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ABSTRACT
TITLE: STUDY PROJECT ON PROTECTION OF
TURBOALTERNATORS AND TRANSFORMERS
IN THERMAL POWER STATION WITH A
SPECIFIC AND BRIEF STUDY OF RELAYS ANDTHEIR APPLICATIONS WITH REFERENCE TO
KTPS C STATION
The entire modern Turbo-Alternators is 2 pole machines and are driven by steam
turbines. Since the efficiency of steam turbine is high at large speeds, the Turbo
Alternators are designed for speed up to 3000 rpm. Modern Turbo Alternators havetypical ratings of 60 MW, 120 MW, and 250 MW, 500 MW, 1000 MW. In this project
we are studying the protection of Turbo Alternator.
The Turbo-Alternator protection can be provided with two types of relays, one is
with single function relays and the other is comprehensive multi function relay having
encapsulated in one single module. The latter has a number of protection functions
within the relay case and does have a number of advantages as compared with single
functions generator protection relays. The relay is more compact and requires less
external connections. Using numerical techniques, enhanced features such as
measurement functions, event and disturbances recording, alternative setting group can
also provide remote communications, printing and testing facilities. These features can
simplify the tasks of commissioning, maintenance, and trouble-shooting and post-fault
analysis.
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In addition, a self monitoring feature, together with remote communications,
allows the monitoring of the relay and the generator on demand, will improve availability
of the protection system.
CONTENTS
PART I:
1. INTRODUCTION
2. BRIEF HISTORY OF K.T.P.S.
3. IMPORTANCE OF PROTECTION.
4. CIRCUIT BREAKERS IN POWER PLANT.
5. OVER VIEW OF RELAYS.
PART II:
1. PROTECTION OF GENERATOR.
2. PROTECTION OF TRANSFORMER.3. PROTECTION OF GENERATOR TRANSFORMER UNIT.
4. CONCLUSION.
BIBILOGRAPHY
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PART I
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INTRODUCTION
THIS PROJECT DEALS WITH THE PROTECTION OF TURBO-
ALTERNATOR & TRANSFORMER.
The generators are the most expensive piece of equipment in A.C. power system
and are subjected to most possible troubles than any other equipment. The Aim of the
protection system is to protect against all these abnormal conditions and yet to keep the
protection simple and reliable has resulted in considerable divergence of opinion on the
choice of the protection. Transformers used in the power system are also subjected to so
many troubles. These are used to step-up or step-down the voltage. In order to have
continuous power supply we have to protect transformer also..
The purpose of the power system is to generate & supply electrical energy to
consumers. The system should be managed to deliver this energy to the utilization points
with both reliability and economy.
The power system represents a very large capital investment so it should be
protected so as to give the best service to the consumers.
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BRIEF HISTORY OF THE KOTHAGUDEM
THERMAL POWER STATION
Kothagudem Thermal Power Station is one of the major power generating station
of the Andhra Pradesh. The main raw material is coal is supplied by Singareni Collieries,
Kothagudem and Water sources is from Kinnerasani project, which is about 12 Kms
from the Paloncha.
Kothagudem power station comprises of four powerhouses namely KTPS A,
KTPS B, KTPS C, KTPS 5 TH stage
KTPS A -- 4 X 60 MW
KTPS B -- 2 X 120 MW
KTPS C -- 2 X 120 MW
KTPS 5 TH Stage -- 2 X 250 MW
Total capacity = 1220MW.
A PROFILE OF KOTHAGUDEM THERMAL POWER STATION
Foundation for Indias power sector was laid in the year 1897 with commencement of
2000 KW micro hydel project at Darjeeling, being the load center.
The Andhra Pradesh state electricity board. Here it is often called as APSEB. The
APSEB is responsible for promoting the coordination of generation, transmission and
distribution of electrical energy through out the state of Andhra Pradesh.
In a step to power sector reforms, APSEB has been divided in to two corporations.
i.e. Andhra Pradesh Power Generation Corporation Limited (APGENCO) and Andhra
Pradesh Power Transmission Corporation Limited respectively on 1 st February, 1999.
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IMPORTANCE OF PROTECTION:
A fail free power system is neither economically justifiable nor technically
feasible. Faults can occur in any power system components generator, transformer,
motors, buses and lines though the transmission lines being exposed to environment
are most vulnerable. Faults fall into two general categories - short circuit faults and open
circuit faults. Short-circuit faults are most ever kind, resulting in a very abnormal high
currents. If allowed to persist even for short period of time, short circuit can lead to
extensive damage to equipment. Undesirable effects of short-circuit faults are unmerited
below:
Arcing faults (most common) can vaporize equipment in vicinity leading to,
possibly, fire and explosion, e.g. in transformers and circuit breakers. Power system
components carrying abnormal currents gets over heated, with the consequent reduction
in the life span of their insulation. Operating voltages can go above or below their
acceptable values, leading to development of another fault or damage to utilization
equipment. Consequent unbalanced system operation causes overheating of generator rotors. Power flow is severely restricted, or even completely blocked, while the short
circuit lasts.
As a consequence of blockage of power flow, power system areas can lose
synchronism. The longer the fault last, the more is possibility of loss of synchronism.
Open circuit faults cause abnormal system operating and danger to personnel. Voltage
tends to rise well beyond acceptable values in certain parts of the system with possibility
of insulation failure and development of short circuit fault. While open circuit faults can
be tolerated for a long period of time than short circuit fault, these cannot be allowed to
persist, and must be removed. We shall devote our attention to most severe type of fault,
i.e. the short circuit faults. There are also other abnormal operating conditions, which
require remedying, but do not fall under two categories of faults mentioned. Two
unbalanced conditions are one is heavily unbalanced generator and the other is loss of
excitation. Faults should be instantly detected and faulty section be isolated from thesection in the shortest possible time. It is obviously not possible to do this manually, and
it must, therefore, be accomplished automatically. Faults are detected automatically by
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by means of relays, and faulty section isolated by C.B. connected to the boundaries of
section. The combination of relays & C.B. is known as protective system.
CIRCUIT BREAKERS IN POWER PLANT
INTRODUCTION
Power switching ON and OFF operations in any electrical system needs
isolation equipments. In a situation where power handled is very low, say a fan or a
lamp, this can be achieved by a simple ON/OFF switch. But as load capacity increases,
for example an ordinary water heater where current is of the order of 7 to 10 amps, the
switch used becomes spring loaded so that the make and break actions becomes fast and
need not depend totally on manual operation. For higher loads, special circuit breaking
equipment called circuit breakers are used. A circuit breaker (CB) consists essentially of
current carrying contacts called electrodes. These contacts remain engaged when the
circuit is ON, but under predetermined conditions, gets separated to interrupt the circuit.
When the contacts are separated in order to interrupt the current, an ARC is struck
between them.
ARC PHENOMENA
The arc consists of a column of noised gas with temperature of about
25000K, in which the molecules have lost one or more of their electrons resulting in
positive ions and electrons. The electrons, which have a negative charge and being light,
are attracted towards the positive contact (the anode) very rapidly and the positive ions
are attracted towards the negative contact (the cathode) relatively slowly. For the
initiation of the arc, electrons must be emitted from the cathode as soon as the contacts
begin to separate and this emission is mainly because of (I) Field Emission and (II)
Thermal Emission. The electrons so liberated from the cathode make many collisions
with the atoms and molecules of the gases and vapour existing between the two contacts
during their travel towards the anode. These collisions cause ionization of atoms and the
molecules thus liberating more electrons.
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For the current to be interrupted at zero passage, two processes are to be
considered. Firstly, cooling of the arc must be strong enough to bring the arc temperature
down to the values where the gas is no longer ionized. The current will then be prevented
from re-establishing in the opposite direction. Secondly, after the current has ceased toflow, the dielectric strength between the arcing contacts must be high enough to
withstand the voltage, which will immediately start to build up.
ARC INTERRUPTION
The common methods used for interruption the arc are
(i) High Resistance Interruption:
In this method the arc is so controlled that its effective resistance increases with time so
that the current is reduced to a value insufficient to maintain it.
