is used Since it does not have any rotating

parts mechanical loses and windage loses This system has fast response and speed

control While preferring this excitation system there are no limitations for the

redundancy of Thyristor bridge circuits

Static excitation has fast field discharge by resistor and inverter operation direct

measurement of field quantity is possible The meaning of excitation is nothing but

continuous supply of DC current (ie field current) to the rotor to buildup required

output voltage in the stator

Field current is changed with respect to the change of load so the digital

automatic voltage regulator (DAVR) is used to regulate the output voltage according to

the load variations

So we conclude that static excitation system with DAVR is preferred since it is

having excellent dynamic performance and better options for R amp M

from the Intermediate Pulse Stage (with pulse transmitters) and are then directed to the

common pulse bus This transmission of the pulse signals to the pulse bus via passive

transmitters ensures a high degree of active channel autonomy Practically no possible

malfunctions on the inactive channel (including for example sustained pulses) affect the

active channel

32 Pulse Monitoring

The ldquoPulse Busrdquo and the pulse signals of the AUTOMATIC channel are

monitored This monitoring device consists of potential isolating stages and the common

monitor If the pulse monitoring of the ldquoPulse Busrdquo responds a switch-over is made to

MANUAL channel The function of the potential isolating stages is to couple the pulse

monitoring device to the pulse circuits without any feedback effect The pulse monitoring

checks the six pulse lines for the following malfunctions continuous or periodic failure

of one or more pulses Periodic occurrence of synchronous or asynchronous false pulses

Continuous pulses the pulse monitoring device can be tested while the machine is in

operation

CHAPTER 4

CONVERTER

Thyristor

The term thyristor usually refers to a family of four layer solid state device having

turn on characteristics that can be externally controlled by either current or voltage They

are also referred to as breakdown device because their working depends on avalanche

breakdown Thyristors have only two stages OFF and ON Thyristors have a similar

function to Uni-junctions they act as switches Thyristors use current flow as a switch

Thyristors have three states

1 Reverse blocking mode mdash Voltage is applied in the direction that would be

blocked by a diode

2 Forward blocking mode mdash Voltage is applied in the direction that would cause

a diode to conduct but the thyristor has not yet been triggered into conduction

3 Forward conducting mode mdash The thyristor has been triggered into conduction

and will remain conducting until the forward current drops below a threshold value

known as the holding current Converter is a semiconductor device which converts ac

input voltage into a constant dc output voltage In present excitation system three phase

fully controlled thyristor converter is used

Because of the following advantages thyristor converters are used

a) Thyristors are used for high power applications ie up to 10Kv3500A1KHz

b) Having high reliability and low losses

c) Uni-directional device like diode

d) Itrsquos operation as a rectifier which are low resistance in forward conduction

mode and high resistance in reverse conduction mode

PROTECTION OF THYRISTORS

For reliable operation of a thyristor demands that its specified ratings are not

exceeded When Subjected to or over voltages During the turn - on of SCR didt

prohibitively large False triggering of SCR by high value of dvdt andSpurious signals

between gate and cathode may leads to unwanted turn ndash on

DIDT AND PROTECTION

When thyristor starts conducting in forward conduction mode and is turned on by

gate pulse The anode current increases rapidly whole area of the gate to Cathode

junction then hot spots will be formed near the gate connection this locality of heating

destroys the thyristor Thyristor thermal time is constant The causes due to faults and

short circuits or surge currents Electronic crowbar protection is used against the over

voltages The rate rise of anode current must be kept at the time of turn on below the

rated or specified limiting value The didt value maintained below limited value by using

a inductor also called ldquodidt inductorrdquo in series with anode circuit The locality of heating

is avoided by applying gate current but not greater the maximum gate current

DVDT AND OVER VOLTAGE PROTECTION

With forward voltage across the anode and cathode of a thyristor the two outer

junctions are forward biased but the inner junction is reverse biased This reverse biased

junction J2 has the characteristics of a capacitor due to charges existing across the

junction In other words space-charges exist in the depletion region around junction J2

and therefore junction J2 behaves like a capacitance If the entire anode to cathode

forward voltage Va appears across J2 junction and the charge is denoted by Q then a

charging current i given by Eq (46) follows

i = dQdt =d(Cj Va )dt

= Cj (d Va dt) + Va(d Cj dt) helliphelliphelliphellip(46 a)

As Cj the capacitance of junction J2 is almost constant the current is given by

i = Cj (d Va dt) helliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip(46 b)

If the rate of rise of forward voltage dVadt is high the charging current i will be

more This charging current plays the role of gate current and turns on the SCR even

when gate signal is zero Such phenomena of turning-on a thyristor called dvdt turn-on

must be avoided as it leads to false operation of the thyristor circuit

For controllable operation of the thyristor the rate of rise of forward anode to

cathode voltage dVadt must be kept below the specified rated limit Typical values of

dvdt are 20 ndash 500 Vμsec False turn-on of a thyristor by large dvdt can be prevented by

using a snubber circuit in parallel with the device thyristor are very sensitive for over

voltage than the semiconductor devices

Over voltage transients are perhaps the main cause of thyristor failure

In thyristor there are mainly two types

1 Internal over voltages

Due to the commutation of the thyristors large voltages are generated internally

Because of the series inductance of the SCR circuit the large transient voltages L didt

produced This voltage several times the break over voltage of the device then thyristor

destroys permanently

2 External over voltages

External over voltages are caused due to the interruptions of current flow in an

inductive circuit and also due to the lightening strokes on the lines feeding the thyristor

system For the reliable operation of thyristor the over voltages must be suppressed by

adopting suitable techniques

Suppression of over voltages

The RC circuit called snubber circuit is connected across the device to protect In

order to keep the protective components to a minimum the thyristors are chosen with

their peak voltages ratings are 25 to 3 times of the normal peak working voltage

ldquoselenium thyrector diodes metal oxide varistors or avalanche diode suppressers are

commonly employed for protecting the thyristor circuit against the over voltages

Gate protection

Gate circuit should also be protected against the over voltages and surges Over

voltage at gate circuit can cause false triggering of the SCR may rises the junction

temperature behind specified limit leading to its damage Protection against over

voltage can be achieved by connecting a ZD across the gate circuit and a resister is

connected in series with gate circuit to protect against the s A capacitor and resister are

connected across gate to cathode to by pass the noise

41 Final Pulse Stages

The Final Pulse Stages adapt the output pulses from the Intermediate Pulse Stage

(pulses on the pulse bus) to the gate currents needed for the thyristors Each thyristor

bridge is equipped with its own Final Pulse Stage Each Final Pulse Stages is provided

with a power supply module The amplified output pulses from the Final Pulse Stages

start as a short strong steep pulse with an amplitude approx 2frac12 times that of the main

pulse This initial pulse edge assures proper firing of the thyristors being triggered

Subsequently the weaker part of main pulse keeps firing conditions steady As already

mentioned the Final Pulse Stages and their associated thyristor bridges form single units

All six pulse outputs from a Final Pulse Stage can be blocked by an external control

signal so that all thyristors in the associated thyristor bridge will block the current A

blocking of the pulses is initiated whenever there is a malfunction in the associated

thyristor bridge

42 Converter Power Section

The thyristor converter consists of three independent parallel rectifier blocks TY1

to TY3 which are all in service Even if one block fails the remaining blocks take over

automatically the full design current of the excitation circuit During normal operation

(with ideal current share) and all three bridges in operation each of these blocks has to

carry only (n-2)n (ie33)of its design current If 2 thyristor bridges fail the excitation

is limited Only when all three bridges fail the excitation is switched off Each thyristor

bridge arm is equipped with current flow monitoring CTrsquos Failure of conduction in any

arm is identified by a Current flow monitoring module

43 Converter Cooling

A cooling system is needed to dissipate heat losses in the converter blocks and

electronics Each converter block has therefore been equipped with a fan supplied with

power from the converterrsquos primary voltage (via transformer ndashT8 in field flashing

cubicle) The fans are protected with motor protection circuit breakers An air flow

monitoring unit is provided for monitoring the air flow through the thyristor bridge If a

circuit breaker failure is detected or if the air flow monitor drops off at one of the

thyristor bridges the bridge involved is immediately set out of operation by blocking its

firing pulses

44 Thyristor Converter Monitoring

A thyristor bridge in which defects occur that could threaten the safety of

operation or cause secondary damage is switched off automatically ie its firing pulses

are blocked This happens whenever A thyristor fuse is blown The fuses are monitored

individually with micro switches The Final Pulse Stage fails which is detected by

internal monitors (supply voltage sustained pulse short-circuit on the output end) The

power supply to the fan fails fan air flow as monitored by the Air flow monitor fails or

is insufficient Isolator on ACDC side is open

CHAPTER 5

Field Current Circuit Excitation Transformer

51 Field Circuit Breaker

The circuit-breaker in the field circuit is used to isolate the field circuit from the

converter It is capable of switching off the synchronous machine from full load under the

maximum conditions of a 3-phase short-circuit In addition to its main contacts the field

circuit-breaker also has a de-excitation contact with which the field energy stored in the

field can be dissipated across the de-excitation resistor The de-excitation contact closes

shortly before the main contacts open so as to ensure proper commutation of the field

current from the main contacts to the de-excitation contact when the breaker is switched

off The field circuit-breaker is switched on by electromagnetic force and is kept switched

on by a mechanical latch When the latch is released by a trip coil the circuit-breaker

opens The circuit-breaker also has auxiliary contacts that report its status

52 Field Flashing

In shunt supplied excitation circuits (excitation transformer connected to the

generator terminals) the generator does not have enough remnant voltage for a generator

voltage build-up via the converter In this case a field flashing circuit is provided It

consists of the field flashing contactor the diode bridge and a transformer used to adapt

the auxiliary input voltage to the voltage needed for field flashing when power is

supplied from the auxiliaries network

Fig Field Flashing

Because the field flashing contactor is not able to switch off the energy stored in

the field the control ensures that the contactor can only reopen if the field circuit breaker

has already been opened (generating the TRIP order) or in a normal field flashing

sequence when the converter has taken over the field current Field flashing occurs in the

following stages

1048729The excitation is switched on closing the field flashing contactor ( Field

Circuit Breaker is already closed )

1048729The start-up excitation current flows through the rotor driving the generator

voltage up to approx 15 U

1048729After about 10 U the firing pulses to the converter are released and it begins

to excite the generator to its rated voltage

1048729After about 30 U the field flashing contactor opens (with no current since

the converter is now supplying the current)

The diode bridge at the input to the field flashing contactor prevents a feed-back

from the converter to the source of field flashing while the contactor is still closed

53 De-excitation

When malfunctions occur the stored field energy must be dissipated as quickly

and safely as possible to protect the generator This is done by the converter the field

circuit-breaker and the de-excitation (discharge) resistor

De-excitation (with opening of the field circuit-breaker) takes place in the following

stages

1048729The converter drives to its inverter limit position (negative ceiling voltage)

recovers a portion of the field energy into the network A trip command is given to the

field circuit breaker

1048729The de-excitation contact closes diverting the field voltage to the de-excitation

resistor

1048729Then immediately the main contacts open building voltage The field voltage

commutates to the de-excitation resistor

1048729The current diminishes at a given time constant TE

(With linear resistance TE = Lf (Rf + Re))

Due to the reversal of the field voltage by the converter the field current

commutates from the main contacts of the field circuit-breaker to the de-excitation

resistor in a very early phase This reversal of the field voltage prevents burn-off on the

main contacts and provides effective protection for the field circuit-breaker Depending

on the operating policy an operational shut-down of the excitation can also be effected

with the field circuit-breaker closed This method is useful mainly when the excitation is

switched on and off frequently In this case the converter is merely driven into the

inverter limit position so that the field energy is recovered into the network The

converter then blocks since it is supplying positive current only

54 Excitation Transformer

The excitation transformer matches the generator voltage to the field voltage

(required ceiling voltage) It also serves as a commutation reactance for the thyristor

converter and as a potential isolator between the network and the excitation circuit In

addition the transformer functions as a current limiter in that it makes it possible to keep

any short circuits in the excitation circuit under better control The excitation transformer

is equipped with temperature monitoring probes which set off an alarm when the

temperature exceeds a first max limit and then trips the excitation if the temperature

continues rising to a second (higher)limit

CHAPTER 6

Monitoring and Protection

61 Excitation Monitoring

611 General Information

The main goal of Excitation Monitoring is to make optimum use of the

redundancies provided in the excitation system and to give alarm whenever a malfunction

makes these redundancies unavailable The field current is monitored to see that it does

not exceed a maximum level and if necessary a switch-over to the MANUAL channel is

initiated In addition the criterion for switching off the field flashing is generated The

excitation Monitoring consists of an autonomous processor system

612 over current Alarms

In the Excitation Monitoring the limits for are set at higher levels than the

settings on the Field Current Maximum Limiter Whenever the current exceeds 110 of

the nominal field current contact R1 and the binary output associated with it are

activated immediately If field current remains gt 110 then after a preset inverse-time

has lapsed relay R2 and - after a further delay - relay R0 and the binary outputs

associated with them are activated Parameters match the measurements for If1 and If2 to

the nominal value for field current so that the internal values can be processed and read as

pu values It can be used to falsify the actual value of the field current If (to raise it) so

as to cause a response from the alarm limits for purposes of testing The processed If

signal is always taken from on the active channel (CHANNEL I OR CHANNEL II) As

long as the field current If is above the threshold value 11 Ifn its peak value is

measured This is stored (until RESET) and can be read at any time on the Micro-

Terminal Once the value of If exceeds 11 Ifn integration of this value starts Whenever

the integrated time-current value (intisup2dt) exceeds the preselected reference value the

alarm OVER CURRENT INVERSE-TIME is set off and a command is simultaneously

issued to switch over to the stand by AUTO channel Software switch F758 enables the

three over current alarm functions (R0 R1 R2) and selects one of three possible inverse-

time curves T1 T2 or T3 Within the characteristic curve (T1 T2 T3) selected the

desired limit curve for response is set using the factor F216

613 Switch-Off Criterion for Field Flashing

The Excitation Monitoring supplies the criterion for switching off the field

flashing Whether this criterion is activated based on the actual value for generator voltage

Ug or for field current If or both depends on the settings of the two threshold values

F200F202 (0 setting means that the output is always ldquological 1rdquo) The measurements

Ug12 and If12 are switched over depending on the present status of the channels

(Channel 1 or Channel 2 ON) Whenever Flag F750 is not activated the binary output is

fixed at ldquological 1rdquo

614 Storage of Alarm Status

The outputs of the over current alarms (R1 R2 R0) and the messages NO

FAILURE MONITORING PARAMETERS CHANGED are stored messages can be

erased by activating the input ldquoGENERAL RESETrdquo or by using the RESET button on the

front of the module Erasure with the input ldquoGENERAL RESETrdquo is effective only if the

situation causing the alarm or the malfunction is no longer present Whenever the self-

diagnosis equipment in the processor detects a malfunction the output NO FAILURE of

MONITORING is set at ldquological 0rdquo (= alarm) The alarm ldquoPARAMETERS CHANGEDrdquo

is activated whenever parameters or settings of software switches have been changed via

the Micro-Terminal

615 Actual Value Monitoring

The actual values for generator voltage Ug synchronous voltage Usyn and field

current If are monitored for malfunctions This monitoring is active regardless of whether

or not the generator is in operation Essentially when the generator is in operation the

measurements are monitored by comparing the signals (the smaller signal reading is

detected as incorrect) When the generator is not in operation the measured data are

monitored for extreme values The percentage of deviation permissible in the

measurement signals being compared

(Ug1ampUsyn1 Ug1ampUg2 Ug2ampUsyn2 If1ampIf2) is defined by parameters F208 and

F20AIf the excitation transformer is being supplied from an auxiliary power source (no

shunt operation) the values of Ug and Usyn will be different in some operational

conditions

In that case Flag F75A can be used to deactivate comparative measurements Ug1 Usyn1

and Ug2 Usyn2 The ldquoprimary monitoringrdquo compares Ug1 with Ug2 and generates the

messages that Channel 1 or Channel 2 has suffered a malfunction Whenever Ug1 lt Ug2

and the binary message from CH1 reports no malfunction a malfunction on

Measurement Channel 1 is reported (Ug1Usyn1 FAILURE) A similar malfunction is

also present whenever the binary message CH1 DISTURBANCE is reported and a

discrepancy is detected between Ug1ampUg2 The generation of the alarm ldquoUg2 Usyn2

FAILURErdquo is analogous to that for Channel 1 The ldquosecondary monitoringrdquo compares

Ug1 with Usyn1 amp Ug2 with Usyn2 This is enabled whenever the binary message of the

comparison channel reports a malfunction or whenever both binary messages report no

malfunction - but both secondary monitors report a malfunction As long as the secondary

monitoring is blocked the differences Ug1neUsyn1 or Ug2neUsyn2 trigger malfunction

signals for the measurement channel involved (suspicion that there is a corresponding

error in Usyn) The measurement channel malfunctions are enabled operationally

whenever after excitation has been switched on generator voltage Ug exceeds the value

set on F204 The voltages Ug1 and Ug2 are checked 16 seconds after the excitation is

switched off to see that they do not exceed the limit value F210 that applies to both of

them At the same time g1Ug2Usyn1Usyn2 are checked for extreme values (gt or lt

the operating range) Monitoring for extreme values is likewise enabled during normal

operation ( Excitation ON and Ug gt F204 ) Flag F754 is used to enable or block the

malfunction signals to the binary outputs Basically the monitoring of the actual values

for If1If2 functions like that of the Ug1Ug2 monitoring

62 Excitation Protection

621 General Information

The Excitation Protection switches off the excitation (and de-excites the machine

rapidly) whenever a danger arises that threatens the excitation transformer the converter

or the generator Generally limiter or monitoring functions precede the emergency trips

and these normally respond before the Excitation Protection must initiate a trip

Protective trip commands are issued directly to the field circuit-breaker from potential

free contacts of the board via the trip relays They are directed redundantly to the

operative field circuit-breaker ldquoOFFrdquo command

622 Protection against Excitation Transformer Overheating

This equipment monitors the excitation transformer for overheating in the

windings that could result from over current short-circuits or inadequate cooling The

monitoring uses temperature monitoring modules in conjunction with temperature

sensors built into the windings Normally the temperature is monitored in two stages the

first stage sets off an alarm the second causes a trip of the excitation

623 Rotor Over voltage Protection

Malfunctions in the generator circuit (eg terminal short-circuit failed

synchronization asynchronous operation) cause induced negative field currents that

produce high voltages in the field circuit These must be restricted to a level with a

sufficient safety margin below the insulation capacity of the field winding (test voltage)

and also below the peak blocking voltage of the converter thyristors The crow bar

employs spark gap elements to detect over voltages in the field circuit Whenever they

respond the associated thyristors are fired immediately switching the de-excitation

resistor parallel to the field The de-excitation current generated thereby initiates an

excitation trip via a supervision circuit causing an immediate opening of the field circuit-

breaker The malfunction isets off an alarm and an internal malfunction is indicated at the

cubicle

TEST VALUES OBTAINED WHEN EXCITATION IS RAISED

TEST

SNO PARTICULARS ACTUAL VALUE OBTAINED VALUE

1 VREF 996 100

2 VACT 997 1003

3 IFACT 735 765

4 IGACT 703 707

5 ACTIVE

POWER

703 705

6 REACTIVE

POWER

101 142

7 POWER

FACTOR

099 IND 098 IND

8 ACTIVE

CURRENT(IR)

705 703

9 REACTIVE

CURRENT(IX)

102 138

10 POWER

ANGLE

566 546

11 FIRING

ANGLE

640 632

VALUES OBTAINED WHEN EXCITATION IS LOWERED

SNO PARTICULARS ACTUAL

VALUE

OBTAINED VALUE

1 VREF 100 997

2 VACT 100 997

3 IFACT 787 761

4 IGACT 839 837

5 ACTIVE

POWER

839 830

6 REACTIVE

POWER

155 110

7 POWER

FACTOR

098 IND 099 IND

8 ACTIVE

CURRENT(IR)

835 830

9 REACTIVE

CURRENT(IX)

140 94

10 POWER

ANGLE

613 DEG 631 DEG

11 FIRING

ANGLE

601 DEG 599 DEG

CONCLUSION

For generating the EMF in stator winding excitation is required to the rotor of a

generator There are two types of excitation

1 Static excitation system

2 Brushless excitation system

A certain disadvantage in brushless excitation system is the slow response time of

the field in case of fast load changes specified No slip-rings and brushes direct

measurements of the field parameters not possible

To avoid all loses static excitation is used Since it does not have any rotating

parts mechanical loses and windage loses This system has fast response and speed

control While preferring this excitation system there are no limitations for the

redundancy of Thyristor bridge circuits

Static excitation has fast field discharge by resistor and inverter operation direct

measurement of field quantity is possible The meaning of excitation is nothing but

continuous supply of DC current (ie field current) to the rotor to buildup required

output voltage in the stator

Field current is changed with respect to the change of load so the digital

automatic voltage regulator (DAVR) is used to regulate the output voltage according to

the load variations

So we conclude that static excitation system with DAVR is preferred since it is

having excellent dynamic performance and better options for R amp M

CHAPTER 4

CONVERTER

Thyristor

The term thyristor usually refers to a family of four layer solid state device having

turn on characteristics that can be externally controlled by either current or voltage They

are also referred to as breakdown device because their working depends on avalanche

breakdown Thyristors have only two stages OFF and ON Thyristors have a similar

function to Uni-junctions they act as switches Thyristors use current flow as a switch

Thyristors have three states

1 Reverse blocking mode mdash Voltage is applied in the direction that would be

blocked by a diode

2 Forward blocking mode mdash Voltage is applied in the direction that would cause

a diode to conduct but the thyristor has not yet been triggered into conduction

3 Forward conducting mode mdash The thyristor has been triggered into conduction

and will remain conducting until the forward current drops below a threshold value

known as the holding current Converter is a semiconductor device which converts ac

input voltage into a constant dc output voltage In present excitation system three phase

fully controlled thyristor converter is used

Because of the following advantages thyristor converters are used

a) Thyristors are used for high power applications ie up to 10Kv3500A1KHz

b) Having high reliability and low losses

c) Uni-directional device like diode

d) Itrsquos operation as a rectifier which are low resistance in forward conduction

mode and high resistance in reverse conduction mode

PROTECTION OF THYRISTORS

For reliable operation of a thyristor demands that its specified ratings are not

exceeded When Subjected to or over voltages During the turn - on of SCR didt

prohibitively large False triggering of SCR by high value of dvdt andSpurious signals

between gate and cathode may leads to unwanted turn ndash on

DIDT AND PROTECTION

When thyristor starts conducting in forward conduction mode and is turned on by

gate pulse The anode current increases rapidly whole area of the gate to Cathode

junction then hot spots will be formed near the gate connection this locality of heating

destroys the thyristor Thyristor thermal time is constant The causes due to faults and

short circuits or surge currents Electronic crowbar protection is used against the over

voltages The rate rise of anode current must be kept at the time of turn on below the

rated or specified limiting value The didt value maintained below limited value by using

a inductor also called ldquodidt inductorrdquo in series with anode circuit The locality of heating

is avoided by applying gate current but not greater the maximum gate current

DVDT AND OVER VOLTAGE PROTECTION

With forward voltage across the anode and cathode of a thyristor the two outer

junctions are forward biased but the inner junction is reverse biased This reverse biased

junction J2 has the characteristics of a capacitor due to charges existing across the

junction In other words space-charges exist in the depletion region around junction J2

and therefore junction J2 behaves like a capacitance If the entire anode to cathode

forward voltage Va appears across J2 junction and the charge is denoted by Q then a

charging current i given by Eq (46) follows

i = dQdt =d(Cj Va )dt

= Cj (d Va dt) + Va(d Cj dt) helliphelliphelliphellip(46 a)

As Cj the capacitance of junction J2 is almost constant the current is given by

i = Cj (d Va dt) helliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip(46 b)

