Automatic Alternator Synch Ron is at Ion

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ABSTRACT

The manual method of synchronization demands a skilled

operator and the method is suitable for no load operation or normal frequency

condition. Under emergency condition such as lowering of frequency or

synchronizing of large machines a very fast action is needed, which may not be

possible for a human operator. Thus there is a need of autosynchroniser in a power

station or in an industrial establishment where generators are employed. This paper

describes a microprocessor based set up for synchronizing a three phase alternator

to a busbar. Also existing methods of synchronization are mentioned.

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CONTENTS

Page no:

1. INTRODUCTION 3

2. EXISTING METHODS OF SYNCHRONISATION 4

3. CRITERIA OF DESIGN 6

4. HARDWARE DETAILS 7

5. PROGRAM STRUCTURE 12

6. FLOWCHART 14

7. SYNCHRONISING 21

8. ADVANTAGES 22

9. RESULT 23

10. CONCLUSION 24

11. REFERENCE 25







INTRODUCTION

It is well known that electrical load on a power system or an

industrial establishment, is never constant but it varies. To meet the requirement of

variable load , economically and also for assuring continuity of supply the number

of generating units connected to a system busbar are varied suitably . The

connection of an incoming alternator to system bus, ie; synchronization requires

fulfillment of the condition like the same phase sequence equality of voltages and

frequency between the incoming machine and frequency between the in coming

machine and busbar. In order to order to overcome the 9 technical drawbacks of the

conventional synchronization methods we can introduce a microprocessor based

system.







EXISTING METHODS OF

SYNCHRONIZATION AND PRINCIPLE

a. Synchronizing Lamp

The operation of connecting an alternator parallel with another

alternator or with a common busbar is known as synchronizing for proper

synchronization of alternators the following three conditions must be satisfied

1. The terminal voltage of incoming machine must be the same as the busbar

voltage.

2. The speed of the incoming machine must be same such that the frequency is equal

to the busbar frequency.

3. The phase of the alternator voltage must be identical to the busbar voltage.

It means that the switch must be closed at the instant the two voltages are

in correct phase.

Condition 1 can be checked with the help of voltmeter, frequency is

adjusted by varying the prime mover speed. In the dark lamp method the lamps are

connected across the alternator and busbar terminal. If the phase sequence is

different, the lamps will brighten in a cyclic manner correct phase sequence is

indicated by simultaneous darkening brightening of lamps. The switch is closed in

the middle of the dark period. Once synchronized properly, the two alternators

continues to run in synchronism.







b. sychroscope

The armature of the sychroscope will align itself so that the axis of

windings are R and F are inclined at an angle equal to phase displacement between

V and V’. If there any difference between the frequencies of V and V’ a pointer

attached to the armature shaft will rotate at slip speed, and the direction of its

rotation will indicate whether the incoming machine is running above or below

synchronism. At synchronism, the pointer will remain stationary, but it must be

brought to the particular position which indicates zero phase displacement between

V and V’ before the main switch of the incoming generator is closed.

AUTOMATIC SYCHRONISATION

Synchronisation by means of manually operated switching served well

enough when the individual generators were relatively small, but with the growth of

system capacity, it becomes necessary to use automatic devices to ensure the closing

of the main switch of the incoming machine at the proper instant.

The scheme introduced here is for the complete automation of

synchronization i.e.; the adjustment of magnitude of voltage and frequency of

incoming alternator is done automatically. When all the requirements of







synchronisation are satisfied, closing of the main switch of the incoming machine is

done by the automatic synchronizer

CRITERIA OF DESIGN

The auto synchronizer has been developed to carry out the following tasks related

to the synchronization such as

I To check if the phase sequence of incoming machine is correct or

otherwise, in case of wrong phase sequence, to terminate the further steps in the

process and also to indicate corrective action.

II To check if frequency of incoming machine is equal to that of busbar and

to adjust it to a value nearly equal to the busbar frequency.

III to check machine voltage is equal to that of busbar and to adjust it to a

value nearly equal to the busbar voltage and

IV After ascertaining the fulfillment of the above condition, to give closing

signal to the circuit breaker so that the breaker will close the exact inphase instant.

In addition, the auto synchronizer has been designed so that the alternator is

started with in minimum voltage and minimum frequency conditions







HARDWARE DETAILS

The hardware has been designed to fulfill all the requirements of the

synchronizing process.

Block diagram of auto synchronizer setup is shown in fig (1)

A microprocessor trainer kit is used as a controller for the setup. Also the figure

showing the auto synchronizer setup consist of

a Frequency control unit

b Voltage control unit

c Potential transformer unit

d Signal conditioning card

e Display card and

f Circuit breaker with the switching circuit.