The resistance can be increased by
Lengthening the arc
Cooing the arc
Splitting the arc Constraining the arc
(ii) Low Resistance Interruption (Current Zero Interruption):
- In this method, the arc resistance is kept low until the current zero where the arc
extinguishes itself naturally and is prevented from re-striking inspite of high re-striking
voltage. A rapid increase of dielectric strength is necessary for successful interruption
and this can be achieved by
Lengthening of the gap
Cooling
Blast effect
Following are the recommended properties for a good arc quenching medium.
1. High Dielectric Strength
2. High Thermal Conductivity
3. Good Physical & Chemical Stability
4. Non-inflammable
5. Good Arc Extinguishing Properties
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TYPES OF CIRCUIT BREAKERS
1. Low Voltage Circuit Breakers:
For voltage level of 1500 volts AC and 3000 volts DC, air is used as the mediumof arc interruption and these circuit breakers are termed as low voltage circuit breakers.
These breakers are fast in operation, free from fire hazards and can be easily maintained.
For arc extinguishion, and arc chute is provided which elongates the arc length thus
increases the resistance in the path, resulting into more cooling effect. These improved
effects help in early arc extinguishion. In manually operated breakers a thermal release is
provided which trips the breaker in case current goes beyond the pre-set value.
Electrically operated breakers are provided with optional separate relay system for added
protection of the circuit.
2. Medium Voltage Circuit Breakers
Generally circuit breakers up to 33 kV are categorized as medium voltage
circuit breakers. Usually these circuit breaker uses Oil, Sulphur Hexaflouride Gas (SF 6)
or Vacuum as medium for arc interruption. These breakers, normally, are of metal clad
design and the form of truck can be raked in or racked out in switchgear. The arc
extinguishing principle differs with different extinguishing medium.
3. High Voltage Circuit Breakers:
Circuit Breaker of 66 kV and above are normally termed as high voltage breakers.
These breakers can be broadly classified into (I) Oil Circuit breakers and (II) Oil lesscircuit breakers. The arc extinguishing medium normally used in these breakers are Oil,
SF 6 and Air Blast etc.
3a. Oil Circuit Breakers
Oil circuit breakers are further sub divided into
(I). Bulk Oil Circuit Breakers and
(II). Minimum Oil Circuit Breakers.
In bulk oil breakers large quantity of oil is used for arc extinguishing as
well as insulating the current carrying parts. In minimum oil breakers oil is used only for
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arc quenching purpose, their current carrying parts are insulated by air and porcelain or
organic insulating materials. During breaker opening operation, the beat generated from
arcing decomposes the oil, which liberates mainly hydrogen, carbon and copper vapour.
More the breaking current, larger the arc, more the hydrogen vapour is generated. Thehydrogen provides the cooling effect and takes away heat of arc as the gas is generated in
a small chamber, gas gets pressurized and its cooling effectiveness increases thus
resulting in arc extinguishion. Further a small vent is provided in arcing chamber which
causes a small blast of hydrogen inside the chamber.
3b. SF6 Circuit Breakers
SF 6 gas is stable up to 500 0C and can be used with insulating materials
such as ceramics, glass, epoxy etc. up to 150 0Cwithout affecting them. At normal
pressure in the interrupters, the gas does not condense up to 40 0C approx. This makes
SF6 gas suitable for use in all temperature ranges. General properties of SF6 gas are
Non toxic
Colourless
OdourlessGenerally two types of SF6 breakers are found.
(a) Flow type generally used in high voltage system
(b) Puffer type used in medium voltage system.
The arc extinguishion in flow type circuit breaker is done by establishing a flow
of gas from breaking contacts. The flow is established at breaking contacts from high-
pressure zone to low pressure zone. For gas vapourization, a compressor and reservoir is
provided on the breaker. In puffer type breaker the gas is enclosed in a sealed chamber.
The chamber is also having breaker contact and piston attached to the moving contact.
When breaker operates the gas on one side of the piston gets pressurized due to
compression effect. This compressed gas passes through circuit breaker contact through a
hole in the piston thus extinguishes the arc.
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3c. Air Blast Circuit Breakers
In these breakers a blast of air is established at breaking contacts for compressed
air system at a pressure of 18-20 Kg/cm2. The blast do the neccessary cooling and thusextinguishes the arc. The advantages of ABCBs over an Oil CB are
Elimination of fire hazard
High speed operation
Short and consistent arc duration
As the arc energy is small, contact burning is less.
Suitable for frequent operation Less maintenance
3d. Vacuum Breakers:
A high vacuum of the order of 10 -8 to 10 -9 torr is established in the arcing
chamber. This increases the mean free path of the particles to several meters there by
avoiding any electron collision and avalanche effect. Thus the arcing is extinguished.
Advantages of Vacuum Breakers are
Vacuum is a superior dielectric medium
Small and compact size of interruption unit
High breakdown for short gaps.
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OVER VIEW OF RELAYS
A protective relay is a device that detects the fault and initiates the operation of
the circuit breaker is isolate the defective element from the rest of the system. The relays
detect the abnormal conditions in the electrical circuits by constantly measuring the
electrical quantities, which are different under normal and fault conditions. As the
technology in the protective system is developed, the relays are also developed in the
complex power system. The different types of relays are
i. Electromagnetic type.
- Attractive type
- Induction type
ii. Static Relays
Microprocessor based relays are
a. RAMDE
b. SPAM 150C
iii. Numerical Relays
ELECTROMAGNETIC RELAY TYPE:i. Attractive Type:
These are the first and fore most types of relays. The relays operate by virtue
of an armature being attracted to the poles of an electromagnet or a plunger
being drawn into a solenoid. Such relays may be actuated by A.C. or D.C.
quantities.
ii. Induction type:
These type of relays operate on the principle of induction motor and are
widely used for protecting relaying purposes involving A.C. quantities. These
are not used for D.C. quantities.
STATIC RELAYS:
Static techniques for power system protection are now well established and have
been developed over a period of more than twenty years use of silicon transistor, MST,
microprocessor and static techniques has made it possible to design high performance
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and more sophisticated characteristics into relays which are necessary to meet the
requirement modern complex power system.
It is now possible for static relays to be designed to replace all functions of
previously performed by electromagnetic relays. The power consumed by static relay islow & are fast in operation there is no moving parts hence it requires less maintenance
use of printed circuit board avoid wiring errors.
DISADVANTAGES:
a. The characteristics vary with the temperature and aging.
b. The reliability of the scheme depends upon a large number of small components
and their electrical connections.
c. The relays have low short time over load capacity compared with electromagnetic
relays.
RAMDE:
RAMDE is an integrated microprocessor controlled RMS measuring motor
protection relay. Ramde can be used for both synchronous & asynchronous motors.
Two types:
RAMDE 1 used for synchronous motors.
RAMDE2 used for asynchronous motors or other applications.
The current measurements are based on RMS values. Therefore taking into
considerations the harmonics common in today industrial environments. The current
unbalance protection is independent of frequency and phase sequence. And therefore also
works for motors fed view frequency converters or for reversible motors.
APPLICATIONS:
For large motors and those which are vital to the process a sensitive differential
protection is recommend for supplementing the short circuit protection in RAMDE.
DISADVANTAGES:
1. No under current protection.
2. No memory locations (registers).
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NUMERICAL RELAYS
These are the latest relay device used in the present world. This numeric relays works on
the numeric values of specified quantity from which it is to be measured. There are based
on the approximation of specified value. There are no mechanical settings in this relays.
THE ADVANTAGES OF NUMERICAL RELAYS
Several setting groups
under range of parameter adjustment
Remote communications built in
Internal Fault diagnosis
Power System measurements available
Distance to fault locator
Disturbance recorder
Auxilia
ry protection functions (broken conductor, negative sequence, etc.)
CB Monitoring (State, condition)
User - definable logic
Backup protection functions in-built
Consistency of operation times - reduced grading margin
It consists of one or more DSP microprocessors, some memory, digital and
analogue input/output (I/O), and a power supply. Where multiple processors are provided,
it is usual for one of them to be dedicated to executing the protection relay algorithms,
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while the remainder implements any associated logic and handles the Human Machine
Interface (HMI). By organizing the I/O on a set of plug-in printed circuit boards (PCB's),
additional I/O up to the limits of the hardware/software can be easily added. The internalcommunications bus links the hardware and therefore is critical component in the design.