If the rate of rise of forward voltage dVadt is high the charging current i will be

more This charging current plays the role of gate current and turns on the SCR even

when gate signal is zero Such phenomena of turning-on a thyristor called dvdt turn-on

must be avoided as it leads to false operation of the thyristor circuit

For controllable operation of the thyristor the rate of rise of forward anode to

cathode voltage dVadt must be kept below the specified rated limit Typical values of

dvdt are 20 ndash 500 Vμsec False turn-on of a thyristor by large dvdt can be prevented by

using a snubber circuit in parallel with the device thyristor are very sensitive for over

voltage than the semiconductor devices

Over voltage transients are perhaps the main cause of thyristor failure

In thyristor there are mainly two types

1 Internal over voltages

Due to the commutation of the thyristors large voltages are generated internally

Because of the series inductance of the SCR circuit the large transient voltages L didt

produced This voltage several times the break over voltage of the device then thyristor

destroys permanently

2 External over voltages

External over voltages are caused due to the interruptions of current flow in an

inductive circuit and also due to the lightening strokes on the lines feeding the thyristor

system For the reliable operation of thyristor the over voltages must be suppressed by

adopting suitable techniques

Suppression of over voltages

The RC circuit called snubber circuit is connected across the device to protect In

order to keep the protective components to a minimum the thyristors are chosen with

their peak voltages ratings are 25 to 3 times of the normal peak working voltage

ldquoselenium thyrector diodes metal oxide varistors or avalanche diode suppressers are

commonly employed for protecting the thyristor circuit against the over voltages

Gate protection

Gate circuit should also be protected against the over voltages and surges Over

voltage at gate circuit can cause false triggering of the SCR may rises the junction

temperature behind specified limit leading to its damage Protection against over

voltage can be achieved by connecting a ZD across the gate circuit and a resister is

connected in series with gate circuit to protect against the s A capacitor and resister are

connected across gate to cathode to by pass the noise

41 Final Pulse Stages

The Final Pulse Stages adapt the output pulses from the Intermediate Pulse Stage

(pulses on the pulse bus) to the gate currents needed for the thyristors Each thyristor

bridge is equipped with its own Final Pulse Stage Each Final Pulse Stages is provided

with a power supply module The amplified output pulses from the Final Pulse Stages

start as a short strong steep pulse with an amplitude approx 2frac12 times that of the main

pulse This initial pulse edge assures proper firing of the thyristors being triggered

Subsequently the weaker part of main pulse keeps firing conditions steady As already

mentioned the Final Pulse Stages and their associated thyristor bridges form single units

All six pulse outputs from a Final Pulse Stage can be blocked by an external control

signal so that all thyristors in the associated thyristor bridge will block the current A

blocking of the pulses is initiated whenever there is a malfunction in the associated

thyristor bridge

42 Converter Power Section

The thyristor converter consists of three independent parallel rectifier blocks TY1

to TY3 which are all in service Even if one block fails the remaining blocks take over

automatically the full design current of the excitation circuit During normal operation

(with ideal current share) and all three bridges in operation each of these blocks has to

carry only (n-2)n (ie33)of its design current If 2 thyristor bridges fail the excitation

is limited Only when all three bridges fail the excitation is switched off Each thyristor

bridge arm is equipped with current flow monitoring CTrsquos Failure of conduction in any

arm is identified by a Current flow monitoring module

43 Converter Cooling

A cooling system is needed to dissipate heat losses in the converter blocks and

electronics Each converter block has therefore been equipped with a fan supplied with

power from the converterrsquos primary voltage (via transformer ndashT8 in field flashing

cubicle) The fans are protected with motor protection circuit breakers An air flow

monitoring unit is provided for monitoring the air flow through the thyristor bridge If a

circuit breaker failure is detected or if the air flow monitor drops off at one of the

thyristor bridges the bridge involved is immediately set out of operation by blocking its

firing pulses

44 Thyristor Converter Monitoring

A thyristor bridge in which defects occur that could threaten the safety of

operation or cause secondary damage is switched off automatically ie its firing pulses

are blocked This happens whenever A thyristor fuse is blown The fuses are monitored

individually with micro switches The Final Pulse Stage fails which is detected by

internal monitors (supply voltage sustained pulse short-circuit on the output end) The

power supply to the fan fails fan air flow as monitored by the Air flow monitor fails or

is insufficient Isolator on ACDC side is open

CHAPTER 5

Field Current Circuit Excitation Transformer

51 Field Circuit Breaker

The circuit-breaker in the field circuit is used to isolate the field circuit from the

converter It is capable of switching off the synchronous machine from full load under the

maximum conditions of a 3-phase short-circuit In addition to its main contacts the field

circuit-breaker also has a de-excitation contact with which the field energy stored in the

field can be dissipated across the de-excitation resistor The de-excitation contact closes

shortly before the main contacts open so as to ensure proper commutation of the field

current from the main contacts to the de-excitation contact when the breaker is switched

off The field circuit-breaker is switched on by electromagnetic force and is kept switched

on by a mechanical latch When the latch is released by a trip coil the circuit-breaker

opens The circuit-breaker also has auxiliary contacts that report its status

52 Field Flashing

In shunt supplied excitation circuits (excitation transformer connected to the

generator terminals) the generator does not have enough remnant voltage for a generator

voltage build-up via the converter In this case a field flashing circuit is provided It

consists of the field flashing contactor the diode bridge and a transformer used to adapt

the auxiliary input voltage to the voltage needed for field flashing when power is

supplied from the auxiliaries network

Fig Field Flashing

Because the field flashing contactor is not able to switch off the energy stored in

the field the control ensures that the contactor can only reopen if the field circuit breaker

has already been opened (generating the TRIP order) or in a normal field flashing

sequence when the converter has taken over the field current Field flashing occurs in the

following stages

1048729The excitation is switched on closing the field flashing contactor ( Field

Circuit Breaker is already closed )

1048729The start-up excitation current flows through the rotor driving the generator

voltage up to approx 15 U

1048729After about 10 U the firing pulses to the converter are released and it begins

to excite the generator to its rated voltage

1048729After about 30 U the field flashing contactor opens (with no current since

the converter is now supplying the current)

The diode bridge at the input to the field flashing contactor prevents a feed-back

from the converter to the source of field flashing while the contactor is still closed

53 De-excitation

When malfunctions occur the stored field energy must be dissipated as quickly

and safely as possible to protect the generator This is done by the converter the field

circuit-breaker and the de-excitation (discharge) resistor

De-excitation (with opening of the field circuit-breaker) takes place in the following

stages

1048729The converter drives to its inverter limit position (negative ceiling voltage)

recovers a portion of the field energy into the network A trip command is given to the

field circuit breaker

1048729The de-excitation contact closes diverting the field voltage to the de-excitation

resistor

1048729Then immediately the main contacts open building voltage The field voltage

commutates to the de-excitation resistor

1048729The current diminishes at a given time constant TE

(With linear resistance TE = Lf (Rf + Re))

Due to the reversal of the field voltage by the converter the field current

commutates from the main contacts of the field circuit-breaker to the de-excitation

resistor in a very early phase This reversal of the field voltage prevents burn-off on the

main contacts and provides effective protection for the field circuit-breaker Depending

on the operating policy an operational shut-down of the excitation can also be effected

with the field circuit-breaker closed This method is useful mainly when the excitation is

switched on and off frequently In this case the converter is merely driven into the

inverter limit position so that the field energy is recovered into the network The

converter then blocks since it is supplying positive current only

54 Excitation Transformer

The excitation transformer matches the generator voltage to the field voltage

(required ceiling voltage) It also serves as a commutation reactance for the thyristor

converter and as a potential isolator between the network and the excitation circuit In

addition the transformer functions as a current limiter in that it makes it possible to keep

any short circuits in the excitation circuit under better control The excitation transformer

is equipped with temperature monitoring probes which set off an alarm when the

temperature exceeds a first max limit and then trips the excitation if the temperature

continues rising to a second (higher)limit

CHAPTER 6

Monitoring and Protection

61 Excitation Monitoring

611 General Information

The main goal of Excitation Monitoring is to make optimum use of the

redundancies provided in the excitation system and to give alarm whenever a malfunction

makes these redundancies unavailable The field current is monitored to see that it does

not exceed a maximum level and if necessary a switch-over to the MANUAL channel is

initiated In addition the criterion for switching off the field flashing is generated The

excitation Monitoring consists of an autonomous processor system

612 over current Alarms

In the Excitation Monitoring the limits for are set at higher levels than the

settings on the Field Current Maximum Limiter Whenever the current exceeds 110 of

the nominal field current contact R1 and the binary output associated with it are

activated immediately If field current remains gt 110 then after a preset inverse-time

has lapsed relay R2 and - after a further delay - relay R0 and the binary outputs

associated with them are activated Parameters match the measurements for If1 and If2 to

the nominal value for field current so that the internal values can be processed and read as

pu values It can be used to falsify the actual value of the field current If (to raise it) so

as to cause a response from the alarm limits for purposes of testing The processed If

signal is always taken from on the active channel (CHANNEL I OR CHANNEL II) As

long as the field current If is above the threshold value 11 Ifn its peak value is

measured This is stored (until RESET) and can be read at any time on the Micro-

Terminal Once the value of If exceeds 11 Ifn integration of this value starts Whenever

the integrated time-current value (intisup2dt) exceeds the preselected reference value the

alarm OVER CURRENT INVERSE-TIME is set off and a command is simultaneously

issued to switch over to the stand by AUTO channel Software switch F758 enables the

three over current alarm functions (R0 R1 R2) and selects one of three possible inverse-

time curves T1 T2 or T3 Within the characteristic curve (T1 T2 T3) selected the

desired limit curve for response is set using the factor F216

613 Switch-Off Criterion for Field Flashing

The Excitation Monitoring supplies the criterion for switching off the field

flashing Whether this criterion is activated based on the actual value for generator voltage

Ug or for field current If or both depends on the settings of the two threshold values

F200F202 (0 setting means that the output is always ldquological 1rdquo) The measurements

Ug12 and If12 are switched over depending on the present status of the channels

(Channel 1 or Channel 2 ON) Whenever Flag F750 is not activated the binary output is

fixed at ldquological 1rdquo

614 Storage of Alarm Status

The outputs of the over current alarms (R1 R2 R0) and the messages NO

FAILURE MONITORING PARAMETERS CHANGED are stored messages can be

erased by activating the input ldquoGENERAL RESETrdquo or by using the RESET button on the

front of the module Erasure with the input ldquoGENERAL RESETrdquo is effective only if the

situation causing the alarm or the malfunction is no longer present Whenever the self-

diagnosis equipment in the processor detects a malfunction the output NO FAILURE of

MONITORING is set at ldquological 0rdquo (= alarm) The alarm ldquoPARAMETERS CHANGEDrdquo

is activated whenever parameters or settings of software switches have been changed via

the Micro-Terminal

615 Actual Value Monitoring

The actual values for generator voltage Ug synchronous voltage Usyn and field

current If are monitored for malfunctions This monitoring is active regardless of whether

or not the generator is in operation Essentially when the generator is in operation the

measurements are monitored by comparing the signals (the smaller signal reading is

detected as incorrect) When the generator is not in operation the measured data are

monitored for extreme values The percentage of deviation permissible in the

measurement signals being compared

(Ug1ampUsyn1 Ug1ampUg2 Ug2ampUsyn2 If1ampIf2) is defined by parameters F208 and

F20AIf the excitation transformer is being supplied from an auxiliary power source (no

shunt operation) the values of Ug and Usyn will be different in some operational

conditions

In that case Flag F75A can be used to deactivate comparative measurements Ug1 Usyn1

and Ug2 Usyn2 The ldquoprimary monitoringrdquo compares Ug1 with Ug2 and generates the

messages that Channel 1 or Channel 2 has suffered a malfunction Whenever Ug1 lt Ug2

and the binary message from CH1 reports no malfunction a malfunction on

Measurement Channel 1 is reported (Ug1Usyn1 FAILURE) A similar malfunction is

also present whenever the binary message CH1 DISTURBANCE is reported and a

discrepancy is detected between Ug1ampUg2 The generation of the alarm ldquoUg2 Usyn2

FAILURErdquo is analogous to that for Channel 1 The ldquosecondary monitoringrdquo compares

Ug1 with Usyn1 amp Ug2 with Usyn2 This is enabled whenever the binary message of the

comparison channel reports a malfunction or whenever both binary messages report no

malfunction - but both secondary monitors report a malfunction As long as the secondary

monitoring is blocked the differences Ug1neUsyn1 or Ug2neUsyn2 trigger malfunction

signals for the measurement channel involved (suspicion that there is a corresponding

error in Usyn) The measurement channel malfunctions are enabled operationally

whenever after excitation has been switched on generator voltage Ug exceeds the value

set on F204 The voltages Ug1 and Ug2 are checked 16 seconds after the excitation is

switched off to see that they do not exceed the limit value F210 that applies to both of

them At the same time g1Ug2Usyn1Usyn2 are checked for extreme values (gt or lt

the operating range) Monitoring for extreme values is likewise enabled during normal

operation ( Excitation ON and Ug gt F204 ) Flag F754 is used to enable or block the

malfunction signals to the binary outputs Basically the monitoring of the actual values

for If1If2 functions like that of the Ug1Ug2 monitoring

62 Excitation Protection

621 General Information

The Excitation Protection switches off the excitation (and de-excites the machine

rapidly) whenever a danger arises that threatens the excitation transformer the converter

or the generator Generally limiter or monitoring functions precede the emergency trips

and these normally respond before the Excitation Protection must initiate a trip

Protective trip commands are issued directly to the field circuit-breaker from potential

free contacts of the board via the trip relays They are directed redundantly to the

operative field circuit-breaker ldquoOFFrdquo command

622 Protection against Excitation Transformer Overheating

This equipment monitors the excitation transformer for overheating in the

windings that could result from over current short-circuits or inadequate cooling The

monitoring uses temperature monitoring modules in conjunction with temperature

sensors built into the windings Normally the temperature is monitored in two stages the

first stage sets off an alarm the second causes a trip of the excitation

623 Rotor Over voltage Protection

Malfunctions in the generator circuit (eg terminal short-circuit failed

synchronization asynchronous operation) cause induced negative field currents that

produce high voltages in the field circuit These must be restricted to a level with a

sufficient safety margin below the insulation capacity of the field winding (test voltage)

and also below the peak blocking voltage of the converter thyristors The crow bar

employs spark gap elements to detect over voltages in the field circuit Whenever they

respond the associated thyristors are fired immediately switching the de-excitation

resistor parallel to the field The de-excitation current generated thereby initiates an

excitation trip via a supervision circuit causing an immediate opening of the field circuit-

breaker The malfunction isets off an alarm and an internal malfunction is indicated at the

cubicle

TEST VALUES OBTAINED WHEN EXCITATION IS RAISED

TEST

SNO PARTICULARS ACTUAL VALUE OBTAINED VALUE

1 VREF 996 100

2 VACT 997 1003

3 IFACT 735 765

4 IGACT 703 707

5 ACTIVE

POWER

703 705

6 REACTIVE

POWER

101 142

7 POWER

FACTOR

099 IND 098 IND

8 ACTIVE

CURRENT(IR)

705 703

9 REACTIVE

CURRENT(IX)

102 138

10 POWER

ANGLE

566 546

11 FIRING

ANGLE

640 632

VALUES OBTAINED WHEN EXCITATION IS LOWERED

SNO PARTICULARS ACTUAL

VALUE

OBTAINED VALUE

1 VREF 100 997

2 VACT 100 997

3 IFACT 787 761

4 IGACT 839 837

5 ACTIVE

POWER

839 830

6 REACTIVE

POWER

155 110

7 POWER

FACTOR

098 IND 099 IND

8 ACTIVE

CURRENT(IR)

835 830

9 REACTIVE

CURRENT(IX)

140 94

10 POWER

ANGLE

613 DEG 631 DEG

11 FIRING

ANGLE

601 DEG 599 DEG

CONCLUSION

For generating the EMF in stator winding excitation is required to the rotor of a

generator There are two types of excitation

1 Static excitation system

2 Brushless excitation system

A certain disadvantage in brushless excitation system is the slow response time of

the field in case of fast load changes specified No slip-rings and brushes direct

measurements of the field parameters not possible

To avoid all loses static excitation is used Since it does not have any rotating

parts mechanical loses and windage loses This system has fast response and speed

control While preferring this excitation system there are no limitations for the

redundancy of Thyristor bridge circuits

Static excitation has fast field discharge by resistor and inverter operation direct

measurement of field quantity is possible The meaning of excitation is nothing but

continuous supply of DC current (ie field current) to the rotor to buildup required

output voltage in the stator

Field current is changed with respect to the change of load so the digital

automatic voltage regulator (DAVR) is used to regulate the output voltage according to

the load variations

So we conclude that static excitation system with DAVR is preferred since it is

having excellent dynamic performance and better options for R amp M

Because of the following advantages thyristor converters are used

a) Thyristors are used for high power applications ie up to 10Kv3500A1KHz

b) Having high reliability and low losses

c) Uni-directional device like diode

d) Itrsquos operation as a rectifier which are low resistance in forward conduction

mode and high resistance in reverse conduction mode

PROTECTION OF THYRISTORS

For reliable operation of a thyristor demands that its specified ratings are not

exceeded When Subjected to or over voltages During the turn - on of SCR didt

prohibitively large False triggering of SCR by high value of dvdt andSpurious signals

between gate and cathode may leads to unwanted turn ndash on

DIDT AND PROTECTION

When thyristor starts conducting in forward conduction mode and is turned on by

gate pulse The anode current increases rapidly whole area of the gate to Cathode

junction then hot spots will be formed near the gate connection this locality of heating

destroys the thyristor Thyristor thermal time is constant The causes due to faults and

short circuits or surge currents Electronic crowbar protection is used against the over

voltages The rate rise of anode current must be kept at the time of turn on below the

rated or specified limiting value The didt value maintained below limited value by using

a inductor also called ldquodidt inductorrdquo in series with anode circuit The locality of heating

is avoided by applying gate current but not greater the maximum gate current

DVDT AND OVER VOLTAGE PROTECTION

With forward voltage across the anode and cathode of a thyristor the two outer

junctions are forward biased but the inner junction is reverse biased This reverse biased

junction J2 has the characteristics of a capacitor due to charges existing across the

junction In other words space-charges exist in the depletion region around junction J2

and therefore junction J2 behaves like a capacitance If the entire anode to cathode

forward voltage Va appears across J2 junction and the charge is denoted by Q then a

charging current i given by Eq (46) follows

i = dQdt =d(Cj Va )dt

= Cj (d Va dt) + Va(d Cj dt) helliphelliphelliphellip(46 a)

As Cj the capacitance of junction J2 is almost constant the current is given by

i = Cj (d Va dt) helliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip(46 b)

If the rate of rise of forward voltage dVadt is high the charging current i will be

more This charging current plays the role of gate current and turns on the SCR even

when gate signal is zero Such phenomena of turning-on a thyristor called dvdt turn-on

must be avoided as it leads to false operation of the thyristor circuit

For controllable operation of the thyristor the rate of rise of forward anode to

cathode voltage dVadt must be kept below the specified rated limit Typical values of

dvdt are 20 ndash 500 Vμsec False turn-on of a thyristor by large dvdt can be prevented by

using a snubber circuit in parallel with the device thyristor are very sensitive for over

voltage than the semiconductor devices

Over voltage transients are perhaps the main cause of thyristor failure

In thyristor there are mainly two types

1 Internal over voltages

Due to the commutation of the thyristors large voltages are generated internally

Because of the series inductance of the SCR circuit the large transient voltages L didt

produced This voltage several times the break over voltage of the device then thyristor

destroys permanently

2 External over voltages

External over voltages are caused due to the interruptions of current flow in an

inductive circuit and also due to the lightening strokes on the lines feeding the thyristor

system For the reliable operation of thyristor the over voltages must be suppressed by

adopting suitable techniques

Suppression of over voltages

The RC circuit called snubber circuit is connected across the device to protect In

order to keep the protective components to a minimum the thyristors are chosen with

their peak voltages ratings are 25 to 3 times of the normal peak working voltage

ldquoselenium thyrector diodes metal oxide varistors or avalanche diode suppressers are

commonly employed for protecting the thyristor circuit against the over voltages

Gate protection

Gate circuit should also be protected against the over voltages and surges Over

voltage at gate circuit can cause false triggering of the SCR may rises the junction

temperature behind specified limit leading to its damage Protection against over

voltage can be achieved by connecting a ZD across the gate circuit and a resister is

connected in series with gate circuit to protect against the s A capacitor and resister are

connected across gate to cathode to by pass the noise

41 Final Pulse Stages

The Final Pulse Stages adapt the output pulses from the Intermediate Pulse Stage

(pulses on the pulse bus) to the gate currents needed for the thyristors Each thyristor

bridge is equipped with its own Final Pulse Stage Each Final Pulse Stages is provided

with a power supply module The amplified output pulses from the Final Pulse Stages

start as a short strong steep pulse with an amplitude approx 2frac12 times that of the main

pulse This initial pulse edge assures proper firing of the thyristors being triggered

Subsequently the weaker part of main pulse keeps firing conditions steady As already

mentioned the Final Pulse Stages and their associated thyristor bridges form single units

All six pulse outputs from a Final Pulse Stage can be blocked by an external control

signal so that all thyristors in the associated thyristor bridge will block the current A

blocking of the pulses is initiated whenever there is a malfunction in the associated

thyristor bridge

42 Converter Power Section

The thyristor converter consists of three independent parallel rectifier blocks TY1

to TY3 which are all in service Even if one block fails the remaining blocks take over

automatically the full design current of the excitation circuit During normal operation

(with ideal current share) and all three bridges in operation each of these blocks has to

carry only (n-2)n (ie33)of its design current If 2 thyristor bridges fail the excitation

is limited Only when all three bridges fail the excitation is switched off Each thyristor

bridge arm is equipped with current flow monitoring CTrsquos Failure of conduction in any

arm is identified by a Current flow monitoring module

43 Converter Cooling

A cooling system is needed to dissipate heat losses in the converter blocks and

electronics Each converter block has therefore been equipped with a fan supplied with

power from the converterrsquos primary voltage (via transformer ndashT8 in field flashing

cubicle) The fans are protected with motor protection circuit breakers An air flow

monitoring unit is provided for monitoring the air flow through the thyristor bridge If a

circuit breaker failure is detected or if the air flow monitor drops off at one of the

thyristor bridges the bridge involved is immediately set out of operation by blocking its

firing pulses

44 Thyristor Converter Monitoring

A thyristor bridge in which defects occur that could threaten the safety of

operation or cause secondary damage is switched off automatically ie its firing pulses

are blocked This happens whenever A thyristor fuse is blown The fuses are monitored

individually with micro switches The Final Pulse Stage fails which is detected by

internal monitors (supply voltage sustained pulse short-circuit on the output end) The

power supply to the fan fails fan air flow as monitored by the Air flow monitor fails or

is insufficient Isolator on ACDC side is open

CHAPTER 5

Field Current Circuit Excitation Transformer

51 Field Circuit Breaker

The circuit-breaker in the field circuit is used to isolate the field circuit from the

converter It is capable of switching off the synchronous machine from full load under the

maximum conditions of a 3-phase short-circuit In addition to its main contacts the field

circuit-breaker also has a de-excitation contact with which the field energy stored in the

field can be dissipated across the de-excitation resistor The de-excitation contact closes

shortly before the main contacts open so as to ensure proper commutation of the field

current from the main contacts to the de-excitation contact when the breaker is switched

off The field circuit-breaker is switched on by electromagnetic force and is kept switched

on by a mechanical latch When the latch is released by a trip coil the circuit-breaker

opens The circuit-breaker also has auxiliary contacts that report its status

52 Field Flashing

In shunt supplied excitation circuits (excitation transformer connected to the

generator terminals) the generator does not have enough remnant voltage for a generator

voltage build-up via the converter In this case a field flashing circuit is provided It

consists of the field flashing contactor the diode bridge and a transformer used to adapt

the auxiliary input voltage to the voltage needed for field flashing when power is

supplied from the auxiliaries network

Fig Field Flashing

Because the field flashing contactor is not able to switch off the energy stored in

the field the control ensures that the contactor can only reopen if the field circuit breaker

has already been opened (generating the TRIP order) or in a normal field flashing

sequence when the converter has taken over the field current Field flashing occurs in the

following stages

1048729The excitation is switched on closing the field flashing contactor ( Field

Circuit Breaker is already closed )

1048729The start-up excitation current flows through the rotor driving the generator

voltage up to approx 15 U

1048729After about 10 U the firing pulses to the converter are released and it begins

to excite the generator to its rated voltage

1048729After about 30 U the field flashing contactor opens (with no current since

the converter is now supplying the current)

The diode bridge at the input to the field flashing contactor prevents a feed-back

from the converter to the source of field flashing while the contactor is still closed

53 De-excitation

When malfunctions occur the stored field energy must be dissipated as quickly

and safely as possible to protect the generator This is done by the converter the field

circuit-breaker and the de-excitation (discharge) resistor

De-excitation (with opening of the field circuit-breaker) takes place in the following

stages

1048729The converter drives to its inverter limit position (negative ceiling voltage)

recovers a portion of the field energy into the network A trip command is given to the

field circuit breaker

1048729The de-excitation contact closes diverting the field voltage to the de-excitation

resistor

1048729Then immediately the main contacts open building voltage The field voltage

commutates to the de-excitation resistor

1048729The current diminishes at a given time constant TE

(With linear resistance TE = Lf (Rf + Re))