1 Frequency Controlling Unit

The frequency of an alternator can be changed by varying the speed

of the prime mover which is a DC shunt motor in this case .A rheostat is provided in

the field circuit of the motor for this purpose The frequency controlling unit is a

lead screw arrangement driven by a stepper motor attached to the variable point on

the rheostat the stepper motor (SM1) is controlled by an 8085 microprocessor

system through a driver circuit.

2 Voltage Controlling Unit







Once frequency of alternator is fixed, or adjusted, its voltage is

controlled by variation of excitation current. This excitation current is varied by

providing a rheostat in the field circuit of the alternator. The automatic variation of

excitation current is obtained by lead screw and stepper motor (SM2) arrangement

similar to the one used for frequency control.

3 Potential Transformer Unit

This unit consists of a bank of four shell type transformer (P.Ts). Out of

the four transformers thee are used for stepping down three phase voltages of

alternator and the remaining one is used for stepping down the voltage of the phase

R of the bus bar. The potential transformers connected to the phase R of the bus

bar and the phase R of the alternator are having two secondaries. Hence one

secondary is used for voltage measurement and the other is used for frequency

measurement .The potential transformers connected to the Y and B phases have

only one secondary each

4 Signal Conditioning Card

It is subdivided into (i) signal conditioning card and (ii) ADC subunit.

The signal conditioning subunit consists of for identical circuits each of

which comprises of a zero crossing detector (ZSD)(for ralt,yalt,balt and rbus) two

rectifier and filter circuits for ralt2 and rbus2 and an inphase sequence detector and

an inphase instant detector as shown in fig.(1).













BLOCK DIAGRAM OF MICROPROCESSOR BASED

ALTERNATOR SYNCHRONISATION







ZCD OUTPUT WAVEFORM

The ZSD converts sinusoidal output of potential transformer secondary to

rectangular signal Fig.3 shows the ZSD output waveforms these square waves are

fed to microprocessor system for measurement of frequency and phase sequence

detection using developed software

The rectifier and filter circuits converts the AC signal of ralt2 and rbus2 to DC

signal compatible for ADC 0809.These are used for the voltage measurements of the

alternator and the bus.

Inphase instant detector circuit is used for detecting the inphase

instant of signals ralt1and rbus1 which is the correct instant for synchronization.

The ADC subunit consists of ADC0809 interfaced with 8085-

microprocessor system. The clock required for this ADC is derived from a

frequency divider circuit made up of three 7490 counter ICs. The clock available on

microprocessor kit of 1.7 MHz, which is divided by further factors 5, 10, 10.

Therefore out of three available outputs, 340 KHz and 3.4 KHz outputs are used

respectively for the ADC 8255. The digital output corresponding to the alternator

and busbar voltages are obtained using separate channels for alternator and busbar

voltages

5 Display Card

Display card has been provided for indication of messages during

alternator synchronization process it uses four seven –segment LED displays to







represent the three inphase synchronization conditions and circuit breaker position.

Also the kit display is used for displaying messages such as ‘HALT’,’DONE’etc.

6 Circuit Breaker With Switching Circuit

The circuit breaker used as a synchronizing switch is in the form of

a direct on line starter .In order to operate the circuit breaker, its operating coil is

connected to 230 V d.c Supply through electromagnetic relay. The relay is activated

at proper instant by the microprocessor so that the circuit breaker is closed at the

correct inphase instant.

PROGRAM STRUCTURE

The main program performs the following functions.

1. Phase sequence detection

2. Alternator frequency measurement and its adjustment

3. Alternator voltage measurement and its adjustment, and

Synchronizing at zero phase difference condition







The following subroutines are developed and called in the main program

1 IN PHASE : The subroutine checks the in phase instant of Ralt and Rbus

Where Ralt refers to the phase R of the incoming alternator

Rbus1 refers that of the bus bar.

2 LSW : This subroutines checks if the limit switch is closed or not

3 SM : Rotates the stepper motor in either direction

4 KCLOSE : Checks the closure of the key handled by the operator

5 PSEQ : Checks the phase sequence of the alternator

6 FRQ : Measures the frequency of the alternator or bus bar

7 VOLM : Measures the voltage of the alternator or bus bar

8 CMPHD : Compares the contents of HL and DE register pair

9 SUBDH : Subtract the contents of DE pair from contents of HL pair

10 In addition the following monitor subroutines are used whenever required :

a. CRLF clears the display

b OUT MSG displays the given message on the display

c delay provides delay in the program

d DONE Displays the message ‘DONE’

Fig (4) shows flowchart of the main program for autosynchronising setup.