It must work at high speed, use low voltage levels and yet be immune to conducted and
radiated interference from the electrically noisy substation environment. Excellent
shielding of the relevant areas is therefore required. Digital inputs are optically isolated to
prevent transients being transmitted to the internal circuits Analogue inputs are isolated
using precision transformers to maintain measurement accuracy while moving harmful
transients. Additionally, the input signals must be amplitude limited to avoid them
exceeding the power supply voltages, as otherwise the waveform will appear distorted.
Analogue signals are converted to digital form using an A/D converter. The cheapest
method is to use a single A/D converter, provided by a multiplexer to connect each of the
input signals in turn to the converter. The signals may be initially input to a number of
simultaneous sample-and-hold circuits prior to multiplexing, or the time relationship
between successive samples must be known if the phase relationship between signals is
important. The alternative is to provide each input with a dedicated A/D converter, and
logic to ensure that all converters perform the measurement simultaneously.
The frequency of sampling must be carefully considered, as the Nyquist criterionapplies :
f s > 2 x f h
Where :
f s
= sampling frequency
f s = highest frequency of interest
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If too low a sampling frequency is chosen, aliasing of the input signal can
occur (Figure 7.12) resulting in high frequencies appearing as part of signal in the frequency
range of interest. Incorrect results will then be obtained. The solution is to apply an anti-aliasing filter, coupled with an appropriate choice of sampling frequency, to the analogue
signal, so those frequency components that could cause aliasing are filtered out. Digital sine
and cosine filters are used, with a frequency response, to extract the real and imaginary
components of the signal. Frequency tracking of the input signals is applied to adjust the
sampling frequency so that the desired number of samples/cycle is always obtained. A
modern numerical relay may sample each analogue input quantity at between 16 and 24
samples per cycle.
All subsequent signal processing is carried out digitally in software, final digital
outputs use relays to provide isolation or arc sent via an external communications bus to
other devices.The relevant software algorithm is when applied. Firstly, the values of the
quantities of interest have to be determined from the available information contained in the
data samples. This is conveniently done by the application of the Discrete Fourier
Transform (DFT), and the result is magnitude and phase information for the selected
quantity. This calculation is repeated for all of the quantities of interest.
The DSP chip in a numerical relay is normally of sufficient processing capacity that
calculation of the relay protection function only occupies part of the processing capacity.The excess capacity is therefore available to perform other functions. Of course, care must
be taken never to load the processor beyond capacity, for if this happens, the protection
algorithm will not complete its calculation in the required time and the protection function
will be compromised.
Typical functions that may be found in a numerical relay besides protection functions
are described in this section. Note that not all functions may be found in a particular relay. In
common with earlier generations of relays, manufacturers, in accordance with their perceived
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market segmentation, will offer different versions offering a different set of functions.
Function parameters will generally be available for display on the front panel of the relay and
also via an external communication port, but some by their nature may only be available atone output interface.
Numerical relays perform their functions by means of software. The process used for
software generation is no different in principle to that for any other device using real-time
software, and includes the difficulties of developing code that is error-free. Manufacturers
must therefore pay particular attention to the methodology used for software generation and
testing to ensure that as far as possible, the code contains no errors. However, it is virtually
impossible to perform internal tests that cover all possible combinations of external effects,
etc., and therefore it must be accepted that errors may exist. In this respect, software used in
relays is no different to any other software, where users accept that field use may uncover
errors that may require changes to the software. Obviously, type testing can be expected to
prove that the protection functions implemented by the relay are carried out properly, but it
has been known for failures or rarely used auxiliary functions to occur under some
conditions.
Where problems are discovered in software subsequent to the release of a numerical
relay for sale, a new version of the software may be considered necessary. This process then
requires some form of software version control to be implemented to keep track of :
a) The different software versions in existence
b) The differences between each version
c) The reasons for the change.
d) Relays fitted with each of the versions.
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With an effective version control system, manufacturers are able to advise
users in the event of reported problems if the problem is a known software related problem
and what remedial action is required. With the aid of suitable software held by a user, it may be possible to download the new software version instead of requiring a visit from a service
engineer.
A numerical relay usually provides many more features than a relay using
static or electromechanical technology. To use these features, the appropriate data must be
entered into the memory of the relay. Users must also keep a record of all of the data, in
case of data loss within the relay, or for use in system studies, etc. The amount of data per
numerical relay may be 10-50 times that of an equivalent electromechanical relay, to which
must be added the possibility of user-defined logic functions. The task of entering the data
correctly into a numerical relay becomes a much more complex task than previously,
which adds to the possibility of a mistake being made. Similarly, the amount of data that
must be recorded is much larger, giving rise potentially to problems of storage.
The problems have been addressed by the provision of software to automate the
preparation and download of relay setting data from a portable computer connected to a
communication port of the relay. As part of the process, the setting data can be read back
from the relay and compared with the desired settings to ensure that the download has
been error-free. A copy of the setting data (including user defined logic schemes whereused) can also be stored on the computer, for later printout and/or upload to the users
database facilities.
Following are the various protections recommended for the generator and generator
transformer protection.
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PART II
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INTER TRIPS
When considering protection of the generator the prime mover must always be
included. For example in the response to a winding failure inside the generator it would
not be sufficient just to trip out the main breaker and disconnect the unit from the
electrical system. We would also need to trip the stop valve and shutdown the prime
mover so as to prevent further damage.
Similarly if a problem occurs within the prime mover which necessitates
tripping the unit, inter trips must be provided to trip out the generator breaker as well.
When the generator is tripped from the system, its excitation system must
also be deenergised by tripping the field breaker.
FIGURE
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CLASSIFICATION OF PROTECTIONS
protection of alternators are classified based on the rating of the m achine and the
purpose for which the machine employed .
Basically protection schemes are categorized into three classes.1. CLASS-A Protection.
2. CLASS-B Protection.
3. CLASS-C Protection.
1.Class-A-Protection:-
This scheme gives the protection against the faults within the unit and auxiliaries
directly connected to the unit where in the entire unit with its auxiliaries and prime
mover to be shutdown instantly.
The unit is tripped in this scheme via reverse power protection.
Class-A-Protection schemes:-
1.Generator differential protection
2.Generator stator 0-to-100% Earth fault protection
3.Buchholz protection (UAT>)
4.Generator Transformer & unit auxiliary Transformer overall differential
protection
5.Protection against interturn faults
6.Generator rotor Earth fault protection
7.Generator loss of excitation
8.Backup impedance protection
9.Protection against motoring
a) Low forward power protectionb) Reverse power protection
10.Generator,Generator Transformer and unit auxiliary over fluxing protection
11.Generator Transformer unit protection
12.Generator over voltage protection
13.Breaker fail protection
2.Class-B-Protection:-
This scheme gives the protection against the fault occurring in the prime mover
or Auxillaries where safe shutdown of the unit is possible .
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Class-B-Schemes: .
1.Generator Transformer oil temperature very high.
2.Commands from automatic voltage regulator control
III. Class-C-Protection:-This scheme gives the protection against the faults occurring in the grid affecting
the unit where the unit can be isolated from the grid but the primemover and auxillaries
retained for synchronizing the unit back to grid at the earliest.
Class-C-Schemes:
1. Generator negative phase sequence protection.
2. Generator overload protection.
3. Generator pole slipping.
4. Under frequency protection .
5. Generator Transformer over current & Earth fault protection.
CLASS-A-PROTECTION SCHEMES
1.Generator differential protection:-
Fig .1 shows the schematic diagram of percentage differential protection. It is
used for the protection of generators above 1MW. It protects against winding faults i.e. phase to phase and phase to ground faults. This is also called biased differential
protection. The polarity of the secondary voltage of C.T. s at a particular moment for an
external fault has been shown in the fig. In the operating coil, the current sent by the
upper C.T. is cancelled by the current sent by the lower C.T. and the relay does not
operate. For an internal fault the polarity of the secondary voltage of the upper C.T. is
reversed as shown in the fig 2. Now the operating coil carries the sum of the currents sent
by the upper C.T. and the lower C.T. and it operates and trip the circuit breaker.
The percentage differential protection does not respond to external faults and overloads.