Due to the reversal of the field voltage by the converter the field current

commutates from the main contacts of the field circuit-breaker to the de-excitation

resistor in a very early phase This reversal of the field voltage prevents burn-off on the

main contacts and provides effective protection for the field circuit-breaker Depending

on the operating policy an operational shut-down of the excitation can also be effected

with the field circuit-breaker closed This method is useful mainly when the excitation is

switched on and off frequently In this case the converter is merely driven into the

inverter limit position so that the field energy is recovered into the network The

converter then blocks since it is supplying positive current only

54 Excitation Transformer

The excitation transformer matches the generator voltage to the field voltage

(required ceiling voltage) It also serves as a commutation reactance for the thyristor

converter and as a potential isolator between the network and the excitation circuit In

addition the transformer functions as a current limiter in that it makes it possible to keep

any short circuits in the excitation circuit under better control The excitation transformer

is equipped with temperature monitoring probes which set off an alarm when the

temperature exceeds a first max limit and then trips the excitation if the temperature

continues rising to a second (higher)limit

CHAPTER 6

Monitoring and Protection

61 Excitation Monitoring

611 General Information

The main goal of Excitation Monitoring is to make optimum use of the

redundancies provided in the excitation system and to give alarm whenever a malfunction

makes these redundancies unavailable The field current is monitored to see that it does

not exceed a maximum level and if necessary a switch-over to the MANUAL channel is

initiated In addition the criterion for switching off the field flashing is generated The

excitation Monitoring consists of an autonomous processor system

612 over current Alarms

In the Excitation Monitoring the limits for are set at higher levels than the

settings on the Field Current Maximum Limiter Whenever the current exceeds 110 of

the nominal field current contact R1 and the binary output associated with it are

activated immediately If field current remains gt 110 then after a preset inverse-time

has lapsed relay R2 and - after a further delay - relay R0 and the binary outputs

associated with them are activated Parameters match the measurements for If1 and If2 to

the nominal value for field current so that the internal values can be processed and read as

pu values It can be used to falsify the actual value of the field current If (to raise it) so

as to cause a response from the alarm limits for purposes of testing The processed If

signal is always taken from on the active channel (CHANNEL I OR CHANNEL II) As

long as the field current If is above the threshold value 11 Ifn its peak value is

measured This is stored (until RESET) and can be read at any time on the Micro-

Terminal Once the value of If exceeds 11 Ifn integration of this value starts Whenever

the integrated time-current value (intisup2dt) exceeds the preselected reference value the

alarm OVER CURRENT INVERSE-TIME is set off and a command is simultaneously

issued to switch over to the stand by AUTO channel Software switch F758 enables the

three over current alarm functions (R0 R1 R2) and selects one of three possible inverse-

time curves T1 T2 or T3 Within the characteristic curve (T1 T2 T3) selected the

desired limit curve for response is set using the factor F216

613 Switch-Off Criterion for Field Flashing

The Excitation Monitoring supplies the criterion for switching off the field

flashing Whether this criterion is activated based on the actual value for generator voltage

Ug or for field current If or both depends on the settings of the two threshold values

F200F202 (0 setting means that the output is always ldquological 1rdquo) The measurements

Ug12 and If12 are switched over depending on the present status of the channels

(Channel 1 or Channel 2 ON) Whenever Flag F750 is not activated the binary output is

fixed at ldquological 1rdquo

614 Storage of Alarm Status

The outputs of the over current alarms (R1 R2 R0) and the messages NO

FAILURE MONITORING PARAMETERS CHANGED are stored messages can be

erased by activating the input ldquoGENERAL RESETrdquo or by using the RESET button on the

front of the module Erasure with the input ldquoGENERAL RESETrdquo is effective only if the

situation causing the alarm or the malfunction is no longer present Whenever the self-

diagnosis equipment in the processor detects a malfunction the output NO FAILURE of

MONITORING is set at ldquological 0rdquo (= alarm) The alarm ldquoPARAMETERS CHANGEDrdquo

is activated whenever parameters or settings of software switches have been changed via

the Micro-Terminal

615 Actual Value Monitoring

The actual values for generator voltage Ug synchronous voltage Usyn and field

current If are monitored for malfunctions This monitoring is active regardless of whether

or not the generator is in operation Essentially when the generator is in operation the

measurements are monitored by comparing the signals (the smaller signal reading is

detected as incorrect) When the generator is not in operation the measured data are

monitored for extreme values The percentage of deviation permissible in the

measurement signals being compared

(Ug1ampUsyn1 Ug1ampUg2 Ug2ampUsyn2 If1ampIf2) is defined by parameters F208 and

F20AIf the excitation transformer is being supplied from an auxiliary power source (no

shunt operation) the values of Ug and Usyn will be different in some operational

conditions

In that case Flag F75A can be used to deactivate comparative measurements Ug1 Usyn1

and Ug2 Usyn2 The ldquoprimary monitoringrdquo compares Ug1 with Ug2 and generates the

messages that Channel 1 or Channel 2 has suffered a malfunction Whenever Ug1 lt Ug2

and the binary message from CH1 reports no malfunction a malfunction on

Measurement Channel 1 is reported (Ug1Usyn1 FAILURE) A similar malfunction is

also present whenever the binary message CH1 DISTURBANCE is reported and a

discrepancy is detected between Ug1ampUg2 The generation of the alarm ldquoUg2 Usyn2

FAILURErdquo is analogous to that for Channel 1 The ldquosecondary monitoringrdquo compares

Ug1 with Usyn1 amp Ug2 with Usyn2 This is enabled whenever the binary message of the

comparison channel reports a malfunction or whenever both binary messages report no

malfunction - but both secondary monitors report a malfunction As long as the secondary

monitoring is blocked the differences Ug1neUsyn1 or Ug2neUsyn2 trigger malfunction

signals for the measurement channel involved (suspicion that there is a corresponding

error in Usyn) The measurement channel malfunctions are enabled operationally

whenever after excitation has been switched on generator voltage Ug exceeds the value

set on F204 The voltages Ug1 and Ug2 are checked 16 seconds after the excitation is

switched off to see that they do not exceed the limit value F210 that applies to both of

them At the same time g1Ug2Usyn1Usyn2 are checked for extreme values (gt or lt

the operating range) Monitoring for extreme values is likewise enabled during normal

operation ( Excitation ON and Ug gt F204 ) Flag F754 is used to enable or block the

malfunction signals to the binary outputs Basically the monitoring of the actual values

for If1If2 functions like that of the Ug1Ug2 monitoring

62 Excitation Protection

621 General Information

The Excitation Protection switches off the excitation (and de-excites the machine

rapidly) whenever a danger arises that threatens the excitation transformer the converter

or the generator Generally limiter or monitoring functions precede the emergency trips

and these normally respond before the Excitation Protection must initiate a trip

Protective trip commands are issued directly to the field circuit-breaker from potential

free contacts of the board via the trip relays They are directed redundantly to the

operative field circuit-breaker ldquoOFFrdquo command

622 Protection against Excitation Transformer Overheating

This equipment monitors the excitation transformer for overheating in the

windings that could result from over current short-circuits or inadequate cooling The

monitoring uses temperature monitoring modules in conjunction with temperature

sensors built into the windings Normally the temperature is monitored in two stages the

first stage sets off an alarm the second causes a trip of the excitation

623 Rotor Over voltage Protection

Malfunctions in the generator circuit (eg terminal short-circuit failed

synchronization asynchronous operation) cause induced negative field currents that

produce high voltages in the field circuit These must be restricted to a level with a

sufficient safety margin below the insulation capacity of the field winding (test voltage)

and also below the peak blocking voltage of the converter thyristors The crow bar

employs spark gap elements to detect over voltages in the field circuit Whenever they

respond the associated thyristors are fired immediately switching the de-excitation

resistor parallel to the field The de-excitation current generated thereby initiates an

excitation trip via a supervision circuit causing an immediate opening of the field circuit-

breaker The malfunction isets off an alarm and an internal malfunction is indicated at the

cubicle

TEST VALUES OBTAINED WHEN EXCITATION IS RAISED

TEST

SNO PARTICULARS ACTUAL VALUE OBTAINED VALUE

1 VREF 996 100

2 VACT 997 1003

3 IFACT 735 765

4 IGACT 703 707

5 ACTIVE

POWER

703 705

6 REACTIVE

POWER

101 142

7 POWER

FACTOR

099 IND 098 IND

8 ACTIVE

CURRENT(IR)

705 703

9 REACTIVE

CURRENT(IX)

102 138

10 POWER

ANGLE

566 546

11 FIRING

ANGLE

640 632

VALUES OBTAINED WHEN EXCITATION IS LOWERED

SNO PARTICULARS ACTUAL

VALUE

OBTAINED VALUE

1 VREF 100 997

2 VACT 100 997

3 IFACT 787 761

4 IGACT 839 837

5 ACTIVE

POWER

839 830

6 REACTIVE

POWER

155 110

7 POWER

FACTOR

098 IND 099 IND

8 ACTIVE

CURRENT(IR)

835 830

9 REACTIVE

CURRENT(IX)

140 94

10 POWER

ANGLE

613 DEG 631 DEG

11 FIRING

ANGLE

601 DEG 599 DEG

CONCLUSION

For generating the EMF in stator winding excitation is required to the rotor of a

generator There are two types of excitation

1 Static excitation system

2 Brushless excitation system

A certain disadvantage in brushless excitation system is the slow response time of

the field in case of fast load changes specified No slip-rings and brushes direct

measurements of the field parameters not possible

To avoid all loses static excitation is used Since it does not have any rotating

parts mechanical loses and windage loses This system has fast response and speed

control While preferring this excitation system there are no limitations for the

redundancy of Thyristor bridge circuits

Static excitation has fast field discharge by resistor and inverter operation direct

measurement of field quantity is possible The meaning of excitation is nothing but

continuous supply of DC current (ie field current) to the rotor to buildup required

output voltage in the stator

Field current is changed with respect to the change of load so the digital

automatic voltage regulator (DAVR) is used to regulate the output voltage according to

the load variations

So we conclude that static excitation system with DAVR is preferred since it is

having excellent dynamic performance and better options for R amp M

destroys the thyristor Thyristor thermal time is constant The causes due to faults and

short circuits or surge currents Electronic crowbar protection is used against the over

voltages The rate rise of anode current must be kept at the time of turn on below the

rated or specified limiting value The didt value maintained below limited value by using

a inductor also called ldquodidt inductorrdquo in series with anode circuit The locality of heating

is avoided by applying gate current but not greater the maximum gate current

DVDT AND OVER VOLTAGE PROTECTION

With forward voltage across the anode and cathode of a thyristor the two outer

junctions are forward biased but the inner junction is reverse biased This reverse biased

junction J2 has the characteristics of a capacitor due to charges existing across the

junction In other words space-charges exist in the depletion region around junction J2

and therefore junction J2 behaves like a capacitance If the entire anode to cathode

forward voltage Va appears across J2 junction and the charge is denoted by Q then a

charging current i given by Eq (46) follows

i = dQdt =d(Cj Va )dt

= Cj (d Va dt) + Va(d Cj dt) helliphelliphelliphellip(46 a)

As Cj the capacitance of junction J2 is almost constant the current is given by

i = Cj (d Va dt) helliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip(46 b)

If the rate of rise of forward voltage dVadt is high the charging current i will be

more This charging current plays the role of gate current and turns on the SCR even

when gate signal is zero Such phenomena of turning-on a thyristor called dvdt turn-on

must be avoided as it leads to false operation of the thyristor circuit

For controllable operation of the thyristor the rate of rise of forward anode to

cathode voltage dVadt must be kept below the specified rated limit Typical values of

dvdt are 20 ndash 500 Vμsec False turn-on of a thyristor by large dvdt can be prevented by

using a snubber circuit in parallel with the device thyristor are very sensitive for over

voltage than the semiconductor devices

Over voltage transients are perhaps the main cause of thyristor failure

In thyristor there are mainly two types

1 Internal over voltages

Due to the commutation of the thyristors large voltages are generated internally

Because of the series inductance of the SCR circuit the large transient voltages L didt

produced This voltage several times the break over voltage of the device then thyristor

destroys permanently

2 External over voltages

External over voltages are caused due to the interruptions of current flow in an

inductive circuit and also due to the lightening strokes on the lines feeding the thyristor

system For the reliable operation of thyristor the over voltages must be suppressed by

adopting suitable techniques

Suppression of over voltages

The RC circuit called snubber circuit is connected across the device to protect In

order to keep the protective components to a minimum the thyristors are chosen with

their peak voltages ratings are 25 to 3 times of the normal peak working voltage

ldquoselenium thyrector diodes metal oxide varistors or avalanche diode suppressers are

commonly employed for protecting the thyristor circuit against the over voltages

Gate protection

Gate circuit should also be protected against the over voltages and surges Over

voltage at gate circuit can cause false triggering of the SCR may rises the junction

temperature behind specified limit leading to its damage Protection against over

voltage can be achieved by connecting a ZD across the gate circuit and a resister is

connected in series with gate circuit to protect against the s A capacitor and resister are

connected across gate to cathode to by pass the noise

41 Final Pulse Stages

The Final Pulse Stages adapt the output pulses from the Intermediate Pulse Stage

(pulses on the pulse bus) to the gate currents needed for the thyristors Each thyristor

bridge is equipped with its own Final Pulse Stage Each Final Pulse Stages is provided

with a power supply module The amplified output pulses from the Final Pulse Stages

start as a short strong steep pulse with an amplitude approx 2frac12 times that of the main

pulse This initial pulse edge assures proper firing of the thyristors being triggered

Subsequently the weaker part of main pulse keeps firing conditions steady As already

mentioned the Final Pulse Stages and their associated thyristor bridges form single units

All six pulse outputs from a Final Pulse Stage can be blocked by an external control

signal so that all thyristors in the associated thyristor bridge will block the current A

blocking of the pulses is initiated whenever there is a malfunction in the associated

thyristor bridge

42 Converter Power Section

The thyristor converter consists of three independent parallel rectifier blocks TY1

to TY3 which are all in service Even if one block fails the remaining blocks take over

automatically the full design current of the excitation circuit During normal operation

(with ideal current share) and all three bridges in operation each of these blocks has to

carry only (n-2)n (ie33)of its design current If 2 thyristor bridges fail the excitation

is limited Only when all three bridges fail the excitation is switched off Each thyristor

bridge arm is equipped with current flow monitoring CTrsquos Failure of conduction in any

arm is identified by a Current flow monitoring module

43 Converter Cooling

A cooling system is needed to dissipate heat losses in the converter blocks and

electronics Each converter block has therefore been equipped with a fan supplied with

power from the converterrsquos primary voltage (via transformer ndashT8 in field flashing

cubicle) The fans are protected with motor protection circuit breakers An air flow

monitoring unit is provided for monitoring the air flow through the thyristor bridge If a

circuit breaker failure is detected or if the air flow monitor drops off at one of the

thyristor bridges the bridge involved is immediately set out of operation by blocking its

firing pulses

44 Thyristor Converter Monitoring

A thyristor bridge in which defects occur that could threaten the safety of

operation or cause secondary damage is switched off automatically ie its firing pulses

are blocked This happens whenever A thyristor fuse is blown The fuses are monitored

individually with micro switches The Final Pulse Stage fails which is detected by

internal monitors (supply voltage sustained pulse short-circuit on the output end) The

power supply to the fan fails fan air flow as monitored by the Air flow monitor fails or

is insufficient Isolator on ACDC side is open

CHAPTER 5

Field Current Circuit Excitation Transformer

51 Field Circuit Breaker

The circuit-breaker in the field circuit is used to isolate the field circuit from the

converter It is capable of switching off the synchronous machine from full load under the

maximum conditions of a 3-phase short-circuit In addition to its main contacts the field

circuit-breaker also has a de-excitation contact with which the field energy stored in the

field can be dissipated across the de-excitation resistor The de-excitation contact closes

shortly before the main contacts open so as to ensure proper commutation of the field

current from the main contacts to the de-excitation contact when the breaker is switched

off The field circuit-breaker is switched on by electromagnetic force and is kept switched

on by a mechanical latch When the latch is released by a trip coil the circuit-breaker

opens The circuit-breaker also has auxiliary contacts that report its status

52 Field Flashing

In shunt supplied excitation circuits (excitation transformer connected to the

generator terminals) the generator does not have enough remnant voltage for a generator

voltage build-up via the converter In this case a field flashing circuit is provided It

consists of the field flashing contactor the diode bridge and a transformer used to adapt

the auxiliary input voltage to the voltage needed for field flashing when power is

supplied from the auxiliaries network

Fig Field Flashing

Because the field flashing contactor is not able to switch off the energy stored in

the field the control ensures that the contactor can only reopen if the field circuit breaker

has already been opened (generating the TRIP order) or in a normal field flashing

sequence when the converter has taken over the field current Field flashing occurs in the

following stages

1048729The excitation is switched on closing the field flashing contactor ( Field

Circuit Breaker is already closed )

1048729The start-up excitation current flows through the rotor driving the generator

voltage up to approx 15 U

1048729After about 10 U the firing pulses to the converter are released and it begins

to excite the generator to its rated voltage

1048729After about 30 U the field flashing contactor opens (with no current since

the converter is now supplying the current)

The diode bridge at the input to the field flashing contactor prevents a feed-back

from the converter to the source of field flashing while the contactor is still closed

53 De-excitation

When malfunctions occur the stored field energy must be dissipated as quickly

and safely as possible to protect the generator This is done by the converter the field

circuit-breaker and the de-excitation (discharge) resistor

De-excitation (with opening of the field circuit-breaker) takes place in the following

stages

1048729The converter drives to its inverter limit position (negative ceiling voltage)

recovers a portion of the field energy into the network A trip command is given to the

field circuit breaker

1048729The de-excitation contact closes diverting the field voltage to the de-excitation

resistor

1048729Then immediately the main contacts open building voltage The field voltage

commutates to the de-excitation resistor

1048729The current diminishes at a given time constant TE

(With linear resistance TE = Lf (Rf + Re))

Due to the reversal of the field voltage by the converter the field current

commutates from the main contacts of the field circuit-breaker to the de-excitation

resistor in a very early phase This reversal of the field voltage prevents burn-off on the

main contacts and provides effective protection for the field circuit-breaker Depending

on the operating policy an operational shut-down of the excitation can also be effected

with the field circuit-breaker closed This method is useful mainly when the excitation is

switched on and off frequently In this case the converter is merely driven into the

inverter limit position so that the field energy is recovered into the network The

converter then blocks since it is supplying positive current only

54 Excitation Transformer

The excitation transformer matches the generator voltage to the field voltage

(required ceiling voltage) It also serves as a commutation reactance for the thyristor

converter and as a potential isolator between the network and the excitation circuit In

addition the transformer functions as a current limiter in that it makes it possible to keep

any short circuits in the excitation circuit under better control The excitation transformer

is equipped with temperature monitoring probes which set off an alarm when the

temperature exceeds a first max limit and then trips the excitation if the temperature

continues rising to a second (higher)limit

CHAPTER 6

Monitoring and Protection

61 Excitation Monitoring

611 General Information

The main goal of Excitation Monitoring is to make optimum use of the

redundancies provided in the excitation system and to give alarm whenever a malfunction

makes these redundancies unavailable The field current is monitored to see that it does

not exceed a maximum level and if necessary a switch-over to the MANUAL channel is

initiated In addition the criterion for switching off the field flashing is generated The

excitation Monitoring consists of an autonomous processor system

612 over current Alarms

In the Excitation Monitoring the limits for are set at higher levels than the

settings on the Field Current Maximum Limiter Whenever the current exceeds 110 of

the nominal field current contact R1 and the binary output associated with it are

activated immediately If field current remains gt 110 then after a preset inverse-time

has lapsed relay R2 and - after a further delay - relay R0 and the binary outputs

associated with them are activated Parameters match the measurements for If1 and If2 to

the nominal value for field current so that the internal values can be processed and read as

pu values It can be used to falsify the actual value of the field current If (to raise it) so

as to cause a response from the alarm limits for purposes of testing The processed If

signal is always taken from on the active channel (CHANNEL I OR CHANNEL II) As

long as the field current If is above the threshold value 11 Ifn its peak value is

measured This is stored (until RESET) and can be read at any time on the Micro-

Terminal Once the value of If exceeds 11 Ifn integration of this value starts Whenever

the integrated time-current value (intisup2dt) exceeds the preselected reference value the

alarm OVER CURRENT INVERSE-TIME is set off and a command is simultaneously

issued to switch over to the stand by AUTO channel Software switch F758 enables the

three over current alarm functions (R0 R1 R2) and selects one of three possible inverse-

time curves T1 T2 or T3 Within the characteristic curve (T1 T2 T3) selected the

desired limit curve for response is set using the factor F216

613 Switch-Off Criterion for Field Flashing

The Excitation Monitoring supplies the criterion for switching off the field

flashing Whether this criterion is activated based on the actual value for generator voltage

Ug or for field current If or both depends on the settings of the two threshold values

F200F202 (0 setting means that the output is always ldquological 1rdquo) The measurements

Ug12 and If12 are switched over depending on the present status of the channels

(Channel 1 or Channel 2 ON) Whenever Flag F750 is not activated the binary output is

fixed at ldquological 1rdquo

614 Storage of Alarm Status

The outputs of the over current alarms (R1 R2 R0) and the messages NO

FAILURE MONITORING PARAMETERS CHANGED are stored messages can be

erased by activating the input ldquoGENERAL RESETrdquo or by using the RESET button on the

front of the module Erasure with the input ldquoGENERAL RESETrdquo is effective only if the

situation causing the alarm or the malfunction is no longer present Whenever the self-

diagnosis equipment in the processor detects a malfunction the output NO FAILURE of

MONITORING is set at ldquological 0rdquo (= alarm) The alarm ldquoPARAMETERS CHANGEDrdquo

is activated whenever parameters or settings of software switches have been changed via

the Micro-Terminal

615 Actual Value Monitoring

The actual values for generator voltage Ug synchronous voltage Usyn and field

current If are monitored for malfunctions This monitoring is active regardless of whether

or not the generator is in operation Essentially when the generator is in operation the

measurements are monitored by comparing the signals (the smaller signal reading is

detected as incorrect) When the generator is not in operation the measured data are

monitored for extreme values The percentage of deviation permissible in the

measurement signals being compared

(Ug1ampUsyn1 Ug1ampUg2 Ug2ampUsyn2 If1ampIf2) is defined by parameters F208 and

F20AIf the excitation transformer is being supplied from an auxiliary power source (no

shunt operation) the values of Ug and Usyn will be different in some operational

conditions

In that case Flag F75A can be used to deactivate comparative measurements Ug1 Usyn1

and Ug2 Usyn2 The ldquoprimary monitoringrdquo compares Ug1 with Ug2 and generates the

messages that Channel 1 or Channel 2 has suffered a malfunction Whenever Ug1 lt Ug2

and the binary message from CH1 reports no malfunction a malfunction on

Measurement Channel 1 is reported (Ug1Usyn1 FAILURE) A similar malfunction is

also present whenever the binary message CH1 DISTURBANCE is reported and a

discrepancy is detected between Ug1ampUg2 The generation of the alarm ldquoUg2 Usyn2

FAILURErdquo is analogous to that for Channel 1 The ldquosecondary monitoringrdquo compares

Ug1 with Usyn1 amp Ug2 with Usyn2 This is enabled whenever the binary message of the

comparison channel reports a malfunction or whenever both binary messages report no

malfunction - but both secondary monitors report a malfunction As long as the secondary

monitoring is blocked the differences Ug1neUsyn1 or Ug2neUsyn2 trigger malfunction

signals for the measurement channel involved (suspicion that there is a corresponding

error in Usyn) The measurement channel malfunctions are enabled operationally

whenever after excitation has been switched on generator voltage Ug exceeds the value

set on F204 The voltages Ug1 and Ug2 are checked 16 seconds after the excitation is

switched off to see that they do not exceed the limit value F210 that applies to both of

them At the same time g1Ug2Usyn1Usyn2 are checked for extreme values (gt or lt

the operating range) Monitoring for extreme values is likewise enabled during normal

operation ( Excitation ON and Ug gt F204 ) Flag F754 is used to enable or block the

malfunction signals to the binary outputs Basically the monitoring of the actual values

for If1If2 functions like that of the Ug1Ug2 monitoring

62 Excitation Protection

621 General Information

The Excitation Protection switches off the excitation (and de-excites the machine

rapidly) whenever a danger arises that threatens the excitation transformer the converter

or the generator Generally limiter or monitoring functions precede the emergency trips

and these normally respond before the Excitation Protection must initiate a trip

Protective trip commands are issued directly to the field circuit-breaker from potential

free contacts of the board via the trip relays They are directed redundantly to the

operative field circuit-breaker ldquoOFFrdquo command

622 Protection against Excitation Transformer Overheating

This equipment monitors the excitation transformer for overheating in the

windings that could result from over current short-circuits or inadequate cooling The

monitoring uses temperature monitoring modules in conjunction with temperature

sensors built into the windings Normally the temperature is monitored in two stages the

first stage sets off an alarm the second causes a trip of the excitation

623 Rotor Over voltage Protection

Malfunctions in the generator circuit (eg terminal short-circuit failed

synchronization asynchronous operation) cause induced negative field currents that

produce high voltages in the field circuit These must be restricted to a level with a

sufficient safety margin below the insulation capacity of the field winding (test voltage)

and also below the peak blocking voltage of the converter thyristors The crow bar

employs spark gap elements to detect over voltages in the field circuit Whenever they

respond the associated thyristors are fired immediately switching the de-excitation

resistor parallel to the field The de-excitation current generated thereby initiates an

excitation trip via a supervision circuit causing an immediate opening of the field circuit-

breaker The malfunction isets off an alarm and an internal malfunction is indicated at the

cubicle

TEST VALUES OBTAINED WHEN EXCITATION IS RAISED

TEST

SNO PARTICULARS ACTUAL VALUE OBTAINED VALUE

1 VREF 996 100

2 VACT 997 1003

3 IFACT 735 765

4 IGACT 703 707

5 ACTIVE

POWER

703 705

6 REACTIVE

POWER

101 142

7 POWER

FACTOR

099 IND 098 IND

8 ACTIVE

CURRENT(IR)

705 703

9 REACTIVE

CURRENT(IX)

102 138

10 POWER

ANGLE

566 546

11 FIRING

ANGLE

640 632

VALUES OBTAINED WHEN EXCITATION IS LOWERED

SNO PARTICULARS ACTUAL

VALUE

OBTAINED VALUE

1 VREF 100 997

2 VACT 100 997

3 IFACT 787 761

4 IGACT 839 837

5 ACTIVE

POWER

839 830

6 REACTIVE

POWER

155 110

7 POWER

FACTOR

098 IND 099 IND

8 ACTIVE

CURRENT(IR)

835 830

9 REACTIVE

CURRENT(IX)

140 94

10 POWER

ANGLE

613 DEG 631 DEG

11 FIRING

ANGLE

601 DEG 599 DEG

CONCLUSION

For generating the EMF in stator winding excitation is required to the rotor of a

generator There are two types of excitation

1 Static excitation system

2 Brushless excitation system

A certain disadvantage in brushless excitation system is the slow response time of

the field in case of fast load changes specified No slip-rings and brushes direct

measurements of the field parameters not possible

To avoid all loses static excitation is used Since it does not have any rotating

parts mechanical loses and windage loses This system has fast response and speed

control While preferring this excitation system there are no limitations for the

redundancy of Thyristor bridge circuits

Static excitation has fast field discharge by resistor and inverter operation direct

measurement of field quantity is possible The meaning of excitation is nothing but

continuous supply of DC current (ie field current) to the rotor to buildup required

output voltage in the stator

Field current is changed with respect to the change of load so the digital

automatic voltage regulator (DAVR) is used to regulate the output voltage according to

the load variations

So we conclude that static excitation system with DAVR is preferred since it is

having excellent dynamic performance and better options for R amp M

produced This voltage several times the break over voltage of the device then thyristor

destroys permanently

2 External over voltages

External over voltages are caused due to the interruptions of current flow in an

inductive circuit and also due to the lightening strokes on the lines feeding the thyristor

system For the reliable operation of thyristor the over voltages must be suppressed by

adopting suitable techniques

Suppression of over voltages

The RC circuit called snubber circuit is connected across the device to protect In

order to keep the protective components to a minimum the thyristors are chosen with

their peak voltages ratings are 25 to 3 times of the normal peak working voltage

ldquoselenium thyrector diodes metal oxide varistors or avalanche diode suppressers are

commonly employed for protecting the thyristor circuit against the over voltages

Gate protection

Gate circuit should also be protected against the over voltages and surges Over

voltage at gate circuit can cause false triggering of the SCR may rises the junction

temperature behind specified limit leading to its damage Protection against over

voltage can be achieved by connecting a ZD across the gate circuit and a resister is

connected in series with gate circuit to protect against the s A capacitor and resister are

connected across gate to cathode to by pass the noise

41 Final Pulse Stages

The Final Pulse Stages adapt the output pulses from the Intermediate Pulse Stage

(pulses on the pulse bus) to the gate currents needed for the thyristors Each thyristor

bridge is equipped with its own Final Pulse Stage Each Final Pulse Stages is provided

with a power supply module The amplified output pulses from the Final Pulse Stages

start as a short strong steep pulse with an amplitude approx 2frac12 times that of the main

pulse This initial pulse edge assures proper firing of the thyristors being triggered

Subsequently the weaker part of main pulse keeps firing conditions steady As already

mentioned the Final Pulse Stages and their associated thyristor bridges form single units

All six pulse outputs from a Final Pulse Stage can be blocked by an external control

signal so that all thyristors in the associated thyristor bridge will block the current A

blocking of the pulses is initiated whenever there is a malfunction in the associated

thyristor bridge

42 Converter Power Section

The thyristor converter consists of three independent parallel rectifier blocks TY1

to TY3 which are all in service Even if one block fails the remaining blocks take over

automatically the full design current of the excitation circuit During normal operation

(with ideal current share) and all three bridges in operation each of these blocks has to

carry only (n-2)n (ie33)of its design current If 2 thyristor bridges fail the excitation

is limited Only when all three bridges fail the excitation is switched off Each thyristor

bridge arm is equipped with current flow monitoring CTrsquos Failure of conduction in any

arm is identified by a Current flow monitoring module

43 Converter Cooling

A cooling system is needed to dissipate heat losses in the converter blocks and

electronics Each converter block has therefore been equipped with a fan supplied with

power from the converterrsquos primary voltage (via transformer ndashT8 in field flashing

cubicle) The fans are protected with motor protection circuit breakers An air flow

monitoring unit is provided for monitoring the air flow through the thyristor bridge If a

circuit breaker failure is detected or if the air flow monitor drops off at one of the

thyristor bridges the bridge involved is immediately set out of operation by blocking its

firing pulses

44 Thyristor Converter Monitoring

A thyristor bridge in which defects occur that could threaten the safety of

operation or cause secondary damage is switched off automatically ie its firing pulses

are blocked This happens whenever A thyristor fuse is blown The fuses are monitored

individually with micro switches The Final Pulse Stage fails which is detected by

internal monitors (supply voltage sustained pulse short-circuit on the output end) The

power supply to the fan fails fan air flow as monitored by the Air flow monitor fails or

is insufficient Isolator on ACDC side is open

CHAPTER 5

Field Current Circuit Excitation Transformer

51 Field Circuit Breaker

The circuit-breaker in the field circuit is used to isolate the field circuit from the

converter It is capable of switching off the synchronous machine from full load under the

maximum conditions of a 3-phase short-circuit In addition to its main contacts the field

circuit-breaker also has a de-excitation contact with which the field energy stored in the

field can be dissipated across the de-excitation resistor The de-excitation contact closes

shortly before the main contacts open so as to ensure proper commutation of the field

current from the main contacts to the de-excitation contact when the breaker is switched

off The field circuit-breaker is switched on by electromagnetic force and is kept switched

on by a mechanical latch When the latch is released by a trip coil the circuit-breaker

opens The circuit-breaker also has auxiliary contacts that report its status

52 Field Flashing

In shunt supplied excitation circuits (excitation transformer connected to the

generator terminals) the generator does not have enough remnant voltage for a generator

voltage build-up via the converter In this case a field flashing circuit is provided It

consists of the field flashing contactor the diode bridge and a transformer used to adapt

the auxiliary input voltage to the voltage needed for field flashing when power is

supplied from the auxiliaries network

Fig Field Flashing

Because the field flashing contactor is not able to switch off the energy stored in

the field the control ensures that the contactor can only reopen if the field circuit breaker

has already been opened (generating the TRIP order) or in a normal field flashing

sequence when the converter has taken over the field current Field flashing occurs in the

following stages

1048729The excitation is switched on closing the field flashing contactor ( Field

Circuit Breaker is already closed )

1048729The start-up excitation current flows through the rotor driving the generator

voltage up to approx 15 U

1048729After about 10 U the firing pulses to the converter are released and it begins

to excite the generator to its rated voltage

1048729After about 30 U the field flashing contactor opens (with no current since

the converter is now supplying the current)

The diode bridge at the input to the field flashing contactor prevents a feed-back

from the converter to the source of field flashing while the contactor is still closed

53 De-excitation

When malfunctions occur the stored field energy must be dissipated as quickly

and safely as possible to protect the generator This is done by the converter the field

circuit-breaker and the de-excitation (discharge) resistor

De-excitation (with opening of the field circuit-breaker) takes place in the following

stages

1048729The converter drives to its inverter limit position (negative ceiling voltage)

recovers a portion of the field energy into the network A trip command is given to the

field circuit breaker

1048729The de-excitation contact closes diverting the field voltage to the de-excitation

resistor

1048729Then immediately the main contacts open building voltage The field voltage

commutates to the de-excitation resistor

1048729The current diminishes at a given time constant TE

(With linear resistance TE = Lf (Rf + Re))

Due to the reversal of the field voltage by the converter the field current

commutates from the main contacts of the field circuit-breaker to the de-excitation

resistor in a very early phase This reversal of the field voltage prevents burn-off on the

main contacts and provides effective protection for the field circuit-breaker Depending

on the operating policy an operational shut-down of the excitation can also be effected

with the field circuit-breaker closed This method is useful mainly when the excitation is

switched on and off frequently In this case the converter is merely driven into the

inverter limit position so that the field energy is recovered into the network The

converter then blocks since it is supplying positive current only

54 Excitation Transformer

The excitation transformer matches the generator voltage to the field voltage

(required ceiling voltage) It also serves as a commutation reactance for the thyristor

converter and as a potential isolator between the network and the excitation circuit In

addition the transformer functions as a current limiter in that it makes it possible to keep

any short circuits in the excitation circuit under better control The excitation transformer

is equipped with temperature monitoring probes which set off an alarm when the

temperature exceeds a first max limit and then trips the excitation if the temperature

continues rising to a second (higher)limit

CHAPTER 6

Monitoring and Protection

61 Excitation Monitoring

611 General Information

The main goal of Excitation Monitoring is to make optimum use of the

redundancies provided in the excitation system and to give alarm whenever a malfunction

makes these redundancies unavailable The field current is monitored to see that it does

not exceed a maximum level and if necessary a switch-over to the MANUAL channel is

initiated In addition the criterion for switching off the field flashing is generated The

excitation Monitoring consists of an autonomous processor system

612 over current Alarms

In the Excitation Monitoring the limits for are set at higher levels than the

settings on the Field Current Maximum Limiter Whenever the current exceeds 110 of

the nominal field current contact R1 and the binary output associated with it are

activated immediately If field current remains gt 110 then after a preset inverse-time

has lapsed relay R2 and - after a further delay - relay R0 and the binary outputs

associated with them are activated Parameters match the measurements for If1 and If2 to

the nominal value for field current so that the internal values can be processed and read as

pu values It can be used to falsify the actual value of the field current If (to raise it) so

as to cause a response from the alarm limits for purposes of testing The processed If

signal is always taken from on the active channel (CHANNEL I OR CHANNEL II) As

long as the field current If is above the threshold value 11 Ifn its peak value is

measured This is stored (until RESET) and can be read at any time on the Micro-

Terminal Once the value of If exceeds 11 Ifn integration of this value starts Whenever

the integrated time-current value (intisup2dt) exceeds the preselected reference value the

alarm OVER CURRENT INVERSE-TIME is set off and a command is simultaneously

issued to switch over to the stand by AUTO channel Software switch F758 enables the

three over current alarm functions (R0 R1 R2) and selects one of three possible inverse-

time curves T1 T2 or T3 Within the characteristic curve (T1 T2 T3) selected the

desired limit curve for response is set using the factor F216

613 Switch-Off Criterion for Field Flashing

The Excitation Monitoring supplies the criterion for switching off the field

flashing Whether this criterion is activated based on the actual value for generator voltage

Ug or for field current If or both depends on the settings of the two threshold values

F200F202 (0 setting means that the output is always ldquological 1rdquo) The measurements

Ug12 and If12 are switched over depending on the present status of the channels

(Channel 1 or Channel 2 ON) Whenever Flag F750 is not activated the binary output is

fixed at ldquological 1rdquo

614 Storage of Alarm Status

The outputs of the over current alarms (R1 R2 R0) and the messages NO

FAILURE MONITORING PARAMETERS CHANGED are stored messages can be

erased by activating the input ldquoGENERAL RESETrdquo or by using the RESET button on the

front of the module Erasure with the input ldquoGENERAL RESETrdquo is effective only if the

situation causing the alarm or the malfunction is no longer present Whenever the self-

diagnosis equipment in the processor detects a malfunction the output NO FAILURE of

MONITORING is set at ldquological 0rdquo (= alarm) The alarm ldquoPARAMETERS CHANGEDrdquo

is activated whenever parameters or settings of software switches have been changed via

the Micro-Terminal

615 Actual Value Monitoring

The actual values for generator voltage Ug synchronous voltage Usyn and field

current If are monitored for malfunctions This monitoring is active regardless of whether

or not the generator is in operation Essentially when the generator is in operation the

measurements are monitored by comparing the signals (the smaller signal reading is

detected as incorrect) When the generator is not in operation the measured data are

monitored for extreme values The percentage of deviation permissible in the

measurement signals being compared

(Ug1ampUsyn1 Ug1ampUg2 Ug2ampUsyn2 If1ampIf2) is defined by parameters F208 and

F20AIf the excitation transformer is being supplied from an auxiliary power source (no

shunt operation) the values of Ug and Usyn will be different in some operational

conditions

In that case Flag F75A can be used to deactivate comparative measurements Ug1 Usyn1

and Ug2 Usyn2 The ldquoprimary monitoringrdquo compares Ug1 with Ug2 and generates the

messages that Channel 1 or Channel 2 has suffered a malfunction Whenever Ug1 lt Ug2

and the binary message from CH1 reports no malfunction a malfunction on

Measurement Channel 1 is reported (Ug1Usyn1 FAILURE) A similar malfunction is

also present whenever the binary message CH1 DISTURBANCE is reported and a

discrepancy is detected between Ug1ampUg2 The generation of the alarm ldquoUg2 Usyn2

FAILURErdquo is analogous to that for Channel 1 The ldquosecondary monitoringrdquo compares

Ug1 with Usyn1 amp Ug2 with Usyn2 This is enabled whenever the binary message of the

comparison channel reports a malfunction or whenever both binary messages report no

malfunction - but both secondary monitors report a malfunction As long as the secondary

monitoring is blocked the differences Ug1neUsyn1 or Ug2neUsyn2 trigger malfunction

signals for the measurement channel involved (suspicion that there is a corresponding

error in Usyn) The measurement channel malfunctions are enabled operationally

whenever after excitation has been switched on generator voltage Ug exceeds the value

set on F204 The voltages Ug1 and Ug2 are checked 16 seconds after the excitation is

switched off to see that they do not exceed the limit value F210 that applies to both of

them At the same time g1Ug2Usyn1Usyn2 are checked for extreme values (gt or lt

the operating range) Monitoring for extreme values is likewise enabled during normal

operation ( Excitation ON and Ug gt F204 ) Flag F754 is used to enable or block the

malfunction signals to the binary outputs Basically the monitoring of the actual values

for If1If2 functions like that of the Ug1Ug2 monitoring

62 Excitation Protection

621 General Information

The Excitation Protection switches off the excitation (and de-excites the machine

rapidly) whenever a danger arises that threatens the excitation transformer the converter

or the generator Generally limiter or monitoring functions precede the emergency trips

and these normally respond before the Excitation Protection must initiate a trip

Protective trip commands are issued directly to the field circuit-breaker from potential

free contacts of the board via the trip relays They are directed redundantly to the

operative field circuit-breaker ldquoOFFrdquo command

622 Protection against Excitation Transformer Overheating

This equipment monitors the excitation transformer for overheating in the

windings that could result from over current short-circuits or inadequate cooling The

monitoring uses temperature monitoring modules in conjunction with temperature

sensors built into the windings Normally the temperature is monitored in two stages the

first stage sets off an alarm the second causes a trip of the excitation

623 Rotor Over voltage Protection

Malfunctions in the generator circuit (eg terminal short-circuit failed

synchronization asynchronous operation) cause induced negative field currents that

produce high voltages in the field circuit These must be restricted to a level with a

sufficient safety margin below the insulation capacity of the field winding (test voltage)

and also below the peak blocking voltage of the converter thyristors The crow bar

employs spark gap elements to detect over voltages in the field circuit Whenever they

respond the associated thyristors are fired immediately switching the de-excitation

resistor parallel to the field The de-excitation current generated thereby initiates an

excitation trip via a supervision circuit causing an immediate opening of the field circuit-

breaker The malfunction isets off an alarm and an internal malfunction is indicated at the

cubicle

TEST VALUES OBTAINED WHEN EXCITATION IS RAISED

TEST

SNO PARTICULARS ACTUAL VALUE OBTAINED VALUE

1 VREF 996 100

2 VACT 997 1003

3 IFACT 735 765

4 IGACT 703 707

5 ACTIVE

POWER

703 705

6 REACTIVE

POWER

101 142

7 POWER

FACTOR

099 IND 098 IND

8 ACTIVE

CURRENT(IR)

705 703

9 REACTIVE

CURRENT(IX)

102 138

10 POWER

ANGLE

566 546

11 FIRING

ANGLE

640 632

VALUES OBTAINED WHEN EXCITATION IS LOWERED

SNO PARTICULARS ACTUAL

VALUE

OBTAINED VALUE

1 VREF 100 997

2 VACT 100 997

3 IFACT 787 761

4 IGACT 839 837

5 ACTIVE

POWER

839 830

6 REACTIVE

POWER

155 110

7 POWER

FACTOR

098 IND 099 IND

8 ACTIVE

CURRENT(IR)

835 830

9 REACTIVE

CURRENT(IX)

140 94

10 POWER

ANGLE

613 DEG 631 DEG

11 FIRING

ANGLE

601 DEG 599 DEG

CONCLUSION

For generating the EMF in stator winding excitation is required to the rotor of a

generator There are two types of excitation

1 Static excitation system

2 Brushless excitation system

A certain disadvantage in brushless excitation system is the slow response time of

the field in case of fast load changes specified No slip-rings and brushes direct

measurements of the field parameters not possible

To avoid all loses static excitation is used Since it does not have any rotating

parts mechanical loses and windage loses This system has fast response and speed

control While preferring this excitation system there are no limitations for the

redundancy of Thyristor bridge circuits

Static excitation has fast field discharge by resistor and inverter operation direct

measurement of field quantity is possible The meaning of excitation is nothing but

continuous supply of DC current (ie field current) to the rotor to buildup required

output voltage in the stator

Field current is changed with respect to the change of load so the digital

automatic voltage regulator (DAVR) is used to regulate the output voltage according to

the load variations

So we conclude that static excitation system with DAVR is preferred since it is

having excellent dynamic performance and better options for R amp M

41 Final Pulse Stages

The Final Pulse Stages adapt the output pulses from the Intermediate Pulse Stage

(pulses on the pulse bus) to the gate currents needed for the thyristors Each thyristor

bridge is equipped with its own Final Pulse Stage Each Final Pulse Stages is provided

with a power supply module The amplified output pulses from the Final Pulse Stages

start as a short strong steep pulse with an amplitude approx 2frac12 times that of the main

pulse This initial pulse edge assures proper firing of the thyristors being triggered

Subsequently the weaker part of main pulse keeps firing conditions steady As already

mentioned the Final Pulse Stages and their associated thyristor bridges form single units

All six pulse outputs from a Final Pulse Stage can be blocked by an external control

signal so that all thyristors in the associated thyristor bridge will block the current A

blocking of the pulses is initiated whenever there is a malfunction in the associated

thyristor bridge

42 Converter Power Section

The thyristor converter consists of three independent parallel rectifier blocks TY1

to TY3 which are all in service Even if one block fails the remaining blocks take over

automatically the full design current of the excitation circuit During normal operation

(with ideal current share) and all three bridges in operation each of these blocks has to

carry only (n-2)n (ie33)of its design current If 2 thyristor bridges fail the excitation

is limited Only when all three bridges fail the excitation is switched off Each thyristor

bridge arm is equipped with current flow monitoring CTrsquos Failure of conduction in any

arm is identified by a Current flow monitoring module

43 Converter Cooling

A cooling system is needed to dissipate heat losses in the converter blocks and

electronics Each converter block has therefore been equipped with a fan supplied with

power from the converterrsquos primary voltage (via transformer ndashT8 in field flashing

cubicle) The fans are protected with motor protection circuit breakers An air flow

monitoring unit is provided for monitoring the air flow through the thyristor bridge If a

circuit breaker failure is detected or if the air flow monitor drops off at one of the

thyristor bridges the bridge involved is immediately set out of operation by blocking its

firing pulses

44 Thyristor Converter Monitoring

A thyristor bridge in which defects occur that could threaten the safety of

operation or cause secondary damage is switched off automatically ie its firing pulses

are blocked This happens whenever A thyristor fuse is blown The fuses are monitored

individually with micro switches The Final Pulse Stage fails which is detected by

internal monitors (supply voltage sustained pulse short-circuit on the output end) The

power supply to the fan fails fan air flow as monitored by the Air flow monitor fails or

is insufficient Isolator on ACDC side is open

CHAPTER 5

Field Current Circuit Excitation Transformer

51 Field Circuit Breaker

The circuit-breaker in the field circuit is used to isolate the field circuit from the

converter It is capable of switching off the synchronous machine from full load under the

maximum conditions of a 3-phase short-circuit In addition to its main contacts the field

circuit-breaker also has a de-excitation contact with which the field energy stored in the

field can be dissipated across the de-excitation resistor The de-excitation contact closes

shortly before the main contacts open so as to ensure proper commutation of the field

current from the main contacts to the de-excitation contact when the breaker is switched

off The field circuit-breaker is switched on by electromagnetic force and is kept switched

on by a mechanical latch When the latch is released by a trip coil the circuit-breaker

opens The circuit-breaker also has auxiliary contacts that report its status

52 Field Flashing

In shunt supplied excitation circuits (excitation transformer connected to the

generator terminals) the generator does not have enough remnant voltage for a generator

voltage build-up via the converter In this case a field flashing circuit is provided It

consists of the field flashing contactor the diode bridge and a transformer used to adapt

the auxiliary input voltage to the voltage needed for field flashing when power is

supplied from the auxiliaries network

Fig Field Flashing

Because the field flashing contactor is not able to switch off the energy stored in

the field the control ensures that the contactor can only reopen if the field circuit breaker

has already been opened (generating the TRIP order) or in a normal field flashing

sequence when the converter has taken over the field current Field flashing occurs in the

following stages

1048729The excitation is switched on closing the field flashing contactor ( Field

Circuit Breaker is already closed )

1048729The start-up excitation current flows through the rotor driving the generator

voltage up to approx 15 U

1048729After about 10 U the firing pulses to the converter are released and it begins

to excite the generator to its rated voltage

1048729After about 30 U the field flashing contactor opens (with no current since

the converter is now supplying the current)

The diode bridge at the input to the field flashing contactor prevents a feed-back

from the converter to the source of field flashing while the contactor is still closed

53 De-excitation

When malfunctions occur the stored field energy must be dissipated as quickly

and safely as possible to protect the generator This is done by the converter the field

circuit-breaker and the de-excitation (discharge) resistor

De-excitation (with opening of the field circuit-breaker) takes place in the following

stages

1048729The converter drives to its inverter limit position (negative ceiling voltage)

recovers a portion of the field energy into the network A trip command is given to the

field circuit breaker

1048729The de-excitation contact closes diverting the field voltage to the de-excitation

resistor

1048729Then immediately the main contacts open building voltage The field voltage

commutates to the de-excitation resistor

1048729The current diminishes at a given time constant TE

(With linear resistance TE = Lf (Rf + Re))