It provides complete protection against phase to phase faults. It provides protection against
ground faults about 80 to 85% of the generator winding it does not provides protection to 100%
of the winding because it is influenced by the magnitude of the earth fault current which
depends upon the method of neutral grounding. When the neutral is grounded through an
impedance, the differential protection supplemented by sensitive earth fault relays.
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Due to the difference in the magnetizing currents of the upper and lower C.T.s the
current through the operating
FIGURE
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The current entering and leaving the protected object are determined current
transformers and compared by relays by means of differential circuit as shown in fig. A
fault inside the protected zone is fed from either one side or both sides depending upon
the current sources present, thus producing a difference current in the differential circuit.If this differential circuit exceeds a set of percentage of the current flowing in the
protected object, the relay picks up.
The relay used is designated 87G & is RADHA & RADSB type. It is to operate
10% (0.5 amp) relay current which corresponds to 1000 Amp fault current.
2. EARTH FAULT PROTECTION:
a. STATOR EARTH FAULT (MAIN):
The generator neutral is earthed through the primary winding of the neutral
grounding transformer of rating 500 KVA, 15.75/.24 kV ratios. The secondary winding
of transformer is shorted through loading resistance of .42 ohms for an earth fault in the
generator the E/F current flow in the primary of the neutral grounding transformer. As a
result a voltage across the resistor is developed which activates stator earth fault sensing
relay. The reason for this kind of protection is due to mechanical damages resulting from
insulation fatigue creep age of conductor bases, vibration of conductor or other fittings of
cooling systems.
The earth fault relay designated is RAGEA type 64G. The relay has an inverse
definite minimum time characteristics. Generally 5% Generator winding starting from
neutral point remains unprotected because a fault in this portion will generate too low
voltage for relay operation.
b. STATOR EARTH FUALT PROTECTION:
The relay is connected across of the generator PT secondary edges. When there is
no E/F the sum of phase voltages of the generator. Hence the voltage across the relay is
zero. The voltage across the point a & b will assume a positive value when one phase
voltage drops because of earth fault on that phase.
The relay designated is 64G , RAGEA static type relay. It has inverse time voltage
characteristics.
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C. ROTOR EARTH FAULT:
First ground leakage in the rotor circuit of the generator does not adversely effect
the operation,Danger arise if a second fault occur causing current to be diverted in part at
least, from the intervening turns which can burn the conductor causing severe damage tothe rotor. If a large portion of winding is shorted the field flux pattern may change the
flux concentration at one pole and wide dispensation at the other the attractive forces
which is proportional to the square of the flux density will be stronger at one pole than
the other which will cause high vibrations and may damage the bearings and may
sufficiently displace the rotor thereby fouling the stator.
Rotor E/F protection is provided by monitoring the I/R value of rotor winding.
< 5.5 K alarm
< 2.2 K Trip
3. Stator Inter Turn Fault:
When leakage occurs between the turns in the same phase of a winding the
induced voltage is reduced and there will be a voltage difference between the center of
the voltage triangle and neutral of the machine.
Therefore, in a generator having one winding per phase, a voltage transformer is
connected between each phase terminals and, the neutral of the winding, the secondary
winding transformer leads being connected in open delta, when inter turn leakage occur
at the ends of the open delta, it is detected by the polarized voltage relay, for generators
having several parallel windings per phase, the neutral ends are connected together to as
many neutrals as parallel windings per phase. These neutrals are then joined through
current transformer to current relay, or though voltage transformer to voltage relay. If an
inter turn fault occur in the machine, the current transformer carries transient current or alternatively voltage transformer produce thereby picking up relay and tripping the
generator.
The relay designated 59 ( U>) and RXEG / RXEDK type static relay is
used for this type of protection.
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GENERATOR INTER-TURN FAULT
4. NEGATIVE PHASE SEQUENCE:
The currents in three-phase machine are normally in balance, but if a fault occurs
on the supplied or supplying system, this balance can be influenced. Single phase andtwo-phase faults, phase rupture or asymmetric loading on the system can give rise to
unbalanced currents, hence negative sequence currents.
Three currents generate a machine stator flux that has the same rotational speed as
the rotor flux but rotates in the opposite direction. Relative to the rotor, the stator flux
therefore rotates at double the power system frequency and generators eddy currents in
the rotor. The high frequency of these eddy currents causes the outer parts of the rotor,
and the winding to become heated. If the negative sequence current is of high
magnitude, or if it persists for long periods of time, these rotor parts can be damaged
due to overheating. It is normally assumed that a generator can sustain negative
sequence currents which exceed a given minimum value for a period of time t, which is
determined from the following equation:
t = K ( I m / I nsc ) 2
where
I m = rated current of the machine
I nsc = negative sequence current
K = a constant in seconds that is characteristic for the generator. This
constant represents the length of time the machine can withstand a negative
phase-sequence current equal to rated current.
The validity of this equation is based on the assumption that all the energygenerated by the negative sequence current is transmitted in the form of heat to the rotor
without any losses, to the surroundings.
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FIGURE
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The relay is used is designated 46G and RARIB static relay is using for this type
of protection.
5. GENERATOR BACKUP IMPEDANCE PROTECTION:
Three-phase zone impedance is provided for the back-up protection of generator
against external three phases and phase to phase fault in 400 KV systems. The zone of
impedance relay should be extended beyond 400 kV switchyard and it should be
connected to trip the generator after a time delay of 1 to 1.5 seconds so that the generator
is tripped only when 400 kV protection has not cleared the fault even in the second zone.
The relay used is designated 21 G and RAKZB type relay is used for this protection..
6. LOSS OF EXCITATION:
Failure of field system leads to losing synchronism and resulting in running above
the synchronous speed. It acts as an induction motor, the main flux being produced by
wattles stator current drawn from the system. Operation as an induction generator
necessitates the flow of slip frequency current in the rotor, damper windings, float
wedges excitation under these conditions requires a large reactive component which
approaches the value of rated out put of the machine. The induced currents in the rotor as
result of this condition, rotor would get over heated due to the slip frequency current.
The magnitude of the reactive power drawn from the system is a function of the machine
reactance and the system source impedance. The resultant current will not be steady
state, but is pulsing, and conventional time delayed over current relays cannot protect thegenerator. Also it could over load the grid, which may not be able to supply the required
MVAR.
When loss of excitation is accompanied by under voltage it will initiate class. A
trip other wise class B trip if the grid is able to sustain the voltage dip.
The relay used is designated 40 G and RAGPC is suitable for all types of
synchronous machines to protect from the loss of excitation.
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7. POLE SLIPPING:
The asynchronous operation of machine while the excitation is still the intact
unlike the loss of excitation, cause sever shock to the both machine and grid due toviolent operation oscillations in the both active power and reactive power. Because of
these machine fall out of step or usually known as pole slipping trip. The oscillations
may disappear in few seconds, in that case it is not desirable to trip the machine. If
however an angular displacement of the rotor exceeds the stability limit of the rotor will
slip a pole pitch. If this disturbances has been sufficiently reduced by the time this has
occurred, the machine should regain synchronism, But if it does not. It must be isolated
from the system.
The swing curves can be detected by the impedance relay. The relay has two
measuring elements set at two values near the impedance as seen from the relay as a
relay impedance seen by the relay changes it comes in the operating zone of the two
relays one after the other. The sequential operation is observed by auxiliary relays. Since
the fault would be with in the 55 ms. However, during pole slipping, two elements would
operate sequentially and a trip command is given when both have operated.
The relay can be set to be in the operation for swing up to + _ 90 deg.
Corresponding to the stability limit of the generator.
The relay used is designated 98 G and is of solid-state design of ZTO type.
In order to discriminate against swing on the grid the tripping is through an
impedance relay (98 GY) set with a reach up to the 400 kV yard. RXZF / RXPE relay is
used for protection against pole slip or out of step.
7. OVER VOLTAGE:The generator winding is rated for 15.75 kV terminal voltage, sustained over
voltage would unduly stress the winding insulation and may lead to failure after some
time. To protect the machine against the over voltage the protection relay senses the
voltage at the secondary of the bus duct PTs. The relay is set to operate at 10% rise in the
terminal voltage. A time delay of 3 seconds is provided to take care of transient over
voltage arising from line charging, switching capacity faults etc. The relay used is
designated 59 G & RXEG type static relay used for this protection.