Due to the reversal of the field voltage by the converter the field current

commutates from the main contacts of the field circuit-breaker to the de-excitation

resistor in a very early phase This reversal of the field voltage prevents burn-off on the

main contacts and provides effective protection for the field circuit-breaker Depending

on the operating policy an operational shut-down of the excitation can also be effected

with the field circuit-breaker closed This method is useful mainly when the excitation is

switched on and off frequently In this case the converter is merely driven into the

inverter limit position so that the field energy is recovered into the network The

converter then blocks since it is supplying positive current only

54 Excitation Transformer

The excitation transformer matches the generator voltage to the field voltage

(required ceiling voltage) It also serves as a commutation reactance for the thyristor

converter and as a potential isolator between the network and the excitation circuit In

addition the transformer functions as a current limiter in that it makes it possible to keep

any short circuits in the excitation circuit under better control The excitation transformer

is equipped with temperature monitoring probes which set off an alarm when the

temperature exceeds a first max limit and then trips the excitation if the temperature

continues rising to a second (higher)limit

CHAPTER 6

Monitoring and Protection

61 Excitation Monitoring

611 General Information

The main goal of Excitation Monitoring is to make optimum use of the

redundancies provided in the excitation system and to give alarm whenever a malfunction

makes these redundancies unavailable The field current is monitored to see that it does

not exceed a maximum level and if necessary a switch-over to the MANUAL channel is

initiated In addition the criterion for switching off the field flashing is generated The

excitation Monitoring consists of an autonomous processor system

612 over current Alarms

In the Excitation Monitoring the limits for are set at higher levels than the

settings on the Field Current Maximum Limiter Whenever the current exceeds 110 of

the nominal field current contact R1 and the binary output associated with it are

activated immediately If field current remains gt 110 then after a preset inverse-time

has lapsed relay R2 and - after a further delay - relay R0 and the binary outputs

associated with them are activated Parameters match the measurements for If1 and If2 to

the nominal value for field current so that the internal values can be processed and read as

pu values It can be used to falsify the actual value of the field current If (to raise it) so

as to cause a response from the alarm limits for purposes of testing The processed If

signal is always taken from on the active channel (CHANNEL I OR CHANNEL II) As

long as the field current If is above the threshold value 11 Ifn its peak value is

measured This is stored (until RESET) and can be read at any time on the Micro-

Terminal Once the value of If exceeds 11 Ifn integration of this value starts Whenever

the integrated time-current value (intisup2dt) exceeds the preselected reference value the

alarm OVER CURRENT INVERSE-TIME is set off and a command is simultaneously

issued to switch over to the stand by AUTO channel Software switch F758 enables the

three over current alarm functions (R0 R1 R2) and selects one of three possible inverse-

time curves T1 T2 or T3 Within the characteristic curve (T1 T2 T3) selected the

desired limit curve for response is set using the factor F216

613 Switch-Off Criterion for Field Flashing

The Excitation Monitoring supplies the criterion for switching off the field

flashing Whether this criterion is activated based on the actual value for generator voltage

Ug or for field current If or both depends on the settings of the two threshold values

F200F202 (0 setting means that the output is always ldquological 1rdquo) The measurements

Ug12 and If12 are switched over depending on the present status of the channels

(Channel 1 or Channel 2 ON) Whenever Flag F750 is not activated the binary output is

fixed at ldquological 1rdquo

614 Storage of Alarm Status

The outputs of the over current alarms (R1 R2 R0) and the messages NO

FAILURE MONITORING PARAMETERS CHANGED are stored messages can be

erased by activating the input ldquoGENERAL RESETrdquo or by using the RESET button on the

front of the module Erasure with the input ldquoGENERAL RESETrdquo is effective only if the

situation causing the alarm or the malfunction is no longer present Whenever the self-

diagnosis equipment in the processor detects a malfunction the output NO FAILURE of

MONITORING is set at ldquological 0rdquo (= alarm) The alarm ldquoPARAMETERS CHANGEDrdquo

is activated whenever parameters or settings of software switches have been changed via

the Micro-Terminal

615 Actual Value Monitoring

The actual values for generator voltage Ug synchronous voltage Usyn and field

current If are monitored for malfunctions This monitoring is active regardless of whether

or not the generator is in operation Essentially when the generator is in operation the

measurements are monitored by comparing the signals (the smaller signal reading is

detected as incorrect) When the generator is not in operation the measured data are

monitored for extreme values The percentage of deviation permissible in the

measurement signals being compared

(Ug1ampUsyn1 Ug1ampUg2 Ug2ampUsyn2 If1ampIf2) is defined by parameters F208 and

F20AIf the excitation transformer is being supplied from an auxiliary power source (no

shunt operation) the values of Ug and Usyn will be different in some operational

conditions

In that case Flag F75A can be used to deactivate comparative measurements Ug1 Usyn1

and Ug2 Usyn2 The ldquoprimary monitoringrdquo compares Ug1 with Ug2 and generates the

messages that Channel 1 or Channel 2 has suffered a malfunction Whenever Ug1 lt Ug2

and the binary message from CH1 reports no malfunction a malfunction on

Measurement Channel 1 is reported (Ug1Usyn1 FAILURE) A similar malfunction is

also present whenever the binary message CH1 DISTURBANCE is reported and a

discrepancy is detected between Ug1ampUg2 The generation of the alarm ldquoUg2 Usyn2

FAILURErdquo is analogous to that for Channel 1 The ldquosecondary monitoringrdquo compares

Ug1 with Usyn1 amp Ug2 with Usyn2 This is enabled whenever the binary message of the

comparison channel reports a malfunction or whenever both binary messages report no

malfunction - but both secondary monitors report a malfunction As long as the secondary

monitoring is blocked the differences Ug1neUsyn1 or Ug2neUsyn2 trigger malfunction

signals for the measurement channel involved (suspicion that there is a corresponding

error in Usyn) The measurement channel malfunctions are enabled operationally

whenever after excitation has been switched on generator voltage Ug exceeds the value

set on F204 The voltages Ug1 and Ug2 are checked 16 seconds after the excitation is

switched off to see that they do not exceed the limit value F210 that applies to both of

them At the same time g1Ug2Usyn1Usyn2 are checked for extreme values (gt or lt

the operating range) Monitoring for extreme values is likewise enabled during normal

operation ( Excitation ON and Ug gt F204 ) Flag F754 is used to enable or block the

malfunction signals to the binary outputs Basically the monitoring of the actual values

for If1If2 functions like that of the Ug1Ug2 monitoring

62 Excitation Protection

621 General Information

The Excitation Protection switches off the excitation (and de-excites the machine

rapidly) whenever a danger arises that threatens the excitation transformer the converter

or the generator Generally limiter or monitoring functions precede the emergency trips

and these normally respond before the Excitation Protection must initiate a trip

Protective trip commands are issued directly to the field circuit-breaker from potential

free contacts of the board via the trip relays They are directed redundantly to the

operative field circuit-breaker ldquoOFFrdquo command

622 Protection against Excitation Transformer Overheating

This equipment monitors the excitation transformer for overheating in the

windings that could result from over current short-circuits or inadequate cooling The

monitoring uses temperature monitoring modules in conjunction with temperature

sensors built into the windings Normally the temperature is monitored in two stages the

first stage sets off an alarm the second causes a trip of the excitation

623 Rotor Over voltage Protection

Malfunctions in the generator circuit (eg terminal short-circuit failed

synchronization asynchronous operation) cause induced negative field currents that

produce high voltages in the field circuit These must be restricted to a level with a

sufficient safety margin below the insulation capacity of the field winding (test voltage)

and also below the peak blocking voltage of the converter thyristors The crow bar

employs spark gap elements to detect over voltages in the field circuit Whenever they

respond the associated thyristors are fired immediately switching the de-excitation

resistor parallel to the field The de-excitation current generated thereby initiates an

excitation trip via a supervision circuit causing an immediate opening of the field circuit-

breaker The malfunction isets off an alarm and an internal malfunction is indicated at the

cubicle

TEST VALUES OBTAINED WHEN EXCITATION IS RAISED

TEST

SNO PARTICULARS ACTUAL VALUE OBTAINED VALUE

1 VREF 996 100

2 VACT 997 1003

3 IFACT 735 765

4 IGACT 703 707

5 ACTIVE

POWER

703 705

6 REACTIVE

POWER

101 142

7 POWER

FACTOR

099 IND 098 IND

8 ACTIVE

CURRENT(IR)

705 703

9 REACTIVE

CURRENT(IX)

102 138

10 POWER

ANGLE

566 546

11 FIRING

ANGLE

640 632

VALUES OBTAINED WHEN EXCITATION IS LOWERED

SNO PARTICULARS ACTUAL

VALUE

OBTAINED VALUE

1 VREF 100 997

2 VACT 100 997

3 IFACT 787 761

4 IGACT 839 837

5 ACTIVE

POWER

839 830

6 REACTIVE

POWER

155 110

7 POWER

FACTOR

098 IND 099 IND

8 ACTIVE

CURRENT(IR)

835 830

9 REACTIVE

CURRENT(IX)

140 94

10 POWER

ANGLE

613 DEG 631 DEG

11 FIRING

ANGLE

601 DEG 599 DEG

CONCLUSION

For generating the EMF in stator winding excitation is required to the rotor of a

generator There are two types of excitation

1 Static excitation system

2 Brushless excitation system

A certain disadvantage in brushless excitation system is the slow response time of

the field in case of fast load changes specified No slip-rings and brushes direct

measurements of the field parameters not possible

To avoid all loses static excitation is used Since it does not have any rotating

parts mechanical loses and windage loses This system has fast response and speed

control While preferring this excitation system there are no limitations for the

redundancy of Thyristor bridge circuits

Static excitation has fast field discharge by resistor and inverter operation direct

measurement of field quantity is possible The meaning of excitation is nothing but

continuous supply of DC current (ie field current) to the rotor to buildup required

output voltage in the stator

Field current is changed with respect to the change of load so the digital

automatic voltage regulator (DAVR) is used to regulate the output voltage according to

the load variations

So we conclude that static excitation system with DAVR is preferred since it is

having excellent dynamic performance and better options for R amp M

44 Thyristor Converter Monitoring

A thyristor bridge in which defects occur that could threaten the safety of

operation or cause secondary damage is switched off automatically ie its firing pulses

are blocked This happens whenever A thyristor fuse is blown The fuses are monitored

individually with micro switches The Final Pulse Stage fails which is detected by

internal monitors (supply voltage sustained pulse short-circuit on the output end) The

power supply to the fan fails fan air flow as monitored by the Air flow monitor fails or

is insufficient Isolator on ACDC side is open

CHAPTER 5

Field Current Circuit Excitation Transformer

51 Field Circuit Breaker

The circuit-breaker in the field circuit is used to isolate the field circuit from the

converter It is capable of switching off the synchronous machine from full load under the

maximum conditions of a 3-phase short-circuit In addition to its main contacts the field

circuit-breaker also has a de-excitation contact with which the field energy stored in the

field can be dissipated across the de-excitation resistor The de-excitation contact closes

shortly before the main contacts open so as to ensure proper commutation of the field

current from the main contacts to the de-excitation contact when the breaker is switched

off The field circuit-breaker is switched on by electromagnetic force and is kept switched

on by a mechanical latch When the latch is released by a trip coil the circuit-breaker

opens The circuit-breaker also has auxiliary contacts that report its status

52 Field Flashing

In shunt supplied excitation circuits (excitation transformer connected to the

generator terminals) the generator does not have enough remnant voltage for a generator

voltage build-up via the converter In this case a field flashing circuit is provided It

consists of the field flashing contactor the diode bridge and a transformer used to adapt

the auxiliary input voltage to the voltage needed for field flashing when power is

supplied from the auxiliaries network

Fig Field Flashing

Because the field flashing contactor is not able to switch off the energy stored in

the field the control ensures that the contactor can only reopen if the field circuit breaker

has already been opened (generating the TRIP order) or in a normal field flashing

sequence when the converter has taken over the field current Field flashing occurs in the

following stages

1048729The excitation is switched on closing the field flashing contactor ( Field

Circuit Breaker is already closed )

1048729The start-up excitation current flows through the rotor driving the generator

voltage up to approx 15 U

1048729After about 10 U the firing pulses to the converter are released and it begins

to excite the generator to its rated voltage

1048729After about 30 U the field flashing contactor opens (with no current since

the converter is now supplying the current)

The diode bridge at the input to the field flashing contactor prevents a feed-back

from the converter to the source of field flashing while the contactor is still closed

53 De-excitation

When malfunctions occur the stored field energy must be dissipated as quickly

and safely as possible to protect the generator This is done by the converter the field

circuit-breaker and the de-excitation (discharge) resistor

De-excitation (with opening of the field circuit-breaker) takes place in the following

stages

1048729The converter drives to its inverter limit position (negative ceiling voltage)

recovers a portion of the field energy into the network A trip command is given to the

field circuit breaker

1048729The de-excitation contact closes diverting the field voltage to the de-excitation

resistor

1048729Then immediately the main contacts open building voltage The field voltage

commutates to the de-excitation resistor

1048729The current diminishes at a given time constant TE

(With linear resistance TE = Lf (Rf + Re))

Due to the reversal of the field voltage by the converter the field current

commutates from the main contacts of the field circuit-breaker to the de-excitation

resistor in a very early phase This reversal of the field voltage prevents burn-off on the

main contacts and provides effective protection for the field circuit-breaker Depending

on the operating policy an operational shut-down of the excitation can also be effected

with the field circuit-breaker closed This method is useful mainly when the excitation is

switched on and off frequently In this case the converter is merely driven into the

inverter limit position so that the field energy is recovered into the network The

converter then blocks since it is supplying positive current only

54 Excitation Transformer

The excitation transformer matches the generator voltage to the field voltage

(required ceiling voltage) It also serves as a commutation reactance for the thyristor

converter and as a potential isolator between the network and the excitation circuit In

addition the transformer functions as a current limiter in that it makes it possible to keep

any short circuits in the excitation circuit under better control The excitation transformer

is equipped with temperature monitoring probes which set off an alarm when the

temperature exceeds a first max limit and then trips the excitation if the temperature

continues rising to a second (higher)limit

CHAPTER 6

Monitoring and Protection

61 Excitation Monitoring

611 General Information

The main goal of Excitation Monitoring is to make optimum use of the

redundancies provided in the excitation system and to give alarm whenever a malfunction

makes these redundancies unavailable The field current is monitored to see that it does

not exceed a maximum level and if necessary a switch-over to the MANUAL channel is

initiated In addition the criterion for switching off the field flashing is generated The

excitation Monitoring consists of an autonomous processor system

612 over current Alarms

In the Excitation Monitoring the limits for are set at higher levels than the

settings on the Field Current Maximum Limiter Whenever the current exceeds 110 of

the nominal field current contact R1 and the binary output associated with it are

activated immediately If field current remains gt 110 then after a preset inverse-time

has lapsed relay R2 and - after a further delay - relay R0 and the binary outputs

associated with them are activated Parameters match the measurements for If1 and If2 to

the nominal value for field current so that the internal values can be processed and read as

pu values It can be used to falsify the actual value of the field current If (to raise it) so

as to cause a response from the alarm limits for purposes of testing The processed If

signal is always taken from on the active channel (CHANNEL I OR CHANNEL II) As

long as the field current If is above the threshold value 11 Ifn its peak value is

measured This is stored (until RESET) and can be read at any time on the Micro-

Terminal Once the value of If exceeds 11 Ifn integration of this value starts Whenever

the integrated time-current value (intisup2dt) exceeds the preselected reference value the

alarm OVER CURRENT INVERSE-TIME is set off and a command is simultaneously

issued to switch over to the stand by AUTO channel Software switch F758 enables the

three over current alarm functions (R0 R1 R2) and selects one of three possible inverse-

time curves T1 T2 or T3 Within the characteristic curve (T1 T2 T3) selected the

desired limit curve for response is set using the factor F216

613 Switch-Off Criterion for Field Flashing

The Excitation Monitoring supplies the criterion for switching off the field

flashing Whether this criterion is activated based on the actual value for generator voltage

Ug or for field current If or both depends on the settings of the two threshold values

F200F202 (0 setting means that the output is always ldquological 1rdquo) The measurements

Ug12 and If12 are switched over depending on the present status of the channels

(Channel 1 or Channel 2 ON) Whenever Flag F750 is not activated the binary output is

fixed at ldquological 1rdquo

614 Storage of Alarm Status

The outputs of the over current alarms (R1 R2 R0) and the messages NO

FAILURE MONITORING PARAMETERS CHANGED are stored messages can be

erased by activating the input ldquoGENERAL RESETrdquo or by using the RESET button on the

front of the module Erasure with the input ldquoGENERAL RESETrdquo is effective only if the

situation causing the alarm or the malfunction is no longer present Whenever the self-

diagnosis equipment in the processor detects a malfunction the output NO FAILURE of

MONITORING is set at ldquological 0rdquo (= alarm) The alarm ldquoPARAMETERS CHANGEDrdquo

is activated whenever parameters or settings of software switches have been changed via

the Micro-Terminal

615 Actual Value Monitoring

The actual values for generator voltage Ug synchronous voltage Usyn and field

current If are monitored for malfunctions This monitoring is active regardless of whether

or not the generator is in operation Essentially when the generator is in operation the

measurements are monitored by comparing the signals (the smaller signal reading is

detected as incorrect) When the generator is not in operation the measured data are

monitored for extreme values The percentage of deviation permissible in the

measurement signals being compared

(Ug1ampUsyn1 Ug1ampUg2 Ug2ampUsyn2 If1ampIf2) is defined by parameters F208 and

F20AIf the excitation transformer is being supplied from an auxiliary power source (no

shunt operation) the values of Ug and Usyn will be different in some operational

conditions

In that case Flag F75A can be used to deactivate comparative measurements Ug1 Usyn1

and Ug2 Usyn2 The ldquoprimary monitoringrdquo compares Ug1 with Ug2 and generates the

messages that Channel 1 or Channel 2 has suffered a malfunction Whenever Ug1 lt Ug2

and the binary message from CH1 reports no malfunction a malfunction on

Measurement Channel 1 is reported (Ug1Usyn1 FAILURE) A similar malfunction is

also present whenever the binary message CH1 DISTURBANCE is reported and a

discrepancy is detected between Ug1ampUg2 The generation of the alarm ldquoUg2 Usyn2

FAILURErdquo is analogous to that for Channel 1 The ldquosecondary monitoringrdquo compares

Ug1 with Usyn1 amp Ug2 with Usyn2 This is enabled whenever the binary message of the

comparison channel reports a malfunction or whenever both binary messages report no

malfunction - but both secondary monitors report a malfunction As long as the secondary

monitoring is blocked the differences Ug1neUsyn1 or Ug2neUsyn2 trigger malfunction

signals for the measurement channel involved (suspicion that there is a corresponding

error in Usyn) The measurement channel malfunctions are enabled operationally

whenever after excitation has been switched on generator voltage Ug exceeds the value

set on F204 The voltages Ug1 and Ug2 are checked 16 seconds after the excitation is

switched off to see that they do not exceed the limit value F210 that applies to both of

them At the same time g1Ug2Usyn1Usyn2 are checked for extreme values (gt or lt

the operating range) Monitoring for extreme values is likewise enabled during normal

operation ( Excitation ON and Ug gt F204 ) Flag F754 is used to enable or block the

malfunction signals to the binary outputs Basically the monitoring of the actual values

for If1If2 functions like that of the Ug1Ug2 monitoring

62 Excitation Protection

621 General Information

The Excitation Protection switches off the excitation (and de-excites the machine

rapidly) whenever a danger arises that threatens the excitation transformer the converter

or the generator Generally limiter or monitoring functions precede the emergency trips

and these normally respond before the Excitation Protection must initiate a trip

Protective trip commands are issued directly to the field circuit-breaker from potential

free contacts of the board via the trip relays They are directed redundantly to the

operative field circuit-breaker ldquoOFFrdquo command

622 Protection against Excitation Transformer Overheating

This equipment monitors the excitation transformer for overheating in the

windings that could result from over current short-circuits or inadequate cooling The

monitoring uses temperature monitoring modules in conjunction with temperature

sensors built into the windings Normally the temperature is monitored in two stages the

first stage sets off an alarm the second causes a trip of the excitation

623 Rotor Over voltage Protection

Malfunctions in the generator circuit (eg terminal short-circuit failed

synchronization asynchronous operation) cause induced negative field currents that

produce high voltages in the field circuit These must be restricted to a level with a

sufficient safety margin below the insulation capacity of the field winding (test voltage)

and also below the peak blocking voltage of the converter thyristors The crow bar

employs spark gap elements to detect over voltages in the field circuit Whenever they

respond the associated thyristors are fired immediately switching the de-excitation

resistor parallel to the field The de-excitation current generated thereby initiates an

excitation trip via a supervision circuit causing an immediate opening of the field circuit-

breaker The malfunction isets off an alarm and an internal malfunction is indicated at the

cubicle

TEST VALUES OBTAINED WHEN EXCITATION IS RAISED

TEST

SNO PARTICULARS ACTUAL VALUE OBTAINED VALUE

1 VREF 996 100

2 VACT 997 1003

3 IFACT 735 765

4 IGACT 703 707

5 ACTIVE

POWER

703 705

6 REACTIVE

POWER

101 142

7 POWER

FACTOR

099 IND 098 IND

8 ACTIVE

CURRENT(IR)

705 703

9 REACTIVE

CURRENT(IX)

102 138

10 POWER

ANGLE

566 546

11 FIRING

ANGLE

640 632

VALUES OBTAINED WHEN EXCITATION IS LOWERED

SNO PARTICULARS ACTUAL

VALUE

OBTAINED VALUE

1 VREF 100 997

2 VACT 100 997

3 IFACT 787 761

4 IGACT 839 837

5 ACTIVE

POWER

839 830

6 REACTIVE

POWER

155 110

7 POWER

FACTOR

098 IND 099 IND

8 ACTIVE

CURRENT(IR)

835 830

9 REACTIVE

CURRENT(IX)

140 94

10 POWER

ANGLE

613 DEG 631 DEG

11 FIRING

ANGLE

601 DEG 599 DEG

CONCLUSION

For generating the EMF in stator winding excitation is required to the rotor of a

generator There are two types of excitation

1 Static excitation system

2 Brushless excitation system

A certain disadvantage in brushless excitation system is the slow response time of

the field in case of fast load changes specified No slip-rings and brushes direct

measurements of the field parameters not possible

To avoid all loses static excitation is used Since it does not have any rotating

parts mechanical loses and windage loses This system has fast response and speed

control While preferring this excitation system there are no limitations for the

redundancy of Thyristor bridge circuits

Static excitation has fast field discharge by resistor and inverter operation direct

measurement of field quantity is possible The meaning of excitation is nothing but

continuous supply of DC current (ie field current) to the rotor to buildup required

output voltage in the stator

Field current is changed with respect to the change of load so the digital

automatic voltage regulator (DAVR) is used to regulate the output voltage according to

the load variations

So we conclude that static excitation system with DAVR is preferred since it is

having excellent dynamic performance and better options for R amp M

CHAPTER 5

Field Current Circuit Excitation Transformer

51 Field Circuit Breaker

The circuit-breaker in the field circuit is used to isolate the field circuit from the

converter It is capable of switching off the synchronous machine from full load under the

maximum conditions of a 3-phase short-circuit In addition to its main contacts the field

circuit-breaker also has a de-excitation contact with which the field energy stored in the

field can be dissipated across the de-excitation resistor The de-excitation contact closes

shortly before the main contacts open so as to ensure proper commutation of the field

current from the main contacts to the de-excitation contact when the breaker is switched

off The field circuit-breaker is switched on by electromagnetic force and is kept switched

on by a mechanical latch When the latch is released by a trip coil the circuit-breaker

opens The circuit-breaker also has auxiliary contacts that report its status

52 Field Flashing

In shunt supplied excitation circuits (excitation transformer connected to the

generator terminals) the generator does not have enough remnant voltage for a generator

voltage build-up via the converter In this case a field flashing circuit is provided It

consists of the field flashing contactor the diode bridge and a transformer used to adapt

the auxiliary input voltage to the voltage needed for field flashing when power is

supplied from the auxiliaries network

Fig Field Flashing

Because the field flashing contactor is not able to switch off the energy stored in

the field the control ensures that the contactor can only reopen if the field circuit breaker

has already been opened (generating the TRIP order) or in a normal field flashing

sequence when the converter has taken over the field current Field flashing occurs in the

following stages

1048729The excitation is switched on closing the field flashing contactor ( Field

Circuit Breaker is already closed )

1048729The start-up excitation current flows through the rotor driving the generator

voltage up to approx 15 U

1048729After about 10 U the firing pulses to the converter are released and it begins

to excite the generator to its rated voltage

1048729After about 30 U the field flashing contactor opens (with no current since

the converter is now supplying the current)