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G.T. OVER FLUXING:
The iron core of the generator transformer carries the flux to produce required
EMF. If the flux increases UN dually the magnetic circuits of the generator, generator
transformer become over saturated resulting in high magnetizing current. This in turnleads to higher to iron losses, which will increase the winding temp of the transformer.
Since core can be damaged because of this over heating, protection has to be provided
against it. The flux is dependent on ration of voltage & frequency.
The condition of over fluxing could arise in the case the voltage at the machine
terminals rise or its frequency drops or both occurring simultaneously. Particularly this
condition will arise if the machine AVR misbehaves thereby unduly increasing voltage
even if the grid frequency is low.
The relay used is designated 99 GT and is GTT 21 type which senses v/f At the
secondary of the bus duct P.T. And it gives alarm and trip signals at different time delay.
The adopted setting for relay if v/f = 1.2 P.U. i.e. 20% higher than rated v/f ratio.
Alarm is set at 0.5 to 1 sec. & trip at 12 sec.
This v/f relay generates an AVR raise block.
Surge voltage originating from lines because of switching or atmospheric
disturbances are dealt with directly by lightening arrested and surge diverter.
8. LOW FORWARD POWER PROTECTION:
When the generator synchronized with the grid, losses it driving force the
generator remains in synchronism. The generator should be isolated from the grid after
the steam flow ceases and the flow of power to the grid reduces to the minimum i.e. the
point when generator starts drawing power from grid and acts as motor.When load on generator drops to less then 0.5%, generator low forward power
relay gets energized and with turbine tripped or stop closed, trip the generator with the
time delay of 2 seconds. This is protection to trip generator on the other than the
electrical fault. And also this protection used for few electrical faults where the generator
trip can be delayed.
However, provision for time lag unit is there to prevent UN desired operation
from transient power reversal.
The power relay used is designated 32 G1 and RXPDK type relay used for this
type of protection.
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9. REVERSE POWER PROTECTION:
The generator must be disconnected from the grid as soon as turbine stop valve is
closed, completely shutting off the steam. Continued full speed turbine rotation causeslot of turbulence of the trapped steam, which result in increase of temperature. Thus
turbine will be subjected to excessive thermal overstress, vibrations and distortion. So
there is back-up arrangement to trip generator if it does not trip with in 2 seconds i.e. on
low forward power (L.F.P.) protection. This is known as reverse power protection, which
acts in two stages.
1.Reverse power relay operates after 5 seconds time delay and includes stop valve
closing/turbine trip.
2.Reverses power relay acts after 15 seconds time delay which trips the generator
irrespective of the either stop valve closing or turbine trip. This acts as a final back up to
L.P.F. protection.
10. UNDER FREQUENCY PROTECTION (81G):
The under frequency protection
Prevents the steam turbine & Generator from exceeding the permissible operating
time at reduced frequencies.
Ensures that the Generating unit is separated from the network at a present value
of frequency that is less than the final stage of system load shedding. Prevents the AVR from exiting the machine at reduced speeds when protective
relays may not perform at all.
Prevents over fluxing of the generator. The over fluxing relay is used to protect
against small over fluxing for long periods while the over voltage and under
frequency relay also protected against large over fluxing for short times.
The stator under frequency relay measures the frequency of the stator terminal
voltage.
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Though under frequency tripping is recommended by turbine manufacturers, care
should be taken by grid operator personal in ensuring, that machines not run at
lower frequencies and instead resort to means like load shedding in the event of over load.
Requirements:
1. Have one alarm stage two tripping stages.
2. Shall have settings of range 45 Hz to 55 Hz with a least count of 0.1 Hz for each
stage.
3. Shall have under voltage blocking.
11. LOCAL BREAKER BACK-UP PROTECTION:
This is protection against the main generator breaker failure which may occur due to-
(1) Mechanical failure
(2) Trip circuit not healthy
Hence, this protection acts as a back up to the main generator by tripping all breakers connected to that particular bus. The relay designated as 51 and RAICA used
for breaker failure protection.
RELAY SENSING:
(1) D.C. to the relay extended through trip command (either 86 G or 286 G or
B/B Protection trip)
(2) Over current element senses actual fault persisting.
When both the above conditions are satisfied LBB protection acts as with a time
(0.2 sec) to trip all other breaker connected to the bus.
The LBB protection initiates bus bar protection.
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PROTECTION OF TRANSFORMERS
NATURE OF TRANSFORMER FAULTS:
Power transformers, being static, totally enclosed and oil immersed develops
faults only rarely but the consequences of even a rare fault may be serious unless the
transformer is quickly disconnected from the system. For the purpose of discussion,
faults can be divided into three main classes:
1) Faults in the auxiliary equipment, which is part of the transformer.
2) Faults in the transformer windings and connections.
3) Overloads and external short circuits.
1. FAULTS IN AUXILIARY EQUIPMENT:
The detection of faults in auxiliary equipment is necessary to prevent ultimate
failure of the main transformer windings. The following can be considered as auxiliary
equipment.
i) Transformer Oil:
Low oil is a dangerous condition in a transformer because live parts and the leads
to bushings, etc. which have to be under oil, gets exposed if the oil drops belowthe specified level. Oil level indicators with alarm contacts are available to give
indication for immediate attention.
ii) Oil pumps and Forced Air Fans:
The top oil temperature normally gives indication of the load on the transformer.
Increased oil temperature might be an indication of an overload or it might be due to
fault in the cooling system, such as failures of the oil pump or the blocking of a
radiator value, or no operation of fans. A thermometer with alarm contacts will
indicate rise in the oil temperature due to any of these faults. An oil flow indicator is
commonly used to indicate proper operation of oil pumps.
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iii) Core and Windings Insulators:
Incipient faults may occur initially, which may develop into major faults if not
taken care of at the initial stages. Insulation failure may develop because of the
following:a) The insulation of laminations and core bolts may be of Poor quality or
has been damaged accidentally during erection.
b) The insulation between the windings, between winding and the core
and the conductor insulations may be of poor quality; may have been damaged
mechanically; may be brittle because of aging or overloading.
c) Badly made joints or connections:
The incipient faults need to be attended to immediately and as soon as possible. Gas
actuated relays described later in this chapter provide indications alarm for incipient
faults.
2. WINDING FAULTS:
Electrical faults, which cause immediate serious damages and are detected by
unbalance current or voltage, may be divided into the following classes:
i) Faults between adjacent turns or parts of coil such as phase-to-phase faults
on HV and LV external terminals or on the windings itself or short-circuit between
turns of HV and LV windings.
ii) Faults to ground or across complete windings such as phase-to-earth faults
on the HV and LV external terminals or on the windings.
A short circuit between turns can start with a point contact resulting from
mechanical forces or insulation deterioration due to excessive overload or a loose
connection, breakdown of transformer insulation by an impulse voltage. The puncture of the turn insulation by an impulse is supposed to cause a path of destruction, through
which normal frequency voltage can maintain an arc. However, if the turn voltage is
insufficient to maintain the arc it will quench by the oil at the first current zero.
Faults to ground, or across a large part of the winding will result in large values of
fault currents as well as the emitting of large amounts of gas due to the decomposition of
oil This type fault is not difficult to detect, but rapid clearance of fault is essential to
avoid excessive damage and to maintain system stability.
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3. OVERLOADS AND EXTERNAL SHORT CIRCUITS:
Excessive overloading will result in deterioration of insulation and subsequent
failure. It is usual to monitor the winding and oil temperature conditions and an alarm is
initiated when the permitted temperature limits are exceeded. External short circuits mayonly be limited by the transformer reactance and where this is low fault currents may be
excessive.
DIFFERENTIAL PROTECTION OF TRANSFORMERS:
Differential protection is the most important type of protection used for
internal phase-to-phase and phase-to-earth faults and is generally applied to transformers
having ratings of 5 MVA and above. Any deviation from the normal ratio of the current
intensities, at the input and output ends, must of necessity be caused by a fault in the
protected part, so that the unbalance current can be employed directly for tripping and
indicating the fault. For this reason current differential protection combines highest
selectivity with the lowest tripping time.