The diode bridge at the input to the field flashing contactor prevents a feed-back

from the converter to the source of field flashing while the contactor is still closed

53 De-excitation

When malfunctions occur the stored field energy must be dissipated as quickly

and safely as possible to protect the generator This is done by the converter the field

circuit-breaker and the de-excitation (discharge) resistor

De-excitation (with opening of the field circuit-breaker) takes place in the following

stages

1048729The converter drives to its inverter limit position (negative ceiling voltage)

recovers a portion of the field energy into the network A trip command is given to the

field circuit breaker

1048729The de-excitation contact closes diverting the field voltage to the de-excitation

resistor

1048729Then immediately the main contacts open building voltage The field voltage

commutates to the de-excitation resistor

1048729The current diminishes at a given time constant TE

(With linear resistance TE = Lf (Rf + Re))

Due to the reversal of the field voltage by the converter the field current

commutates from the main contacts of the field circuit-breaker to the de-excitation

resistor in a very early phase This reversal of the field voltage prevents burn-off on the

main contacts and provides effective protection for the field circuit-breaker Depending

on the operating policy an operational shut-down of the excitation can also be effected

with the field circuit-breaker closed This method is useful mainly when the excitation is

switched on and off frequently In this case the converter is merely driven into the

inverter limit position so that the field energy is recovered into the network The

converter then blocks since it is supplying positive current only

54 Excitation Transformer

The excitation transformer matches the generator voltage to the field voltage

(required ceiling voltage) It also serves as a commutation reactance for the thyristor

converter and as a potential isolator between the network and the excitation circuit In

addition the transformer functions as a current limiter in that it makes it possible to keep

any short circuits in the excitation circuit under better control The excitation transformer

is equipped with temperature monitoring probes which set off an alarm when the

temperature exceeds a first max limit and then trips the excitation if the temperature

continues rising to a second (higher)limit

CHAPTER 6

Monitoring and Protection

61 Excitation Monitoring

611 General Information

The main goal of Excitation Monitoring is to make optimum use of the

redundancies provided in the excitation system and to give alarm whenever a malfunction

makes these redundancies unavailable The field current is monitored to see that it does

not exceed a maximum level and if necessary a switch-over to the MANUAL channel is

initiated In addition the criterion for switching off the field flashing is generated The

excitation Monitoring consists of an autonomous processor system

612 over current Alarms

In the Excitation Monitoring the limits for are set at higher levels than the

settings on the Field Current Maximum Limiter Whenever the current exceeds 110 of

the nominal field current contact R1 and the binary output associated with it are

activated immediately If field current remains gt 110 then after a preset inverse-time

has lapsed relay R2 and - after a further delay - relay R0 and the binary outputs

associated with them are activated Parameters match the measurements for If1 and If2 to

the nominal value for field current so that the internal values can be processed and read as

pu values It can be used to falsify the actual value of the field current If (to raise it) so

as to cause a response from the alarm limits for purposes of testing The processed If

signal is always taken from on the active channel (CHANNEL I OR CHANNEL II) As

long as the field current If is above the threshold value 11 Ifn its peak value is

measured This is stored (until RESET) and can be read at any time on the Micro-

Terminal Once the value of If exceeds 11 Ifn integration of this value starts Whenever

the integrated time-current value (intisup2dt) exceeds the preselected reference value the

alarm OVER CURRENT INVERSE-TIME is set off and a command is simultaneously

issued to switch over to the stand by AUTO channel Software switch F758 enables the

three over current alarm functions (R0 R1 R2) and selects one of three possible inverse-

time curves T1 T2 or T3 Within the characteristic curve (T1 T2 T3) selected the

desired limit curve for response is set using the factor F216

613 Switch-Off Criterion for Field Flashing

The Excitation Monitoring supplies the criterion for switching off the field

flashing Whether this criterion is activated based on the actual value for generator voltage

Ug or for field current If or both depends on the settings of the two threshold values

F200F202 (0 setting means that the output is always ldquological 1rdquo) The measurements

Ug12 and If12 are switched over depending on the present status of the channels

(Channel 1 or Channel 2 ON) Whenever Flag F750 is not activated the binary output is

fixed at ldquological 1rdquo

614 Storage of Alarm Status

The outputs of the over current alarms (R1 R2 R0) and the messages NO

FAILURE MONITORING PARAMETERS CHANGED are stored messages can be

erased by activating the input ldquoGENERAL RESETrdquo or by using the RESET button on the

front of the module Erasure with the input ldquoGENERAL RESETrdquo is effective only if the

situation causing the alarm or the malfunction is no longer present Whenever the self-

diagnosis equipment in the processor detects a malfunction the output NO FAILURE of

MONITORING is set at ldquological 0rdquo (= alarm) The alarm ldquoPARAMETERS CHANGEDrdquo

is activated whenever parameters or settings of software switches have been changed via

the Micro-Terminal

615 Actual Value Monitoring

The actual values for generator voltage Ug synchronous voltage Usyn and field

current If are monitored for malfunctions This monitoring is active regardless of whether

or not the generator is in operation Essentially when the generator is in operation the

measurements are monitored by comparing the signals (the smaller signal reading is

detected as incorrect) When the generator is not in operation the measured data are

monitored for extreme values The percentage of deviation permissible in the

measurement signals being compared

(Ug1ampUsyn1 Ug1ampUg2 Ug2ampUsyn2 If1ampIf2) is defined by parameters F208 and

F20AIf the excitation transformer is being supplied from an auxiliary power source (no

shunt operation) the values of Ug and Usyn will be different in some operational

conditions

In that case Flag F75A can be used to deactivate comparative measurements Ug1 Usyn1

and Ug2 Usyn2 The ldquoprimary monitoringrdquo compares Ug1 with Ug2 and generates the

messages that Channel 1 or Channel 2 has suffered a malfunction Whenever Ug1 lt Ug2

and the binary message from CH1 reports no malfunction a malfunction on

Measurement Channel 1 is reported (Ug1Usyn1 FAILURE) A similar malfunction is

also present whenever the binary message CH1 DISTURBANCE is reported and a

discrepancy is detected between Ug1ampUg2 The generation of the alarm ldquoUg2 Usyn2

FAILURErdquo is analogous to that for Channel 1 The ldquosecondary monitoringrdquo compares

Ug1 with Usyn1 amp Ug2 with Usyn2 This is enabled whenever the binary message of the

comparison channel reports a malfunction or whenever both binary messages report no

malfunction - but both secondary monitors report a malfunction As long as the secondary

monitoring is blocked the differences Ug1neUsyn1 or Ug2neUsyn2 trigger malfunction

signals for the measurement channel involved (suspicion that there is a corresponding

error in Usyn) The measurement channel malfunctions are enabled operationally

whenever after excitation has been switched on generator voltage Ug exceeds the value

set on F204 The voltages Ug1 and Ug2 are checked 16 seconds after the excitation is

switched off to see that they do not exceed the limit value F210 that applies to both of

them At the same time g1Ug2Usyn1Usyn2 are checked for extreme values (gt or lt

the operating range) Monitoring for extreme values is likewise enabled during normal

operation ( Excitation ON and Ug gt F204 ) Flag F754 is used to enable or block the

malfunction signals to the binary outputs Basically the monitoring of the actual values

for If1If2 functions like that of the Ug1Ug2 monitoring

62 Excitation Protection

621 General Information

The Excitation Protection switches off the excitation (and de-excites the machine

rapidly) whenever a danger arises that threatens the excitation transformer the converter

or the generator Generally limiter or monitoring functions precede the emergency trips

and these normally respond before the Excitation Protection must initiate a trip

Protective trip commands are issued directly to the field circuit-breaker from potential

free contacts of the board via the trip relays They are directed redundantly to the

operative field circuit-breaker ldquoOFFrdquo command

622 Protection against Excitation Transformer Overheating

This equipment monitors the excitation transformer for overheating in the

windings that could result from over current short-circuits or inadequate cooling The

monitoring uses temperature monitoring modules in conjunction with temperature

sensors built into the windings Normally the temperature is monitored in two stages the

first stage sets off an alarm the second causes a trip of the excitation

623 Rotor Over voltage Protection

Malfunctions in the generator circuit (eg terminal short-circuit failed

synchronization asynchronous operation) cause induced negative field currents that

produce high voltages in the field circuit These must be restricted to a level with a

sufficient safety margin below the insulation capacity of the field winding (test voltage)

and also below the peak blocking voltage of the converter thyristors The crow bar

employs spark gap elements to detect over voltages in the field circuit Whenever they

respond the associated thyristors are fired immediately switching the de-excitation

resistor parallel to the field The de-excitation current generated thereby initiates an

excitation trip via a supervision circuit causing an immediate opening of the field circuit-

breaker The malfunction isets off an alarm and an internal malfunction is indicated at the

cubicle

TEST VALUES OBTAINED WHEN EXCITATION IS RAISED

TEST

SNO PARTICULARS ACTUAL VALUE OBTAINED VALUE

1 VREF 996 100

2 VACT 997 1003

3 IFACT 735 765

4 IGACT 703 707

5 ACTIVE

POWER

703 705

6 REACTIVE

POWER

101 142

7 POWER

FACTOR

099 IND 098 IND

8 ACTIVE

CURRENT(IR)

705 703

9 REACTIVE

CURRENT(IX)

102 138

10 POWER

ANGLE

566 546

11 FIRING

ANGLE

640 632

VALUES OBTAINED WHEN EXCITATION IS LOWERED

SNO PARTICULARS ACTUAL

VALUE

OBTAINED VALUE

1 VREF 100 997

2 VACT 100 997

3 IFACT 787 761

4 IGACT 839 837

5 ACTIVE

POWER

839 830

6 REACTIVE

POWER

155 110

7 POWER

FACTOR

098 IND 099 IND

8 ACTIVE

CURRENT(IR)

835 830

9 REACTIVE

CURRENT(IX)

140 94

10 POWER

ANGLE

613 DEG 631 DEG

11 FIRING

ANGLE

601 DEG 599 DEG

CONCLUSION

For generating the EMF in stator winding excitation is required to the rotor of a

generator There are two types of excitation

1 Static excitation system

2 Brushless excitation system

A certain disadvantage in brushless excitation system is the slow response time of

the field in case of fast load changes specified No slip-rings and brushes direct

measurements of the field parameters not possible

To avoid all loses static excitation is used Since it does not have any rotating

parts mechanical loses and windage loses This system has fast response and speed

control While preferring this excitation system there are no limitations for the

redundancy of Thyristor bridge circuits

Static excitation has fast field discharge by resistor and inverter operation direct

measurement of field quantity is possible The meaning of excitation is nothing but

continuous supply of DC current (ie field current) to the rotor to buildup required

output voltage in the stator

Field current is changed with respect to the change of load so the digital

automatic voltage regulator (DAVR) is used to regulate the output voltage according to

the load variations

So we conclude that static excitation system with DAVR is preferred since it is

having excellent dynamic performance and better options for R amp M

52 Field Flashing

In shunt supplied excitation circuits (excitation transformer connected to the

generator terminals) the generator does not have enough remnant voltage for a generator

voltage build-up via the converter In this case a field flashing circuit is provided It

consists of the field flashing contactor the diode bridge and a transformer used to adapt

the auxiliary input voltage to the voltage needed for field flashing when power is

supplied from the auxiliaries network

Fig Field Flashing

Because the field flashing contactor is not able to switch off the energy stored in

the field the control ensures that the contactor can only reopen if the field circuit breaker

has already been opened (generating the TRIP order) or in a normal field flashing

sequence when the converter has taken over the field current Field flashing occurs in the

following stages

1048729The excitation is switched on closing the field flashing contactor ( Field

Circuit Breaker is already closed )

1048729The start-up excitation current flows through the rotor driving the generator

voltage up to approx 15 U

1048729After about 10 U the firing pulses to the converter are released and it begins

to excite the generator to its rated voltage

1048729After about 30 U the field flashing contactor opens (with no current since

the converter is now supplying the current)

The diode bridge at the input to the field flashing contactor prevents a feed-back

from the converter to the source of field flashing while the contactor is still closed

53 De-excitation

When malfunctions occur the stored field energy must be dissipated as quickly

and safely as possible to protect the generator This is done by the converter the field

circuit-breaker and the de-excitation (discharge) resistor

De-excitation (with opening of the field circuit-breaker) takes place in the following

stages

1048729The converter drives to its inverter limit position (negative ceiling voltage)

recovers a portion of the field energy into the network A trip command is given to the

field circuit breaker

1048729The de-excitation contact closes diverting the field voltage to the de-excitation

resistor

1048729Then immediately the main contacts open building voltage The field voltage

commutates to the de-excitation resistor

1048729The current diminishes at a given time constant TE

(With linear resistance TE = Lf (Rf + Re))

Due to the reversal of the field voltage by the converter the field current

commutates from the main contacts of the field circuit-breaker to the de-excitation

resistor in a very early phase This reversal of the field voltage prevents burn-off on the

main contacts and provides effective protection for the field circuit-breaker Depending

on the operating policy an operational shut-down of the excitation can also be effected

with the field circuit-breaker closed This method is useful mainly when the excitation is

switched on and off frequently In this case the converter is merely driven into the

inverter limit position so that the field energy is recovered into the network The

converter then blocks since it is supplying positive current only

54 Excitation Transformer

The excitation transformer matches the generator voltage to the field voltage

(required ceiling voltage) It also serves as a commutation reactance for the thyristor

converter and as a potential isolator between the network and the excitation circuit In

addition the transformer functions as a current limiter in that it makes it possible to keep

any short circuits in the excitation circuit under better control The excitation transformer

is equipped with temperature monitoring probes which set off an alarm when the

temperature exceeds a first max limit and then trips the excitation if the temperature

continues rising to a second (higher)limit

CHAPTER 6

Monitoring and Protection

61 Excitation Monitoring

611 General Information

The main goal of Excitation Monitoring is to make optimum use of the

redundancies provided in the excitation system and to give alarm whenever a malfunction

makes these redundancies unavailable The field current is monitored to see that it does

not exceed a maximum level and if necessary a switch-over to the MANUAL channel is

initiated In addition the criterion for switching off the field flashing is generated The

excitation Monitoring consists of an autonomous processor system

612 over current Alarms

In the Excitation Monitoring the limits for are set at higher levels than the

settings on the Field Current Maximum Limiter Whenever the current exceeds 110 of

the nominal field current contact R1 and the binary output associated with it are

activated immediately If field current remains gt 110 then after a preset inverse-time

has lapsed relay R2 and - after a further delay - relay R0 and the binary outputs

associated with them are activated Parameters match the measurements for If1 and If2 to

the nominal value for field current so that the internal values can be processed and read as

pu values It can be used to falsify the actual value of the field current If (to raise it) so

as to cause a response from the alarm limits for purposes of testing The processed If

signal is always taken from on the active channel (CHANNEL I OR CHANNEL II) As

long as the field current If is above the threshold value 11 Ifn its peak value is

measured This is stored (until RESET) and can be read at any time on the Micro-

Terminal Once the value of If exceeds 11 Ifn integration of this value starts Whenever

the integrated time-current value (intisup2dt) exceeds the preselected reference value the

alarm OVER CURRENT INVERSE-TIME is set off and a command is simultaneously

issued to switch over to the stand by AUTO channel Software switch F758 enables the

three over current alarm functions (R0 R1 R2) and selects one of three possible inverse-

time curves T1 T2 or T3 Within the characteristic curve (T1 T2 T3) selected the

desired limit curve for response is set using the factor F216

613 Switch-Off Criterion for Field Flashing

The Excitation Monitoring supplies the criterion for switching off the field

flashing Whether this criterion is activated based on the actual value for generator voltage

Ug or for field current If or both depends on the settings of the two threshold values

F200F202 (0 setting means that the output is always ldquological 1rdquo) The measurements

Ug12 and If12 are switched over depending on the present status of the channels

(Channel 1 or Channel 2 ON) Whenever Flag F750 is not activated the binary output is

fixed at ldquological 1rdquo

614 Storage of Alarm Status

The outputs of the over current alarms (R1 R2 R0) and the messages NO

FAILURE MONITORING PARAMETERS CHANGED are stored messages can be

erased by activating the input ldquoGENERAL RESETrdquo or by using the RESET button on the

front of the module Erasure with the input ldquoGENERAL RESETrdquo is effective only if the

situation causing the alarm or the malfunction is no longer present Whenever the self-

diagnosis equipment in the processor detects a malfunction the output NO FAILURE of

MONITORING is set at ldquological 0rdquo (= alarm) The alarm ldquoPARAMETERS CHANGEDrdquo

is activated whenever parameters or settings of software switches have been changed via

the Micro-Terminal

615 Actual Value Monitoring

The actual values for generator voltage Ug synchronous voltage Usyn and field

current If are monitored for malfunctions This monitoring is active regardless of whether

or not the generator is in operation Essentially when the generator is in operation the

measurements are monitored by comparing the signals (the smaller signal reading is

detected as incorrect) When the generator is not in operation the measured data are

monitored for extreme values The percentage of deviation permissible in the

measurement signals being compared

(Ug1ampUsyn1 Ug1ampUg2 Ug2ampUsyn2 If1ampIf2) is defined by parameters F208 and

F20AIf the excitation transformer is being supplied from an auxiliary power source (no

shunt operation) the values of Ug and Usyn will be different in some operational

conditions

In that case Flag F75A can be used to deactivate comparative measurements Ug1 Usyn1

and Ug2 Usyn2 The ldquoprimary monitoringrdquo compares Ug1 with Ug2 and generates the

messages that Channel 1 or Channel 2 has suffered a malfunction Whenever Ug1 lt Ug2

and the binary message from CH1 reports no malfunction a malfunction on

Measurement Channel 1 is reported (Ug1Usyn1 FAILURE) A similar malfunction is

also present whenever the binary message CH1 DISTURBANCE is reported and a

discrepancy is detected between Ug1ampUg2 The generation of the alarm ldquoUg2 Usyn2

FAILURErdquo is analogous to that for Channel 1 The ldquosecondary monitoringrdquo compares

Ug1 with Usyn1 amp Ug2 with Usyn2 This is enabled whenever the binary message of the

comparison channel reports a malfunction or whenever both binary messages report no

malfunction - but both secondary monitors report a malfunction As long as the secondary

monitoring is blocked the differences Ug1neUsyn1 or Ug2neUsyn2 trigger malfunction

signals for the measurement channel involved (suspicion that there is a corresponding

error in Usyn) The measurement channel malfunctions are enabled operationally

whenever after excitation has been switched on generator voltage Ug exceeds the value

set on F204 The voltages Ug1 and Ug2 are checked 16 seconds after the excitation is

switched off to see that they do not exceed the limit value F210 that applies to both of

them At the same time g1Ug2Usyn1Usyn2 are checked for extreme values (gt or lt

the operating range) Monitoring for extreme values is likewise enabled during normal

operation ( Excitation ON and Ug gt F204 ) Flag F754 is used to enable or block the

malfunction signals to the binary outputs Basically the monitoring of the actual values

for If1If2 functions like that of the Ug1Ug2 monitoring

62 Excitation Protection

621 General Information

The Excitation Protection switches off the excitation (and de-excites the machine

rapidly) whenever a danger arises that threatens the excitation transformer the converter

or the generator Generally limiter or monitoring functions precede the emergency trips

and these normally respond before the Excitation Protection must initiate a trip

Protective trip commands are issued directly to the field circuit-breaker from potential

free contacts of the board via the trip relays They are directed redundantly to the

operative field circuit-breaker ldquoOFFrdquo command

622 Protection against Excitation Transformer Overheating

This equipment monitors the excitation transformer for overheating in the

windings that could result from over current short-circuits or inadequate cooling The

monitoring uses temperature monitoring modules in conjunction with temperature

sensors built into the windings Normally the temperature is monitored in two stages the

first stage sets off an alarm the second causes a trip of the excitation

623 Rotor Over voltage Protection

Malfunctions in the generator circuit (eg terminal short-circuit failed

synchronization asynchronous operation) cause induced negative field currents that

produce high voltages in the field circuit These must be restricted to a level with a

sufficient safety margin below the insulation capacity of the field winding (test voltage)

and also below the peak blocking voltage of the converter thyristors The crow bar

employs spark gap elements to detect over voltages in the field circuit Whenever they

respond the associated thyristors are fired immediately switching the de-excitation

resistor parallel to the field The de-excitation current generated thereby initiates an

excitation trip via a supervision circuit causing an immediate opening of the field circuit-

breaker The malfunction isets off an alarm and an internal malfunction is indicated at the

cubicle

TEST VALUES OBTAINED WHEN EXCITATION IS RAISED

TEST

SNO PARTICULARS ACTUAL VALUE OBTAINED VALUE

1 VREF 996 100

2 VACT 997 1003

3 IFACT 735 765

4 IGACT 703 707

5 ACTIVE

POWER

703 705

6 REACTIVE

POWER

101 142

7 POWER

FACTOR

099 IND 098 IND

8 ACTIVE

CURRENT(IR)

705 703

9 REACTIVE

CURRENT(IX)

102 138

10 POWER

ANGLE

566 546

11 FIRING

ANGLE

640 632

VALUES OBTAINED WHEN EXCITATION IS LOWERED

SNO PARTICULARS ACTUAL

VALUE

OBTAINED VALUE

1 VREF 100 997

2 VACT 100 997

3 IFACT 787 761

4 IGACT 839 837

5 ACTIVE

POWER

839 830

6 REACTIVE

POWER

155 110

7 POWER

FACTOR

098 IND 099 IND

8 ACTIVE

CURRENT(IR)

835 830

9 REACTIVE

CURRENT(IX)

140 94

10 POWER

ANGLE

613 DEG 631 DEG

11 FIRING

ANGLE

601 DEG 599 DEG

CONCLUSION

For generating the EMF in stator winding excitation is required to the rotor of a

generator There are two types of excitation

1 Static excitation system

2 Brushless excitation system

A certain disadvantage in brushless excitation system is the slow response time of

the field in case of fast load changes specified No slip-rings and brushes direct

measurements of the field parameters not possible

To avoid all loses static excitation is used Since it does not have any rotating

parts mechanical loses and windage loses This system has fast response and speed

control While preferring this excitation system there are no limitations for the

redundancy of Thyristor bridge circuits

Static excitation has fast field discharge by resistor and inverter operation direct

measurement of field quantity is possible The meaning of excitation is nothing but

continuous supply of DC current (ie field current) to the rotor to buildup required

output voltage in the stator

Field current is changed with respect to the change of load so the digital

automatic voltage regulator (DAVR) is used to regulate the output voltage according to

the load variations

So we conclude that static excitation system with DAVR is preferred since it is

having excellent dynamic performance and better options for R amp M

Fig Field Flashing

Because the field flashing contactor is not able to switch off the energy stored in

the field the control ensures that the contactor can only reopen if the field circuit breaker

has already been opened (generating the TRIP order) or in a normal field flashing

sequence when the converter has taken over the field current Field flashing occurs in the

following stages

1048729The excitation is switched on closing the field flashing contactor ( Field

Circuit Breaker is already closed )

1048729The start-up excitation current flows through the rotor driving the generator

voltage up to approx 15 U

1048729After about 10 U the firing pulses to the converter are released and it begins

to excite the generator to its rated voltage

1048729After about 30 U the field flashing contactor opens (with no current since

the converter is now supplying the current)

The diode bridge at the input to the field flashing contactor prevents a feed-back

from the converter to the source of field flashing while the contactor is still closed

53 De-excitation

When malfunctions occur the stored field energy must be dissipated as quickly

and safely as possible to protect the generator This is done by the converter the field

circuit-breaker and the de-excitation (discharge) resistor

De-excitation (with opening of the field circuit-breaker) takes place in the following

stages

1048729The converter drives to its inverter limit position (negative ceiling voltage)

recovers a portion of the field energy into the network A trip command is given to the

field circuit breaker

1048729The de-excitation contact closes diverting the field voltage to the de-excitation

resistor

1048729Then immediately the main contacts open building voltage The field voltage

commutates to the de-excitation resistor

1048729The current diminishes at a given time constant TE

(With linear resistance TE = Lf (Rf + Re))