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The differential protection of transformer is also known as Merz-Price
Protection for the transformer. Figure shown an ordinary differential protection for a
three-phase star-delta power transformer. In a star-delta transformer, the load currents in
the two windings are not in direct phase transformer, the load currents in the twowindings are not in direct phase opposition, but are displaced by 30 degree centigrade
and to allow for this the CT secondaries are connected in delta on the star side and in star
on the delta side.
PROBLEMS ENCOUNTERED IN DIFFERENTIAL
PROTECTION OF TRANSFORMER:
The differential scheme described suffers from the following drawbacks:
i) Unmatched characteristics of current transformer.
ii) Ratio change as a result of tapping.
iii) Magnetizing inrush current.
CT Characteristics:
Unless saturation is avoided, the difference in CT characteristic due to different
ratios being required in the circuits of different voltages may cause appreciabledifference in the respective secondary currents whenever through-faults occur. This
trouble is aggravated in the case of transformers due to unequal ration current
transformers being employed on either side of protected transformer. A source of ratio
error, which results in circulating currents under through-fault condition, is unequal
burden imposed on the current transformers due to unequal lead lengths.
Ratio Change as a Result of Tapping:
Tap changing equipment is a common feature of a power transformer that
effectively alters the turns ratio. Compensating for this effect by varying the tapings on
differential-protection current transformers is impracticable.
Based or percentage differential relays ensure stability with the amount with the
amount of unbalance occurring at the extremities of tap-change range. Based relays are
better suited to the overall protection of variable-ratio transformers.
Magnetizing Inrush Current:
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When the transformer is energized, the transient inrush of magnetizing current
following into the transformer may be as great as ten times full-load current and it decays
relatively slowly. This is bound to operate the differential protection of the transformer
falsely. This magnitude of the magnetizing inrush current is a function of the permanentflux trapped in the transformer core and the instant on the voltage cycle when it is
switched on.
There are a number of ways of ensuring immunity from operation by magnetizing
surges. Firstly, the relay may be given a setting higher than maximum inrush current;
secondly the time setting may be made long enough for the magnetizing current to fall to
a value below the primary operating current before the relays operates. The simple
remedies are incompatible with high speed and low primary operating current. In third
method the harmonic content of the current flowing in the operating circuit is filtered out
and through a restraining coil.
Gas Actuated Relays:
When a fault occurs inside the transformer tank gas is usually generated, slowly
for an incipiently fault and violently for heavy faults most short circuits developed either
by impulse breakdown between adjacent turns at the turns of the winding or as a very
poor initial contact which will immediately heat to arcing temperature the heat
produced by the high local current causes the transformer oil to decompose and produce
a gas which can be made use of to detect the winding faults.Based on this following
relays are available:
1. Gas accumulator relay popularly known as buchholz relay actuated by the gas
formed.
2. Rate of pressure rise relay that acts on the measurement of the rate o9f formationof gas.
3. Pressure relays and pressures relief devices, which have for their actuation a
measurement of the total accumulated pressure.
4. Gas analyzers, which act on the analysis of products of decomposition.
Buchholz relay is the simplest for protection, which is commonly used in all
transformers provided with conservator. It consists of a chamber connected in the
upper side of the pipe run between the oil conservator and the transformer tank, and
containing two cylindrical floats, one near the top of the chamber and other opposite
orifice of the pipe to the transformer. Under the normal conditions the floats are up,
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but on the occurrence of, say a between-turns fault gas bubbles produced by the
breakdown of the oil flow out of the transformer in the direction of the conservator.
On reaching the Buchholz relay they are trapped and there by reduce the oil level in
the chamber and cause the upper float to fall. This is usually a slowly fault and whenthe float has fallen through a pre determined distance a pair of contacts, which are
controlled by the float, close and give an audible or visual warning.
However, if the fault is heavy, the surge of the gas and oil up the pipe towards the
conservator engages the lower float, which is pushed over instantaneously and engages
its associated contacts, which in turn the circuit breaker. Leakage of oil causes the upper
float to operate. Care must be taken during oil changing to avoid spurious signals.
The main advantage of the Buchholz relays are that they indicate incipient faults,
for e.g., between turns faults or core heating and so many enables a transformer to be
taken out of service before serious damage occurs. One important limitation, however, is
that they do not protect the connecting cables which must therefore have a separate
protection.
Hermetically sealed transformers are common in the United States. These
transformers are some times fitted with hydraulic relays. Which respond to the rate of
change of pressure in the gas cushion above the oil.
ON LOAD TAP CHANGER (OLTC):
On load tap changer for use with power transformers could be in their own tank
suitable for housing in the same tank along with transformers.
On load tap changer suitable for mounting in the same tank along with the
transformers are available in three phase unit for neutral end connection and single phase
Unit for connection in the middle of the winding. The design of this on load tap changer is based on modular construction and can be used at neutral end of windings up to 400
KV system or at the line end of windings up to 220 KV system.
The important features of the OLTC are:
i) The tap changer is of resistor type and this ensures minimum arcing at the
diverter switch.
ii) The main current is never interrupted during a tap change.
iii) The ohmic values of the transition resistors are chosen to reduce the voltage
variations during tap a change to a minimum.
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PROTECTION OFGENERATOR TRANSFORMER UNITS
In large high voltage systems generators are connected through step-up power
transformers to the main transmission circuit. For supplying generator auxiliaries a unit
of transformer is connected at the generator terminals.
It is usual to provide common differential protection for both generator and step-
up transformer and the intervening connections. This differential protection which covers
a zone from generator neutral to HV circuit breaker must take into account the phase
shift and current transformation in the step-up transformer. Magentizing inrush surges
due to restoration of voltage on fault clearance may cause unwanted operation of
protection scheme unless precautions are taken. Also the effective setting will differ
depending on position type of fault, i.e.HV phase or earth.
A typical overall differential protective scheme is shown in fig. A biased
differential relay with a setting of 20% and a bias of 20% is generally satisfactorily. This
overall differential protection does not include the until transformer, which has separate
differential protection.
For protection against earth-faults an earth-fault relay can be put in the generator
neutral or in the secondary winding of the step-up transformer. The differential
protection is effective against earth-faults in case the HV star side of the generator-
transformer is resistance earthen and thus restricted earth-fault protection provided. In
case it is solidly earthen the differential protection is quite adequate, but still a separate
restricted earth-fault protection of the instantaneous type is preferred and the differential
protection under such a case acts as a backup protection to the restricted earth-fault
protection. A scheme of restricted E/F protection along with differential protection for a
resistance earthen transformer on the HV side shown in the fig.
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FIGURE
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MOTOR PROTECTION
INDUCTION MOTORS
In order to understand the type of protection required, it is necessary first to
review the main characteristics of both the induction motor and the synchronous motor.
In the induction motor the stator windings are placed around the stator in such a
manner that when a three-phase voltage is applied, a rotating magnetic field is set-up. For
example, with the applied voltage having a frequency of 60 Hz on a four-pole machine,
the speed of the field is 1800 rpm. This is called synchronous speed. This rotating field
cuts the rotor conductors. The rotor consists of a series of solid conductor bars short-circuited at each end to form what looks like a squirrel cage. In order to improve the
magnetic circuit, the conductors are embedded in a laminated iron core.
The flux of the rotating magnetic field induces current in the rotor
conductors. Interaction of this current with the flux produces a torque, which causes the
rotor to rotate. The magnetic field rotates at synchronous speed but the rotor turns at a
slightly lower speed. This is essential in order to maintain some induced current in the
rotor conductors. The actual difference between operating speed and synchronous speed
is known as slip. Typically, the value of slip will vary between 18 and 33 as the load
increases from no load to full load.
When the stator winding is first When the stator winding is first energized, the
rotor is stationary, so a very large current is induced into the rotor conductors as the rate
of cutting by the rotating flux is maximum. Correspondingly, a very large stator current
is drawn, perhaps 7 or 8 times full load current. As soon as the rotor begins to accelerate
the current drops rapidly to about 5 or 6 times full load current. As soon as the rotor
begins to accelerate the current drops rapidly to about 5 or 6 times full load current and
remain fairly Constant at this level until the speed reaches about 80 percent of
synchronous speed. At this point, as the rotor approaches synchronous speed and the slip
rate reduces, the stator current falls of rapidly until at 100 percent synchronous speed it is
zero.