Due to the reversal of the field voltage by the converter the field current

commutates from the main contacts of the field circuit-breaker to the de-excitation

resistor in a very early phase This reversal of the field voltage prevents burn-off on the

main contacts and provides effective protection for the field circuit-breaker Depending

on the operating policy an operational shut-down of the excitation can also be effected

with the field circuit-breaker closed This method is useful mainly when the excitation is

switched on and off frequently In this case the converter is merely driven into the

inverter limit position so that the field energy is recovered into the network The

converter then blocks since it is supplying positive current only

54 Excitation Transformer

The excitation transformer matches the generator voltage to the field voltage

(required ceiling voltage) It also serves as a commutation reactance for the thyristor

converter and as a potential isolator between the network and the excitation circuit In

addition the transformer functions as a current limiter in that it makes it possible to keep

any short circuits in the excitation circuit under better control The excitation transformer

is equipped with temperature monitoring probes which set off an alarm when the

temperature exceeds a first max limit and then trips the excitation if the temperature

continues rising to a second (higher)limit

CHAPTER 6

Monitoring and Protection

61 Excitation Monitoring

611 General Information

The main goal of Excitation Monitoring is to make optimum use of the

redundancies provided in the excitation system and to give alarm whenever a malfunction

makes these redundancies unavailable The field current is monitored to see that it does

not exceed a maximum level and if necessary a switch-over to the MANUAL channel is

initiated In addition the criterion for switching off the field flashing is generated The

excitation Monitoring consists of an autonomous processor system

612 over current Alarms

In the Excitation Monitoring the limits for are set at higher levels than the

settings on the Field Current Maximum Limiter Whenever the current exceeds 110 of

the nominal field current contact R1 and the binary output associated with it are

activated immediately If field current remains gt 110 then after a preset inverse-time

has lapsed relay R2 and - after a further delay - relay R0 and the binary outputs

associated with them are activated Parameters match the measurements for If1 and If2 to

the nominal value for field current so that the internal values can be processed and read as

pu values It can be used to falsify the actual value of the field current If (to raise it) so

as to cause a response from the alarm limits for purposes of testing The processed If

signal is always taken from on the active channel (CHANNEL I OR CHANNEL II) As

long as the field current If is above the threshold value 11 Ifn its peak value is

measured This is stored (until RESET) and can be read at any time on the Micro-

Terminal Once the value of If exceeds 11 Ifn integration of this value starts Whenever

the integrated time-current value (intisup2dt) exceeds the preselected reference value the

alarm OVER CURRENT INVERSE-TIME is set off and a command is simultaneously

issued to switch over to the stand by AUTO channel Software switch F758 enables the

three over current alarm functions (R0 R1 R2) and selects one of three possible inverse-

time curves T1 T2 or T3 Within the characteristic curve (T1 T2 T3) selected the

desired limit curve for response is set using the factor F216

613 Switch-Off Criterion for Field Flashing

The Excitation Monitoring supplies the criterion for switching off the field

flashing Whether this criterion is activated based on the actual value for generator voltage

Ug or for field current If or both depends on the settings of the two threshold values

F200F202 (0 setting means that the output is always ldquological 1rdquo) The measurements

Ug12 and If12 are switched over depending on the present status of the channels

(Channel 1 or Channel 2 ON) Whenever Flag F750 is not activated the binary output is

fixed at ldquological 1rdquo

614 Storage of Alarm Status

The outputs of the over current alarms (R1 R2 R0) and the messages NO

FAILURE MONITORING PARAMETERS CHANGED are stored messages can be

erased by activating the input ldquoGENERAL RESETrdquo or by using the RESET button on the

front of the module Erasure with the input ldquoGENERAL RESETrdquo is effective only if the

situation causing the alarm or the malfunction is no longer present Whenever the self-

diagnosis equipment in the processor detects a malfunction the output NO FAILURE of

MONITORING is set at ldquological 0rdquo (= alarm) The alarm ldquoPARAMETERS CHANGEDrdquo

is activated whenever parameters or settings of software switches have been changed via

the Micro-Terminal

615 Actual Value Monitoring

The actual values for generator voltage Ug synchronous voltage Usyn and field

current If are monitored for malfunctions This monitoring is active regardless of whether

or not the generator is in operation Essentially when the generator is in operation the

measurements are monitored by comparing the signals (the smaller signal reading is

detected as incorrect) When the generator is not in operation the measured data are

monitored for extreme values The percentage of deviation permissible in the

measurement signals being compared

(Ug1ampUsyn1 Ug1ampUg2 Ug2ampUsyn2 If1ampIf2) is defined by parameters F208 and

F20AIf the excitation transformer is being supplied from an auxiliary power source (no

shunt operation) the values of Ug and Usyn will be different in some operational

conditions

In that case Flag F75A can be used to deactivate comparative measurements Ug1 Usyn1

and Ug2 Usyn2 The ldquoprimary monitoringrdquo compares Ug1 with Ug2 and generates the

messages that Channel 1 or Channel 2 has suffered a malfunction Whenever Ug1 lt Ug2

and the binary message from CH1 reports no malfunction a malfunction on

Measurement Channel 1 is reported (Ug1Usyn1 FAILURE) A similar malfunction is

also present whenever the binary message CH1 DISTURBANCE is reported and a

discrepancy is detected between Ug1ampUg2 The generation of the alarm ldquoUg2 Usyn2

FAILURErdquo is analogous to that for Channel 1 The ldquosecondary monitoringrdquo compares

Ug1 with Usyn1 amp Ug2 with Usyn2 This is enabled whenever the binary message of the

comparison channel reports a malfunction or whenever both binary messages report no

malfunction - but both secondary monitors report a malfunction As long as the secondary

monitoring is blocked the differences Ug1neUsyn1 or Ug2neUsyn2 trigger malfunction

signals for the measurement channel involved (suspicion that there is a corresponding

error in Usyn) The measurement channel malfunctions are enabled operationally

whenever after excitation has been switched on generator voltage Ug exceeds the value

set on F204 The voltages Ug1 and Ug2 are checked 16 seconds after the excitation is

switched off to see that they do not exceed the limit value F210 that applies to both of

them At the same time g1Ug2Usyn1Usyn2 are checked for extreme values (gt or lt

the operating range) Monitoring for extreme values is likewise enabled during normal

operation ( Excitation ON and Ug gt F204 ) Flag F754 is used to enable or block the

malfunction signals to the binary outputs Basically the monitoring of the actual values

for If1If2 functions like that of the Ug1Ug2 monitoring

62 Excitation Protection

621 General Information

The Excitation Protection switches off the excitation (and de-excites the machine

rapidly) whenever a danger arises that threatens the excitation transformer the converter

or the generator Generally limiter or monitoring functions precede the emergency trips

and these normally respond before the Excitation Protection must initiate a trip

Protective trip commands are issued directly to the field circuit-breaker from potential

free contacts of the board via the trip relays They are directed redundantly to the

operative field circuit-breaker ldquoOFFrdquo command

622 Protection against Excitation Transformer Overheating

This equipment monitors the excitation transformer for overheating in the

windings that could result from over current short-circuits or inadequate cooling The

monitoring uses temperature monitoring modules in conjunction with temperature

sensors built into the windings Normally the temperature is monitored in two stages the

first stage sets off an alarm the second causes a trip of the excitation

623 Rotor Over voltage Protection

Malfunctions in the generator circuit (eg terminal short-circuit failed

synchronization asynchronous operation) cause induced negative field currents that

produce high voltages in the field circuit These must be restricted to a level with a

sufficient safety margin below the insulation capacity of the field winding (test voltage)

and also below the peak blocking voltage of the converter thyristors The crow bar

employs spark gap elements to detect over voltages in the field circuit Whenever they

respond the associated thyristors are fired immediately switching the de-excitation

resistor parallel to the field The de-excitation current generated thereby initiates an

excitation trip via a supervision circuit causing an immediate opening of the field circuit-

breaker The malfunction isets off an alarm and an internal malfunction is indicated at the

cubicle

TEST VALUES OBTAINED WHEN EXCITATION IS RAISED

TEST

SNO PARTICULARS ACTUAL VALUE OBTAINED VALUE

1 VREF 996 100

2 VACT 997 1003

3 IFACT 735 765

4 IGACT 703 707

5 ACTIVE

POWER

703 705

6 REACTIVE

POWER

101 142

7 POWER

FACTOR

099 IND 098 IND

8 ACTIVE

CURRENT(IR)

705 703

9 REACTIVE

CURRENT(IX)

102 138

10 POWER

ANGLE

566 546

11 FIRING

ANGLE

640 632

VALUES OBTAINED WHEN EXCITATION IS LOWERED

SNO PARTICULARS ACTUAL

VALUE

OBTAINED VALUE

1 VREF 100 997

2 VACT 100 997

3 IFACT 787 761

4 IGACT 839 837

5 ACTIVE

POWER

839 830

6 REACTIVE

POWER

155 110

7 POWER

FACTOR

098 IND 099 IND

8 ACTIVE

CURRENT(IR)

835 830

9 REACTIVE

CURRENT(IX)

140 94

10 POWER

ANGLE

613 DEG 631 DEG

11 FIRING

ANGLE

601 DEG 599 DEG

CONCLUSION

For generating the EMF in stator winding excitation is required to the rotor of a

generator There are two types of excitation

1 Static excitation system

2 Brushless excitation system

A certain disadvantage in brushless excitation system is the slow response time of

the field in case of fast load changes specified No slip-rings and brushes direct

measurements of the field parameters not possible

To avoid all loses static excitation is used Since it does not have any rotating

parts mechanical loses and windage loses This system has fast response and speed

control While preferring this excitation system there are no limitations for the

redundancy of Thyristor bridge circuits

Static excitation has fast field discharge by resistor and inverter operation direct

measurement of field quantity is possible The meaning of excitation is nothing but

continuous supply of DC current (ie field current) to the rotor to buildup required

output voltage in the stator

Field current is changed with respect to the change of load so the digital

automatic voltage regulator (DAVR) is used to regulate the output voltage according to

the load variations

So we conclude that static excitation system with DAVR is preferred since it is

having excellent dynamic performance and better options for R amp M

1048729The converter drives to its inverter limit position (negative ceiling voltage)

recovers a portion of the field energy into the network A trip command is given to the

field circuit breaker

1048729The de-excitation contact closes diverting the field voltage to the de-excitation

resistor

1048729Then immediately the main contacts open building voltage The field voltage

commutates to the de-excitation resistor

1048729The current diminishes at a given time constant TE

(With linear resistance TE = Lf (Rf + Re))

Due to the reversal of the field voltage by the converter the field current

commutates from the main contacts of the field circuit-breaker to the de-excitation

resistor in a very early phase This reversal of the field voltage prevents burn-off on the

main contacts and provides effective protection for the field circuit-breaker Depending

on the operating policy an operational shut-down of the excitation can also be effected

with the field circuit-breaker closed This method is useful mainly when the excitation is

switched on and off frequently In this case the converter is merely driven into the

inverter limit position so that the field energy is recovered into the network The

converter then blocks since it is supplying positive current only

54 Excitation Transformer

The excitation transformer matches the generator voltage to the field voltage

(required ceiling voltage) It also serves as a commutation reactance for the thyristor

converter and as a potential isolator between the network and the excitation circuit In

addition the transformer functions as a current limiter in that it makes it possible to keep

any short circuits in the excitation circuit under better control The excitation transformer

is equipped with temperature monitoring probes which set off an alarm when the

temperature exceeds a first max limit and then trips the excitation if the temperature

continues rising to a second (higher)limit

CHAPTER 6

Monitoring and Protection

61 Excitation Monitoring

611 General Information

The main goal of Excitation Monitoring is to make optimum use of the

redundancies provided in the excitation system and to give alarm whenever a malfunction

makes these redundancies unavailable The field current is monitored to see that it does

not exceed a maximum level and if necessary a switch-over to the MANUAL channel is

initiated In addition the criterion for switching off the field flashing is generated The

excitation Monitoring consists of an autonomous processor system

612 over current Alarms

In the Excitation Monitoring the limits for are set at higher levels than the

settings on the Field Current Maximum Limiter Whenever the current exceeds 110 of

the nominal field current contact R1 and the binary output associated with it are

activated immediately If field current remains gt 110 then after a preset inverse-time

has lapsed relay R2 and - after a further delay - relay R0 and the binary outputs

associated with them are activated Parameters match the measurements for If1 and If2 to

the nominal value for field current so that the internal values can be processed and read as

pu values It can be used to falsify the actual value of the field current If (to raise it) so

as to cause a response from the alarm limits for purposes of testing The processed If

signal is always taken from on the active channel (CHANNEL I OR CHANNEL II) As

long as the field current If is above the threshold value 11 Ifn its peak value is

measured This is stored (until RESET) and can be read at any time on the Micro-

Terminal Once the value of If exceeds 11 Ifn integration of this value starts Whenever

the integrated time-current value (intisup2dt) exceeds the preselected reference value the

alarm OVER CURRENT INVERSE-TIME is set off and a command is simultaneously

issued to switch over to the stand by AUTO channel Software switch F758 enables the

three over current alarm functions (R0 R1 R2) and selects one of three possible inverse-

time curves T1 T2 or T3 Within the characteristic curve (T1 T2 T3) selected the

desired limit curve for response is set using the factor F216

613 Switch-Off Criterion for Field Flashing

The Excitation Monitoring supplies the criterion for switching off the field

flashing Whether this criterion is activated based on the actual value for generator voltage

Ug or for field current If or both depends on the settings of the two threshold values

F200F202 (0 setting means that the output is always ldquological 1rdquo) The measurements

Ug12 and If12 are switched over depending on the present status of the channels

(Channel 1 or Channel 2 ON) Whenever Flag F750 is not activated the binary output is

fixed at ldquological 1rdquo

614 Storage of Alarm Status

The outputs of the over current alarms (R1 R2 R0) and the messages NO

FAILURE MONITORING PARAMETERS CHANGED are stored messages can be

erased by activating the input ldquoGENERAL RESETrdquo or by using the RESET button on the

front of the module Erasure with the input ldquoGENERAL RESETrdquo is effective only if the

situation causing the alarm or the malfunction is no longer present Whenever the self-

diagnosis equipment in the processor detects a malfunction the output NO FAILURE of

MONITORING is set at ldquological 0rdquo (= alarm) The alarm ldquoPARAMETERS CHANGEDrdquo

is activated whenever parameters or settings of software switches have been changed via

the Micro-Terminal

615 Actual Value Monitoring

The actual values for generator voltage Ug synchronous voltage Usyn and field

current If are monitored for malfunctions This monitoring is active regardless of whether

or not the generator is in operation Essentially when the generator is in operation the

measurements are monitored by comparing the signals (the smaller signal reading is

detected as incorrect) When the generator is not in operation the measured data are

monitored for extreme values The percentage of deviation permissible in the

measurement signals being compared

(Ug1ampUsyn1 Ug1ampUg2 Ug2ampUsyn2 If1ampIf2) is defined by parameters F208 and

F20AIf the excitation transformer is being supplied from an auxiliary power source (no

shunt operation) the values of Ug and Usyn will be different in some operational

conditions

In that case Flag F75A can be used to deactivate comparative measurements Ug1 Usyn1

and Ug2 Usyn2 The ldquoprimary monitoringrdquo compares Ug1 with Ug2 and generates the

messages that Channel 1 or Channel 2 has suffered a malfunction Whenever Ug1 lt Ug2

and the binary message from CH1 reports no malfunction a malfunction on

Measurement Channel 1 is reported (Ug1Usyn1 FAILURE) A similar malfunction is

also present whenever the binary message CH1 DISTURBANCE is reported and a

discrepancy is detected between Ug1ampUg2 The generation of the alarm ldquoUg2 Usyn2

FAILURErdquo is analogous to that for Channel 1 The ldquosecondary monitoringrdquo compares

Ug1 with Usyn1 amp Ug2 with Usyn2 This is enabled whenever the binary message of the

comparison channel reports a malfunction or whenever both binary messages report no

malfunction - but both secondary monitors report a malfunction As long as the secondary

monitoring is blocked the differences Ug1neUsyn1 or Ug2neUsyn2 trigger malfunction

signals for the measurement channel involved (suspicion that there is a corresponding

error in Usyn) The measurement channel malfunctions are enabled operationally

whenever after excitation has been switched on generator voltage Ug exceeds the value

set on F204 The voltages Ug1 and Ug2 are checked 16 seconds after the excitation is

switched off to see that they do not exceed the limit value F210 that applies to both of

them At the same time g1Ug2Usyn1Usyn2 are checked for extreme values (gt or lt

the operating range) Monitoring for extreme values is likewise enabled during normal

operation ( Excitation ON and Ug gt F204 ) Flag F754 is used to enable or block the

malfunction signals to the binary outputs Basically the monitoring of the actual values

for If1If2 functions like that of the Ug1Ug2 monitoring

62 Excitation Protection

621 General Information

The Excitation Protection switches off the excitation (and de-excites the machine

rapidly) whenever a danger arises that threatens the excitation transformer the converter

or the generator Generally limiter or monitoring functions precede the emergency trips

and these normally respond before the Excitation Protection must initiate a trip

Protective trip commands are issued directly to the field circuit-breaker from potential

free contacts of the board via the trip relays They are directed redundantly to the

operative field circuit-breaker ldquoOFFrdquo command

622 Protection against Excitation Transformer Overheating

This equipment monitors the excitation transformer for overheating in the

windings that could result from over current short-circuits or inadequate cooling The

monitoring uses temperature monitoring modules in conjunction with temperature

sensors built into the windings Normally the temperature is monitored in two stages the

first stage sets off an alarm the second causes a trip of the excitation

623 Rotor Over voltage Protection

Malfunctions in the generator circuit (eg terminal short-circuit failed

synchronization asynchronous operation) cause induced negative field currents that

produce high voltages in the field circuit These must be restricted to a level with a

sufficient safety margin below the insulation capacity of the field winding (test voltage)

and also below the peak blocking voltage of the converter thyristors The crow bar

employs spark gap elements to detect over voltages in the field circuit Whenever they

respond the associated thyristors are fired immediately switching the de-excitation

resistor parallel to the field The de-excitation current generated thereby initiates an

excitation trip via a supervision circuit causing an immediate opening of the field circuit-

breaker The malfunction isets off an alarm and an internal malfunction is indicated at the

cubicle

TEST VALUES OBTAINED WHEN EXCITATION IS RAISED

TEST

SNO PARTICULARS ACTUAL VALUE OBTAINED VALUE

1 VREF 996 100

2 VACT 997 1003

3 IFACT 735 765

4 IGACT 703 707

5 ACTIVE

POWER

703 705

6 REACTIVE

POWER

101 142

7 POWER

FACTOR

099 IND 098 IND

8 ACTIVE

CURRENT(IR)

705 703

9 REACTIVE

CURRENT(IX)

102 138

10 POWER

ANGLE

566 546

11 FIRING

ANGLE

640 632

VALUES OBTAINED WHEN EXCITATION IS LOWERED

SNO PARTICULARS ACTUAL

VALUE

OBTAINED VALUE

1 VREF 100 997

2 VACT 100 997

3 IFACT 787 761

4 IGACT 839 837

5 ACTIVE

POWER

839 830

6 REACTIVE

POWER

155 110

7 POWER

FACTOR

098 IND 099 IND

8 ACTIVE

CURRENT(IR)

835 830

9 REACTIVE

CURRENT(IX)

140 94

10 POWER

ANGLE

613 DEG 631 DEG

11 FIRING

ANGLE

601 DEG 599 DEG

CONCLUSION

For generating the EMF in stator winding excitation is required to the rotor of a

generator There are two types of excitation

1 Static excitation system

2 Brushless excitation system

A certain disadvantage in brushless excitation system is the slow response time of

the field in case of fast load changes specified No slip-rings and brushes direct

measurements of the field parameters not possible

To avoid all loses static excitation is used Since it does not have any rotating

parts mechanical loses and windage loses This system has fast response and speed

control While preferring this excitation system there are no limitations for the

redundancy of Thyristor bridge circuits

Static excitation has fast field discharge by resistor and inverter operation direct

measurement of field quantity is possible The meaning of excitation is nothing but

continuous supply of DC current (ie field current) to the rotor to buildup required

output voltage in the stator

Field current is changed with respect to the change of load so the digital

automatic voltage regulator (DAVR) is used to regulate the output voltage according to

the load variations

So we conclude that static excitation system with DAVR is preferred since it is

having excellent dynamic performance and better options for R amp M

Monitoring and Protection

61 Excitation Monitoring

611 General Information

The main goal of Excitation Monitoring is to make optimum use of the

redundancies provided in the excitation system and to give alarm whenever a malfunction

makes these redundancies unavailable The field current is monitored to see that it does

not exceed a maximum level and if necessary a switch-over to the MANUAL channel is

initiated In addition the criterion for switching off the field flashing is generated The

excitation Monitoring consists of an autonomous processor system

612 over current Alarms

In the Excitation Monitoring the limits for are set at higher levels than the

settings on the Field Current Maximum Limiter Whenever the current exceeds 110 of

the nominal field current contact R1 and the binary output associated with it are

activated immediately If field current remains gt 110 then after a preset inverse-time

has lapsed relay R2 and - after a further delay - relay R0 and the binary outputs

associated with them are activated Parameters match the measurements for If1 and If2 to

the nominal value for field current so that the internal values can be processed and read as

pu values It can be used to falsify the actual value of the field current If (to raise it) so

as to cause a response from the alarm limits for purposes of testing The processed If

signal is always taken from on the active channel (CHANNEL I OR CHANNEL II) As

long as the field current If is above the threshold value 11 Ifn its peak value is

measured This is stored (until RESET) and can be read at any time on the Micro-

Terminal Once the value of If exceeds 11 Ifn integration of this value starts Whenever

the integrated time-current value (intisup2dt) exceeds the preselected reference value the

alarm OVER CURRENT INVERSE-TIME is set off and a command is simultaneously

issued to switch over to the stand by AUTO channel Software switch F758 enables the

three over current alarm functions (R0 R1 R2) and selects one of three possible inverse-

time curves T1 T2 or T3 Within the characteristic curve (T1 T2 T3) selected the

desired limit curve for response is set using the factor F216

613 Switch-Off Criterion for Field Flashing

The Excitation Monitoring supplies the criterion for switching off the field

flashing Whether this criterion is activated based on the actual value for generator voltage

Ug or for field current If or both depends on the settings of the two threshold values

F200F202 (0 setting means that the output is always ldquological 1rdquo) The measurements

Ug12 and If12 are switched over depending on the present status of the channels

(Channel 1 or Channel 2 ON) Whenever Flag F750 is not activated the binary output is

fixed at ldquological 1rdquo

614 Storage of Alarm Status

The outputs of the over current alarms (R1 R2 R0) and the messages NO

FAILURE MONITORING PARAMETERS CHANGED are stored messages can be

erased by activating the input ldquoGENERAL RESETrdquo or by using the RESET button on the

front of the module Erasure with the input ldquoGENERAL RESETrdquo is effective only if the

situation causing the alarm or the malfunction is no longer present Whenever the self-

diagnosis equipment in the processor detects a malfunction the output NO FAILURE of

MONITORING is set at ldquological 0rdquo (= alarm) The alarm ldquoPARAMETERS CHANGEDrdquo

is activated whenever parameters or settings of software switches have been changed via

the Micro-Terminal

615 Actual Value Monitoring

The actual values for generator voltage Ug synchronous voltage Usyn and field

current If are monitored for malfunctions This monitoring is active regardless of whether

or not the generator is in operation Essentially when the generator is in operation the

measurements are monitored by comparing the signals (the smaller signal reading is

detected as incorrect) When the generator is not in operation the measured data are

monitored for extreme values The percentage of deviation permissible in the

measurement signals being compared

(Ug1ampUsyn1 Ug1ampUg2 Ug2ampUsyn2 If1ampIf2) is defined by parameters F208 and

F20AIf the excitation transformer is being supplied from an auxiliary power source (no

shunt operation) the values of Ug and Usyn will be different in some operational

conditions

In that case Flag F75A can be used to deactivate comparative measurements Ug1 Usyn1

and Ug2 Usyn2 The ldquoprimary monitoringrdquo compares Ug1 with Ug2 and generates the

messages that Channel 1 or Channel 2 has suffered a malfunction Whenever Ug1 lt Ug2

and the binary message from CH1 reports no malfunction a malfunction on

Measurement Channel 1 is reported (Ug1Usyn1 FAILURE) A similar malfunction is

also present whenever the binary message CH1 DISTURBANCE is reported and a

discrepancy is detected between Ug1ampUg2 The generation of the alarm ldquoUg2 Usyn2

FAILURErdquo is analogous to that for Channel 1 The ldquosecondary monitoringrdquo compares