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Torque to the amount of mechanical turning force, which is available to drive the
load, says a large fan. The torque required by the fan is almost a straight line from zero
up to operating speed. When the motor is first switched on, the small amount of excess
torque causes the rotor to accelerate. At about 40% synchronous speed.This point is known as the pullout torque. From this point, as the speed
approaches synchronous speed, the amount of torque available rapidly falls, until it is
zero at synchronous speed. This is because the magnitude of current induced in the rotor
conductors decreases as it approaches synchronous speed. The motor will operate at the
point where the torque developed by the motor, equals the torque required by the fan.
The actual speed at which pullout torque is determined by the design of the motor
and depends upon the ratio of the resistance to the reactance of the rotor conductors. The
characteristic of the motor can be changed to meet the requirements of certain loads by
adding or subtracting resistance in the rotor winding. This is achieved by using a wound
rotor instead of the squirrel cage. The rotor winding is no longer short circuited, instead,
it is brought out to slip rings and these in turn are attached to an external resistance
which can be varied to suit requirements. Another effect of this is to provide variable
speed control.
i. INSULATION FAILURE:
The most common type of motor fault that can occur is insulation failure on the
stator windings. This causes a phase to ground fault, as the insulated conductors are fitted
tightly into the slots of the iron core. In many cases this ground fault leads to a phase to
phase short circuit.
ii. OVER HEATING:
Another common problem with motors is overheating, which may lead todeterioration and eventual failure of the insulation. Overheating can be caused by
overload, where the driven machine, say, a fan or pump, is working above the motors
rated capacity.
iii. STALLING OR LOCKED ROTOR:
If the overload is very great, perhaps due to a mechanical defect, then the motor will
stall and, the stator current will increase to a high value, perhaps 7 times normal rated
current. This condition is called locked rotor and can quickly cause severe damage to
the rotor and stator windings because there is no cooling with a stationary rotor.
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iv. LOSS OF VOLTAGE:
Another cause of overheating is low voltage. Even when the driven load is
normal, with low voltage, excess current will be drawn by the stator windings,
overheating can also be caused by the loss of one phase of the supply voltage. For example, if a single fuse were to fail the remaining two phases would draw excessive
current to continue driving the load. Even a difference between the three phase voltages
of the power supply can cause overheating. The unbalanced system conditions cause
negative sequence currents to circulate in the stator windings and this produces
overheating in the iron core.
v. RAPIC RESTART AT REDUCED SPEEDS:
A hazardous condition can occur with complete loss of voltage, particularly on
large, heavy machines. If the voltage is rapidly restored while the machine is still running
down in speed, an extremely large current will be drawn by the motor as it attempts to
arrest the fall in speed and then accelerate the machine up to normal speed again. In
some cases, due to the inertia of the heavy machine, this acceleration has even caused the
driveshaft to break.
BASIC MOTOR PROTECTION:
Generally speaking, motors up to about 1,000 horsepower operate at 600 volts
or less, while larger motors commonly operate at 4,000 volts or higher.
Small motors are generally energized through a contractor switch, which provides
some built-in protection. The contactor is held closed by the contractor coil. If the supply
voltage falls, the contractor opens and remains open until the operator effects a manualstart. A thermal overcorrect relay is usually built-in to protect the motor against over
currents. For medium size motors, say, 300 horse power and up, the contractor switch is
operated by external protection relays. In order to close the contractor contacts and start
the motor, the contractor coil is energized by closing the push button. This will close the
seal-in circuit allowing the push button to be released. As long as there is voltage on the
contractor coil, the contractor remains closed and the motor will run. To stop the motor
we must press the stop button to open the circuit. Protective relays operate in the same
manner that is, by opening their contacts in the contractor coil circuit.
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The thermal relay (49) provides overcorrect protection. It consists of a heater
element, which operates a bi-metallic switch. Excessive current flow will cause the
switch to open and thus interrupt the power supply to the contactor coil.
The thermal relay must be chosen and set to march the damage (thermal limit)curve of the motor as provided by the manufacturer.
In practice, we try to adjust the thermal relay so that its tripping curve is quite to
the damage curve in order to prevent inadvertent tripping. However, it is impossible to
get the curves to match exactly, and we will probably find that, for high values of
current, the bi-metallic switch is too slow.
One way around this problem is to add a time overcurrent relay. The time
overcurrent relay is set for a higher pick-up. The bi-metallic relay protects for lower
values of overload, while the overcurrent relay protects for higher values.
An improved method of protection against overheating directly measures the
actual temperature of the winding and stator core. When the motor is constructed,
resistance temperature detectors (RTDs) are embedded in the stator slots along with the
stator winding. These RTDs (also known as search coils) are connected detector. The
detector is made of material, which has a linear change in resistance as temperature
changes. Hence, by measuring the value of resistance, the relay can accurately determine
the temperature inside the motor. When the temperature exceeds a specified limit the
relay operates and de-energizes the motor.
Overheating can be caused by phase unbalance. Protection is often provided
against unbalance by the installation of a negative sequence overvoltage relay (47). The
relay is connected to measure negative sequence only. This is normally zero or at least
very low under balanced conditions. The relay is set to operate when negative sequencevoltage rises to, say 4% of normal terminal voltage. When the relay operates it trips all of
the motors on the bus. This relay also protects against phase reversal of the power supply
which could occur, for during a maintenance period.
For a high magnitude fault current, both the time overcurrent relay and the
thermal relay take over 10 seconds to operate. Moreover, in many instances the
contractor does not have sufficient interrupting capacity for high fault current. The motor
is normally protected against this condition by a set of fuses which are located up-stream
of the contractor. Typically, if the rated current of the motor is, say 120 amps. It operates
before the motor damage curve is reached.
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SYNCHRONOUS MOTORS:
The main feature of the synchronous motor is that the rotor winding must be
provided with DC current in order to set-up its own magnetic field. The rotating
magnetic field, produced by the stator, locks together with the rotor field and causes therotor to rotate at precisely synchronous speed. Synchronous speed depends upon
frequency, and on the number of poles in the rotor and stator. For example, at 60 Hz a 2-
pole motor has a synchronous speed of 3600 rpm; a 4-pole motor 1800 rpm, and so on.
As load increases, the rotor will continue to rotate at synchronous speed but the stator
will draw more current from the power supply system. The DC supply for the rotor may
be produced by a DC generator, which is coupled to the motor, or perhaps the supply
may be from an outside DC source. Synchronous motors are used where a precise,
constant speed is require. Another advantage of the synchronous motor is that any
adjustment of the DC excitation current adjusts the power factor of the motor. Reducing
the field current of the synchronous motor increases the demand for VARs from the
system and so causes the lagging power factor to fall. Increasing the field current
actually delivers VAR to the power system, so the motor operate at leading power factor.
This feature is most useful in large industrial installations where the synchronous motor
is commonly employed for power factor correction, thus reducing electricity bills.
Protection for the synchronous motor is similar to that for the induction motor, but
with the addition of a few special relays. One particular condition to protect against is
that of over-excitation of the rotor. Conceivably automatic power factor control or
voltage control could result in excessive DC current flowing through the rotor field
windings leading to overheating of the rotor. Protection is provided by either an
overexcitation relay (76) or a thermal bi-metallic relay (49) connection into the fieldwinding circuit. Another more direct method measures the resistance of the field winding
and from this calculates the temperature. These relays are wired to sound an alarm, or
perhaps in an unattended station, to trip the unit.Overheating of the stator winding is
protected by overcurrent relays, thermal relays, and current balance relays, just as with
the large induction motor. Another relay often found on synchronous motors is the loss
of excitation relay (40). If field current is interrupted for some reason or another, the
motor will operate as an induction motor, at lower speed, drawing a heavy reactive load
from the power system. This will cause serious overheating of both the rotor and the
stator windings. The loss of excitation relay operates by measuring current in the
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FIGURE
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excitation circuit on by measuring the flow of vars into the stator windings.
In power factor relay (5) is often used which is set to operate the power factor
falls below about 0.5 lagging. This also protects against loss of excitation.
Alternatively, an impedance relay may be used, as described in an earlier tape on
generator protection. The loss of excitation relay may be connected to only alarm the
operator, so that he may restore excitation. Alternatively, in an unattended station it will
trip the unit.