Ug1 with Usyn1 amp Ug2 with Usyn2 This is enabled whenever the binary message of the

comparison channel reports a malfunction or whenever both binary messages report no

malfunction - but both secondary monitors report a malfunction As long as the secondary

monitoring is blocked the differences Ug1neUsyn1 or Ug2neUsyn2 trigger malfunction

signals for the measurement channel involved (suspicion that there is a corresponding

error in Usyn) The measurement channel malfunctions are enabled operationally

whenever after excitation has been switched on generator voltage Ug exceeds the value

set on F204 The voltages Ug1 and Ug2 are checked 16 seconds after the excitation is

switched off to see that they do not exceed the limit value F210 that applies to both of

them At the same time g1Ug2Usyn1Usyn2 are checked for extreme values (gt or lt

the operating range) Monitoring for extreme values is likewise enabled during normal

operation ( Excitation ON and Ug gt F204 ) Flag F754 is used to enable or block the

malfunction signals to the binary outputs Basically the monitoring of the actual values

for If1If2 functions like that of the Ug1Ug2 monitoring

62 Excitation Protection

621 General Information

The Excitation Protection switches off the excitation (and de-excites the machine

rapidly) whenever a danger arises that threatens the excitation transformer the converter

or the generator Generally limiter or monitoring functions precede the emergency trips

and these normally respond before the Excitation Protection must initiate a trip

Protective trip commands are issued directly to the field circuit-breaker from potential

free contacts of the board via the trip relays They are directed redundantly to the

operative field circuit-breaker ldquoOFFrdquo command

622 Protection against Excitation Transformer Overheating

This equipment monitors the excitation transformer for overheating in the

windings that could result from over current short-circuits or inadequate cooling The

monitoring uses temperature monitoring modules in conjunction with temperature

sensors built into the windings Normally the temperature is monitored in two stages the

first stage sets off an alarm the second causes a trip of the excitation

623 Rotor Over voltage Protection

Malfunctions in the generator circuit (eg terminal short-circuit failed

synchronization asynchronous operation) cause induced negative field currents that

produce high voltages in the field circuit These must be restricted to a level with a

sufficient safety margin below the insulation capacity of the field winding (test voltage)

and also below the peak blocking voltage of the converter thyristors The crow bar

employs spark gap elements to detect over voltages in the field circuit Whenever they

respond the associated thyristors are fired immediately switching the de-excitation

resistor parallel to the field The de-excitation current generated thereby initiates an

excitation trip via a supervision circuit causing an immediate opening of the field circuit-

breaker The malfunction isets off an alarm and an internal malfunction is indicated at the

cubicle

TEST VALUES OBTAINED WHEN EXCITATION IS RAISED

TEST

SNO PARTICULARS ACTUAL VALUE OBTAINED VALUE

1 VREF 996 100

2 VACT 997 1003

3 IFACT 735 765

4 IGACT 703 707

5 ACTIVE

POWER

703 705

6 REACTIVE

POWER

101 142

7 POWER

FACTOR

099 IND 098 IND

8 ACTIVE

CURRENT(IR)

705 703

9 REACTIVE

CURRENT(IX)

102 138

10 POWER

ANGLE

566 546

11 FIRING

ANGLE

640 632

VALUES OBTAINED WHEN EXCITATION IS LOWERED

SNO PARTICULARS ACTUAL

VALUE

OBTAINED VALUE

1 VREF 100 997

2 VACT 100 997

3 IFACT 787 761

4 IGACT 839 837

5 ACTIVE

POWER

839 830

6 REACTIVE

POWER

155 110

7 POWER

FACTOR

098 IND 099 IND

8 ACTIVE

CURRENT(IR)

835 830

9 REACTIVE

CURRENT(IX)

140 94

10 POWER

ANGLE

613 DEG 631 DEG

11 FIRING

ANGLE

601 DEG 599 DEG

CONCLUSION

For generating the EMF in stator winding excitation is required to the rotor of a

generator There are two types of excitation

1 Static excitation system

2 Brushless excitation system

A certain disadvantage in brushless excitation system is the slow response time of

the field in case of fast load changes specified No slip-rings and brushes direct

measurements of the field parameters not possible

To avoid all loses static excitation is used Since it does not have any rotating

parts mechanical loses and windage loses This system has fast response and speed

control While preferring this excitation system there are no limitations for the

redundancy of Thyristor bridge circuits

Static excitation has fast field discharge by resistor and inverter operation direct

measurement of field quantity is possible The meaning of excitation is nothing but

continuous supply of DC current (ie field current) to the rotor to buildup required

output voltage in the stator

Field current is changed with respect to the change of load so the digital

automatic voltage regulator (DAVR) is used to regulate the output voltage according to

the load variations

So we conclude that static excitation system with DAVR is preferred since it is

having excellent dynamic performance and better options for R amp M

The Excitation Monitoring supplies the criterion for switching off the field

flashing Whether this criterion is activated based on the actual value for generator voltage

Ug or for field current If or both depends on the settings of the two threshold values

F200F202 (0 setting means that the output is always ldquological 1rdquo) The measurements

Ug12 and If12 are switched over depending on the present status of the channels

(Channel 1 or Channel 2 ON) Whenever Flag F750 is not activated the binary output is

fixed at ldquological 1rdquo

614 Storage of Alarm Status

The outputs of the over current alarms (R1 R2 R0) and the messages NO

FAILURE MONITORING PARAMETERS CHANGED are stored messages can be

erased by activating the input ldquoGENERAL RESETrdquo or by using the RESET button on the

front of the module Erasure with the input ldquoGENERAL RESETrdquo is effective only if the

situation causing the alarm or the malfunction is no longer present Whenever the self-

diagnosis equipment in the processor detects a malfunction the output NO FAILURE of

MONITORING is set at ldquological 0rdquo (= alarm) The alarm ldquoPARAMETERS CHANGEDrdquo

is activated whenever parameters or settings of software switches have been changed via

the Micro-Terminal

615 Actual Value Monitoring

The actual values for generator voltage Ug synchronous voltage Usyn and field

current If are monitored for malfunctions This monitoring is active regardless of whether

or not the generator is in operation Essentially when the generator is in operation the

measurements are monitored by comparing the signals (the smaller signal reading is

detected as incorrect) When the generator is not in operation the measured data are

monitored for extreme values The percentage of deviation permissible in the

measurement signals being compared

(Ug1ampUsyn1 Ug1ampUg2 Ug2ampUsyn2 If1ampIf2) is defined by parameters F208 and

F20AIf the excitation transformer is being supplied from an auxiliary power source (no

shunt operation) the values of Ug and Usyn will be different in some operational

conditions

In that case Flag F75A can be used to deactivate comparative measurements Ug1 Usyn1

and Ug2 Usyn2 The ldquoprimary monitoringrdquo compares Ug1 with Ug2 and generates the

messages that Channel 1 or Channel 2 has suffered a malfunction Whenever Ug1 lt Ug2

and the binary message from CH1 reports no malfunction a malfunction on

Measurement Channel 1 is reported (Ug1Usyn1 FAILURE) A similar malfunction is

also present whenever the binary message CH1 DISTURBANCE is reported and a

discrepancy is detected between Ug1ampUg2 The generation of the alarm ldquoUg2 Usyn2

FAILURErdquo is analogous to that for Channel 1 The ldquosecondary monitoringrdquo compares

Ug1 with Usyn1 amp Ug2 with Usyn2 This is enabled whenever the binary message of the

comparison channel reports a malfunction or whenever both binary messages report no

malfunction - but both secondary monitors report a malfunction As long as the secondary

monitoring is blocked the differences Ug1neUsyn1 or Ug2neUsyn2 trigger malfunction

signals for the measurement channel involved (suspicion that there is a corresponding

error in Usyn) The measurement channel malfunctions are enabled operationally

whenever after excitation has been switched on generator voltage Ug exceeds the value

set on F204 The voltages Ug1 and Ug2 are checked 16 seconds after the excitation is

switched off to see that they do not exceed the limit value F210 that applies to both of

them At the same time g1Ug2Usyn1Usyn2 are checked for extreme values (gt or lt

the operating range) Monitoring for extreme values is likewise enabled during normal

operation ( Excitation ON and Ug gt F204 ) Flag F754 is used to enable or block the

malfunction signals to the binary outputs Basically the monitoring of the actual values

for If1If2 functions like that of the Ug1Ug2 monitoring

62 Excitation Protection

621 General Information

The Excitation Protection switches off the excitation (and de-excites the machine

rapidly) whenever a danger arises that threatens the excitation transformer the converter

or the generator Generally limiter or monitoring functions precede the emergency trips

and these normally respond before the Excitation Protection must initiate a trip

Protective trip commands are issued directly to the field circuit-breaker from potential

free contacts of the board via the trip relays They are directed redundantly to the

operative field circuit-breaker ldquoOFFrdquo command

622 Protection against Excitation Transformer Overheating

This equipment monitors the excitation transformer for overheating in the

windings that could result from over current short-circuits or inadequate cooling The

monitoring uses temperature monitoring modules in conjunction with temperature

sensors built into the windings Normally the temperature is monitored in two stages the

first stage sets off an alarm the second causes a trip of the excitation

623 Rotor Over voltage Protection

Malfunctions in the generator circuit (eg terminal short-circuit failed

synchronization asynchronous operation) cause induced negative field currents that

produce high voltages in the field circuit These must be restricted to a level with a

sufficient safety margin below the insulation capacity of the field winding (test voltage)

and also below the peak blocking voltage of the converter thyristors The crow bar

employs spark gap elements to detect over voltages in the field circuit Whenever they

respond the associated thyristors are fired immediately switching the de-excitation

resistor parallel to the field The de-excitation current generated thereby initiates an

excitation trip via a supervision circuit causing an immediate opening of the field circuit-

breaker The malfunction isets off an alarm and an internal malfunction is indicated at the

cubicle

TEST VALUES OBTAINED WHEN EXCITATION IS RAISED

TEST

SNO PARTICULARS ACTUAL VALUE OBTAINED VALUE

1 VREF 996 100

2 VACT 997 1003

3 IFACT 735 765

4 IGACT 703 707

5 ACTIVE

POWER

703 705

6 REACTIVE

POWER

101 142

7 POWER

FACTOR

099 IND 098 IND

8 ACTIVE

CURRENT(IR)

705 703

9 REACTIVE

CURRENT(IX)

102 138

10 POWER

ANGLE

566 546

11 FIRING

ANGLE

640 632

VALUES OBTAINED WHEN EXCITATION IS LOWERED

SNO PARTICULARS ACTUAL

VALUE

OBTAINED VALUE

1 VREF 100 997

2 VACT 100 997

3 IFACT 787 761

4 IGACT 839 837

5 ACTIVE

POWER

839 830

6 REACTIVE

POWER

155 110

7 POWER

FACTOR

098 IND 099 IND

8 ACTIVE

CURRENT(IR)

835 830

9 REACTIVE

CURRENT(IX)

140 94

10 POWER

ANGLE

613 DEG 631 DEG

11 FIRING

ANGLE

601 DEG 599 DEG

CONCLUSION

For generating the EMF in stator winding excitation is required to the rotor of a

generator There are two types of excitation

1 Static excitation system

2 Brushless excitation system

A certain disadvantage in brushless excitation system is the slow response time of

the field in case of fast load changes specified No slip-rings and brushes direct

measurements of the field parameters not possible

To avoid all loses static excitation is used Since it does not have any rotating

parts mechanical loses and windage loses This system has fast response and speed

control While preferring this excitation system there are no limitations for the

redundancy of Thyristor bridge circuits

Static excitation has fast field discharge by resistor and inverter operation direct

measurement of field quantity is possible The meaning of excitation is nothing but

continuous supply of DC current (ie field current) to the rotor to buildup required

output voltage in the stator

Field current is changed with respect to the change of load so the digital

automatic voltage regulator (DAVR) is used to regulate the output voltage according to

the load variations

So we conclude that static excitation system with DAVR is preferred since it is

having excellent dynamic performance and better options for R amp M

In that case Flag F75A can be used to deactivate comparative measurements Ug1 Usyn1

and Ug2 Usyn2 The ldquoprimary monitoringrdquo compares Ug1 with Ug2 and generates the

messages that Channel 1 or Channel 2 has suffered a malfunction Whenever Ug1 lt Ug2

and the binary message from CH1 reports no malfunction a malfunction on

Measurement Channel 1 is reported (Ug1Usyn1 FAILURE) A similar malfunction is

also present whenever the binary message CH1 DISTURBANCE is reported and a

discrepancy is detected between Ug1ampUg2 The generation of the alarm ldquoUg2 Usyn2

FAILURErdquo is analogous to that for Channel 1 The ldquosecondary monitoringrdquo compares

Ug1 with Usyn1 amp Ug2 with Usyn2 This is enabled whenever the binary message of the

comparison channel reports a malfunction or whenever both binary messages report no

malfunction - but both secondary monitors report a malfunction As long as the secondary

monitoring is blocked the differences Ug1neUsyn1 or Ug2neUsyn2 trigger malfunction

signals for the measurement channel involved (suspicion that there is a corresponding

error in Usyn) The measurement channel malfunctions are enabled operationally

whenever after excitation has been switched on generator voltage Ug exceeds the value

set on F204 The voltages Ug1 and Ug2 are checked 16 seconds after the excitation is

switched off to see that they do not exceed the limit value F210 that applies to both of

them At the same time g1Ug2Usyn1Usyn2 are checked for extreme values (gt or lt

the operating range) Monitoring for extreme values is likewise enabled during normal

operation ( Excitation ON and Ug gt F204 ) Flag F754 is used to enable or block the

malfunction signals to the binary outputs Basically the monitoring of the actual values

for If1If2 functions like that of the Ug1Ug2 monitoring

62 Excitation Protection

621 General Information

The Excitation Protection switches off the excitation (and de-excites the machine

rapidly) whenever a danger arises that threatens the excitation transformer the converter

or the generator Generally limiter or monitoring functions precede the emergency trips

and these normally respond before the Excitation Protection must initiate a trip

Protective trip commands are issued directly to the field circuit-breaker from potential

free contacts of the board via the trip relays They are directed redundantly to the

operative field circuit-breaker ldquoOFFrdquo command

622 Protection against Excitation Transformer Overheating

This equipment monitors the excitation transformer for overheating in the

windings that could result from over current short-circuits or inadequate cooling The

monitoring uses temperature monitoring modules in conjunction with temperature

sensors built into the windings Normally the temperature is monitored in two stages the

first stage sets off an alarm the second causes a trip of the excitation

623 Rotor Over voltage Protection

Malfunctions in the generator circuit (eg terminal short-circuit failed

synchronization asynchronous operation) cause induced negative field currents that

produce high voltages in the field circuit These must be restricted to a level with a

sufficient safety margin below the insulation capacity of the field winding (test voltage)

and also below the peak blocking voltage of the converter thyristors The crow bar

employs spark gap elements to detect over voltages in the field circuit Whenever they

respond the associated thyristors are fired immediately switching the de-excitation

resistor parallel to the field The de-excitation current generated thereby initiates an

excitation trip via a supervision circuit causing an immediate opening of the field circuit-

breaker The malfunction isets off an alarm and an internal malfunction is indicated at the

cubicle

TEST VALUES OBTAINED WHEN EXCITATION IS RAISED

TEST

SNO PARTICULARS ACTUAL VALUE OBTAINED VALUE

1 VREF 996 100

2 VACT 997 1003

3 IFACT 735 765

4 IGACT 703 707

5 ACTIVE

POWER

703 705

6 REACTIVE

POWER

101 142

7 POWER

FACTOR

099 IND 098 IND

8 ACTIVE

CURRENT(IR)

705 703

9 REACTIVE

CURRENT(IX)

102 138

10 POWER

ANGLE

566 546

11 FIRING

ANGLE

640 632

VALUES OBTAINED WHEN EXCITATION IS LOWERED

SNO PARTICULARS ACTUAL

VALUE

OBTAINED VALUE

1 VREF 100 997

2 VACT 100 997

3 IFACT 787 761

4 IGACT 839 837

5 ACTIVE

POWER

839 830

6 REACTIVE

POWER

155 110

7 POWER

FACTOR

098 IND 099 IND

8 ACTIVE

CURRENT(IR)

835 830

9 REACTIVE

CURRENT(IX)

140 94

10 POWER

ANGLE

613 DEG 631 DEG

11 FIRING

ANGLE

601 DEG 599 DEG

CONCLUSION

For generating the EMF in stator winding excitation is required to the rotor of a

generator There are two types of excitation

1 Static excitation system

2 Brushless excitation system

A certain disadvantage in brushless excitation system is the slow response time of

the field in case of fast load changes specified No slip-rings and brushes direct

measurements of the field parameters not possible

To avoid all loses static excitation is used Since it does not have any rotating

parts mechanical loses and windage loses This system has fast response and speed

control While preferring this excitation system there are no limitations for the

redundancy of Thyristor bridge circuits

Static excitation has fast field discharge by resistor and inverter operation direct

measurement of field quantity is possible The meaning of excitation is nothing but

continuous supply of DC current (ie field current) to the rotor to buildup required

output voltage in the stator

Field current is changed with respect to the change of load so the digital

automatic voltage regulator (DAVR) is used to regulate the output voltage according to

the load variations

So we conclude that static excitation system with DAVR is preferred since it is

having excellent dynamic performance and better options for R amp M

622 Protection against Excitation Transformer Overheating

This equipment monitors the excitation transformer for overheating in the

windings that could result from over current short-circuits or inadequate cooling The

monitoring uses temperature monitoring modules in conjunction with temperature

sensors built into the windings Normally the temperature is monitored in two stages the

first stage sets off an alarm the second causes a trip of the excitation

623 Rotor Over voltage Protection

Malfunctions in the generator circuit (eg terminal short-circuit failed

synchronization asynchronous operation) cause induced negative field currents that

produce high voltages in the field circuit These must be restricted to a level with a

sufficient safety margin below the insulation capacity of the field winding (test voltage)

and also below the peak blocking voltage of the converter thyristors The crow bar

employs spark gap elements to detect over voltages in the field circuit Whenever they

respond the associated thyristors are fired immediately switching the de-excitation

resistor parallel to the field The de-excitation current generated thereby initiates an

excitation trip via a supervision circuit causing an immediate opening of the field circuit-

breaker The malfunction isets off an alarm and an internal malfunction is indicated at the

cubicle

TEST VALUES OBTAINED WHEN EXCITATION IS RAISED

TEST

SNO PARTICULARS ACTUAL VALUE OBTAINED VALUE

1 VREF 996 100

2 VACT 997 1003

3 IFACT 735 765

4 IGACT 703 707

5 ACTIVE

POWER

703 705

6 REACTIVE

POWER

101 142

7 POWER

FACTOR

099 IND 098 IND

8 ACTIVE

CURRENT(IR)

705 703

9 REACTIVE

CURRENT(IX)

102 138

10 POWER

ANGLE

566 546

11 FIRING

ANGLE

640 632

VALUES OBTAINED WHEN EXCITATION IS LOWERED

SNO PARTICULARS ACTUAL

VALUE

OBTAINED VALUE

1 VREF 100 997

2 VACT 100 997

3 IFACT 787 761

4 IGACT 839 837

5 ACTIVE

POWER

839 830

6 REACTIVE

POWER

155 110

7 POWER

FACTOR

098 IND 099 IND

8 ACTIVE

CURRENT(IR)

835 830

9 REACTIVE

CURRENT(IX)

140 94

10 POWER

ANGLE

613 DEG 631 DEG

11 FIRING

ANGLE

601 DEG 599 DEG

CONCLUSION

For generating the EMF in stator winding excitation is required to the rotor of a

generator There are two types of excitation

1 Static excitation system

2 Brushless excitation system

A certain disadvantage in brushless excitation system is the slow response time of

the field in case of fast load changes specified No slip-rings and brushes direct

measurements of the field parameters not possible

To avoid all loses static excitation is used Since it does not have any rotating

parts mechanical loses and windage loses This system has fast response and speed

control While preferring this excitation system there are no limitations for the

redundancy of Thyristor bridge circuits

Static excitation has fast field discharge by resistor and inverter operation direct

measurement of field quantity is possible The meaning of excitation is nothing but

continuous supply of DC current (ie field current) to the rotor to buildup required

output voltage in the stator

Field current is changed with respect to the change of load so the digital

automatic voltage regulator (DAVR) is used to regulate the output voltage according to

the load variations

So we conclude that static excitation system with DAVR is preferred since it is

having excellent dynamic performance and better options for R amp M

TEST VALUES OBTAINED WHEN EXCITATION IS RAISED

TEST

SNO PARTICULARS ACTUAL VALUE OBTAINED VALUE

1 VREF 996 100

2 VACT 997 1003

3 IFACT 735 765

4 IGACT 703 707

5 ACTIVE

POWER

703 705

6 REACTIVE

POWER

101 142

7 POWER

FACTOR

099 IND 098 IND

8 ACTIVE

CURRENT(IR)

705 703

9 REACTIVE

CURRENT(IX)

102 138

10 POWER

ANGLE

566 546

11 FIRING

ANGLE

640 632

VALUES OBTAINED WHEN EXCITATION IS LOWERED

SNO PARTICULARS ACTUAL

VALUE

OBTAINED VALUE

1 VREF 100 997

2 VACT 100 997

3 IFACT 787 761

4 IGACT 839 837

5 ACTIVE

POWER

839 830

6 REACTIVE

POWER

155 110

7 POWER

FACTOR

098 IND 099 IND

8 ACTIVE

CURRENT(IR)

835 830

9 REACTIVE

CURRENT(IX)

140 94

10 POWER

ANGLE

613 DEG 631 DEG

11 FIRING

ANGLE

601 DEG 599 DEG

CONCLUSION

For generating the EMF in stator winding excitation is required to the rotor of a

generator There are two types of excitation

1 Static excitation system

2 Brushless excitation system

A certain disadvantage in brushless excitation system is the slow response time of

the field in case of fast load changes specified No slip-rings and brushes direct

measurements of the field parameters not possible

To avoid all loses static excitation is used Since it does not have any rotating

parts mechanical loses and windage loses This system has fast response and speed

control While preferring this excitation system there are no limitations for the

redundancy of Thyristor bridge circuits

Static excitation has fast field discharge by resistor and inverter operation direct

measurement of field quantity is possible The meaning of excitation is nothing but

continuous supply of DC current (ie field current) to the rotor to buildup required

output voltage in the stator

Field current is changed with respect to the change of load so the digital

automatic voltage regulator (DAVR) is used to regulate the output voltage according to

the load variations

So we conclude that static excitation system with DAVR is preferred since it is

having excellent dynamic performance and better options for R amp M

VALUES OBTAINED WHEN EXCITATION IS LOWERED

SNO PARTICULARS ACTUAL

VALUE

OBTAINED VALUE

1 VREF 100 997

2 VACT 100 997

3 IFACT 787 761

4 IGACT 839 837

5 ACTIVE

POWER

839 830

6 REACTIVE

POWER

155 110

7 POWER

FACTOR

098 IND 099 IND

8 ACTIVE

CURRENT(IR)

835 830

9 REACTIVE

CURRENT(IX)

140 94

10 POWER

ANGLE

613 DEG 631 DEG

11 FIRING

ANGLE

601 DEG 599 DEG

CONCLUSION

For generating the EMF in stator winding excitation is required to the rotor of a

generator There are two types of excitation

1 Static excitation system

2 Brushless excitation system

A certain disadvantage in brushless excitation system is the slow response time of

the field in case of fast load changes specified No slip-rings and brushes direct

measurements of the field parameters not possible

To avoid all loses static excitation is used Since it does not have any rotating

parts mechanical loses and windage loses This system has fast response and speed

control While preferring this excitation system there are no limitations for the

redundancy of Thyristor bridge circuits

Static excitation has fast field discharge by resistor and inverter operation direct

measurement of field quantity is possible The meaning of excitation is nothing but

continuous supply of DC current (ie field current) to the rotor to buildup required

output voltage in the stator

Field current is changed with respect to the change of load so the digital

automatic voltage regulator (DAVR) is used to regulate the output voltage according to

the load variations

So we conclude that static excitation system with DAVR is preferred since it is

having excellent dynamic performance and better options for R amp M

CONCLUSION

For generating the EMF in stator winding excitation is required to the rotor of a

generator There are two types of excitation

1 Static excitation system

2 Brushless excitation system

A certain disadvantage in brushless excitation system is the slow response time of

the field in case of fast load changes specified No slip-rings and brushes direct

measurements of the field parameters not possible

To avoid all loses static excitation is used Since it does not have any rotating

parts mechanical loses and windage loses This system has fast response and speed

control While preferring this excitation system there are no limitations for the

redundancy of Thyristor bridge circuits

Static excitation has fast field discharge by resistor and inverter operation direct

measurement of field quantity is possible The meaning of excitation is nothing but

continuous supply of DC current (ie field current) to the rotor to buildup required

output voltage in the stator

Field current is changed with respect to the change of load so the digital

automatic voltage regulator (DAVR) is used to regulate the output voltage according to

the load variations

So we conclude that static excitation system with DAVR is preferred since it is

having excellent dynamic performance and better options for R amp M