In special cases the synchronous motor is protected against loss of synchronism.
The motor could fall out of step if to correct power factor, the excitation was at a
minimum setting and the mechanical load increased. This could also happen if there gas
a sudden reduction in supply voltage. When pole slipping does occur, the state current
increases and the lagging powerfactor falls to a very low value, about.25.
If the motor continues to slip, the stator current and power factor will fluctuate
over a wide range as the rotor tries to pull into step.
Then out-of-step relay (78) detects the initial decrease in power factor and trips
the motor during the first slip cycle.
Synchronous motors are fitted with undervoltage relays (27) so as to prevent rapid
re-start if loss of voltage occurs. This is the same as in induction motors. Additional
protection for this condition in sometimes provided by an underpower relay (37). Thisrelay measures power being fed into the synchronous motor and it is usually set to
operate when power falls below 38 of non load maximum operation, that is, close to
zero.
Alternatively, a reverse power relay (32) could be installed. This relay will only
operate if the motor actually feeds power back into the bus and it can only do this if the
bus is connected to other motors which are drawing power. In all probability this could
be the case as most synchronous motors are used in large industrial installations.
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SUMMARY OF MOTOR PROTECTION:
1) STATOR FAULT
Instantaneous Overcurrent Relays 50Instantaneous Ground Relays 50 G
Inverse Time Ground Relays 51 G
Differential Protection Relay 87
2) OVERLOAD
Thermal Relays 49
Time Overcurrent Relays 51
3) LOCKED ROTOR
Time Overcurrent Relay 51
Impedance Relay 21
Acceleration Relay 18
Supervised Instantaneous Overcurrent 50
4) PHASE UNBALANCE
Negative sequence Overvoltage Relay 47
Current Balance Relay 46
5) LOSS OF VOLTAGE
Undervoltage Relay 27
6) EXCITATION
Loss of Excitation Relay 40
Overexcitation Relay 76
Rotor Winding Temperature Relay 49Power Factor Relay 55
7) SYNCHRONISM
Out-of-step Relay 78
Underpower Relay 37
Reverse Power Relay 32
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GENERATOR PROTECTION AGAINST SYSTEM DISTURBANCES
As the generator is synchronized to the power system, it is responsive to
disturbances, which occur on the system. As shown in fig. E-1, certain protective devices
are installed to protect against these conditions. One typical example is that of frequency.Large steam turbines are designed to operate within a very narrow range of speed i.e.
between 49.5 and 50.5 Hz.
High frequency can occur as a result of load rejection, perhaps as a consequence
of tripping transmission lines or load feeders. However the turbine governor will
normally control the turbine speed and maintain frequency close to normal. In case the
governor loses control, the turbine is fitted with an over speed trip, which is set to
operate at 110 per cent, say 55 Hz.
Low frequency can occur as a result of system overload. If the turbine generator
operates below 49.5 Hz, serious vibration and consequent damage may occur to the
large, low-pressure turbine blades. The turbine is permitted to operate at low frequency
only for very short periods of time typically:
48.5 -- 49.5 Hz - 60 Minutes accumulated
47.5 -- 48.5 Hz - 10 Minutes accumulated
A frequency relay (81) is installed to alarm or trips.
In practice, in a large interconnected power system, the frequency rarely falls
outside normal limits. However, such an extreme situation can occur, if the power system
becomes disconnected into separate areas or islands, so that each generation of group of
generators is supplying its own block of load. In some areas we will have too much
generation available, hence the frequency will initially rise. In other areas there will be
insufficient generation, and if load shedding is not rapidly initiated, the generator will become overloaded. The consequences will be:
1) A fall in frequency;
2) A fall in voltage;
3) A rise in stator current.
The voltage regulator will increase excitation on the generator in order to maintain
line voltage, and this may lead to overheating in the rotor. To protect the rotor,
overcurrent protection is sometimes installed in the excitation circuit. This relay is set to
alarm the operator.
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The stator winding may be protected from overheating by the installation of an
extremely inverse time overcurrent relay (50/51) set to operate just before the stator
winding short time thermal limit is reached. To present this relay operating during
normal operation, a combined instantaneous element is usually connected as a permissivefor the time overcurrent contacts. This will prevent operation of the unit below 115 per
cent of maximum rated current.
During a cold start-up, several hours are required to bring a steam turbine
generator unit up to speed. During this low speed period of the generator voltage at this
low frequency may be high enough to overexcite the main transformer primary. To avoid
this problem, an overvoltage relay (59F) may be installed to compare voltage and
frequency; this is known as a volts-bertz relay. This relay will operate at about 115% of
rated voltage when frequency is normal. At low frequency the voltage trip point will be
proportionally lower. This relay will also protect the stator insulation against overvoltage
at normal frequency.
Protection against closing the breaker out of phase is provided by connecting a
directional time overcurrent relay (67). When power flows into the generator, this relay
will operate and trip.
Another type of relay, which is often installed, is a synchronizing relay. This type
will not allow the breaker to close unless the phase angle is within a determined range
(usually 10 degrees either side of synchronism).
Operation of the generator is subject to the following limits:
1) Minimum excitation;
2) Overheating of the stator winding due to overload; and3) Overheating of the rotor winding due to excessive excitation on current.
The limits of generator operation are indicated by the units capability curve. A
typical units curve is shown in Fig.E-2. This shows the combination of megawatts and
mega vars that can be produced by the generator at different power factors. Positive vars
are vars supplied by the generator. Negative vars are fed into the generator from the
power system.
We cannot maintain the same MVA at lower power factor, due to the temperature
limit of the rotor winding. The capability of the generator is reduced at low lagging poor
factor.
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On the leading power factor side, very low excitation current may cause the rotor
to fall out of step, due to loss of magnetic torque. This is the steady-state stability limit.
There is yet another limit beyond this the overheating of stator iron which results
from excessive flow of capacitive currents. Usually the excitation to a dangerous level.
What would happen if the generator suffered a complete loss of field perhaps due
to a defect in the excitation circuit? In this situation, remember, the generator is still
connected to the power system, and is still delivering megawatts because it is still being
driven by its prime mover. However, it will no longer supply vars. On the contrary, it
will draw var from then system in order to maintain excitation. The power factor will
move to, say, 0.5 leading. So the generator will continue running and producing power as
an induction generator. However, this will probably lead to low voltage at the generator.
However, this will probably lead to low voltage at the generator terminals, and, more
importantly, serious overheating will occur in the stator iron. If the field cannot be
restored promptly, the unit should be down. The loss of field relay (40) may be used for
alarm or to initiate tripping of the unit.
Earlier loss of field relays worked by measuring current in the excitation circuit.
When this fall below a pre-set level, the relay operated after a time delay.
Nowadays, loss of field is detected by measurement on the generator high voltage
side. One method is to use a mega var meter set to operate when the imported (that is
negative) mega vars reach a high level, implying that the unit is operating as an induction
generator.
A more common method is to install an impedance relay, which compares the
state of voltage and current. The impedance characteristic is shown in Fig. E-3.
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FIGURE
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Following are the various protections recommended for the generator and generator
transformer protection:
Type of Fault ANSI Device No. Protection FunctionsGENERATOR STATOR Short Circuits 87 g
87 GT
21 G
51/27 G
Generator differential
Overall differential
Minimum impedance (or alternatively Over
Current / Under voltage)Asymmetry
Stator Overload
Earth Fault Stator
46 g
51 G
64 G1
64 G2
Negative sequence
Overload
95% Stator earth fault
100% Stator earth faultLoss of excitation 40 G Loss of excitationOut of step 98 G Pole SlipMonitoring 32 G / 37 G Low forward power/reverse power
(double protection for large generators)Blade fatigue 81 G Minimum frequencyInter turn fault 59 G / 87 GT Over voltage or over currentMag. Circuits 99 G Over fluxing volt/Hz.Higher Voltage 59 G Over VoltageAccidental Energisation 27/50 G Dead machineMonitoring 60 G PT fuse failureGENERATOR ROTOR:
Rotor Ground 64 F Rotor earth faultGENERATOR TRANSFORMER Short Circuits 87 GT
51 GT
87 T
Overall differential
Ove