The status of the limit switches LS1and LS2 are checked. These are provided

with the field circuit rheostats of exciter and driving motor. Accordingly the

stepper motors are rotated in appropriate directions to obtain initial

positions respectively of field rheostat (Rf) and exciter rheostat (Rex) . ht

emessage ‘START’ is displayed indicating operator to start the DC motor

(prime mover). When the operator sees the prompt, he switches ‘ON’ the DC

motor of the alternator. Once the alternator is started, it develops some

voltage at some frequency; following sequence of events will take place

automatically.

1. Detection of phase sequence

2. Frequency measurement and control







3. Voltage measurement and control

4. Synchronizing

FLOWCHART







DETECTION OF PHASE SEQUENCE

Before alternator is connected to the busbar first of all we have to ensure that the

phase sequence of the incoming alternator is the same as that of the busbar. The

program checks the ZCD outputs corresponding to Ralt and Yalt phases for their

low to high transitions and count corresponding to time T1 as shown in fig (8) is

obtained using subroutine ‘PSEQ’. Similarly the ZCD outputs corresponding to

Ralt and Balt are measured or checked for their zero to one transition and count

corresponding to time T2 is obtained. To check the phase sequence, T1 and T2 are

compared . When T1 is greater than T2, the phase sequence is not correct. This

condition is indicated by ‘N’ and the display of message ‘HALT’ will be there and

the program execution is stopped on the other hand, if T1 is less than T2, the phase

sequence is ‘OK’ or correct and is indicated by ‘O’. There after the program control

is transferred to frequency measurement and control part.

FREQUENCY MEASUREMENT AND CONTROL

The subroutine FRQ written for frequency measurement of bus 0or alternator

checks their respective ZCD outputs for low to high transitions







In software, the register HL(for busbar signal) or DE (for alternator signal)

initialized with zero components are incremented till the ZCD outputs are in a high

to low transition . This count in HL is equivalent to the time period corresponding

to the half cycle of alternator signal . The counts obtained inHL and DE pairs are

compared. If the count in HL is less than that of DE , it indicates that alternator

frequency is less than the busbar frequency. The difference in frequency is checked

and if the difference is greater than allowed difference (0.1Hz), then the stepper

motor (SM2) is rotated to bring the difference with in the limit, and ‘FE’ is

displayed when this condition is achieved.

On the other hand, if the count in the HL pair is greater than that in DE,

alternator frequency is high and is indicated by ‘FH’. The stepper motor (SM2) is

rotated in reverse direction to bring the difference in frequency within limit till ‘FE’

is displayed.

VOLTAGE MEASUREMENT AND CONTROL

The digital output corresponding to the alternator and bus voltages are obtained by

the following method. The busbar output and the incoming alternator output are

first stepped down in the same ratio using P.T unit. These step-down transformer

signals are fed to the rectifier and filter circuits. The output from it is given to ADC

through separate channels. ADC output ie; the digital outputs are compared and the

difference of these is obtained. When the difference is less than the allowed

difference,(1%) the ‘VE’ is displayed and the program execution is continued.

When the difference is greater than allowed difference, either ‘VH’or ‘VL’

is displayed to indicate high or low voltage of alternator respectively. The stepper







motor (SM1) is rotated in appropriate direction to bring the difference with in the

limit till ‘VE’ is displayed.

SYNCHRONISING

After satisfying all these condition, the time (Ti) between consecutive inphase

instants of Rbus and Ralt (obtained from inphase instant detector) is measured

using 8253 in mode ‘0’. The time interval (Ti-To) where T0 is operating time of

switching circuits, is obtained.







The closure of circuit breaker is achieved by sensing next inphase instant with delay

of (Ti-T0) which will enable to switch on the circuit exactly at the next inphase

instant.

ADVANTAGES OF MICROPROCESSOR BASED ALTERNATOR SYNCHRONISATION

1. Anticipatory close signal provides smooth synchronizing with minimum system impact.2. Patented real-time adaptive proportional speed control algorithm provides fast, reliable frequency matching while eliminating overshoots and hunting. Includes smart target pulse to prevent hung scope condition. (Patent #5,761,073)3. Highly flexible design can be configured for optimum performance over







a wide range of system characteristics from sensitive, asymmetrical low inertia to high inertia hydro systems.4. One unit can control multiple systems with up to six different sets of breaker closing parameters.5. Test module facilitates testing via front panel terminals.6. Standard 19 inch rack-mounted case with front cover.

RESULT







The phase sequence has been checked by using developed prototype. When phase

sequence is R.Y.B the auto ‘synchronizer’ gives a prompt to the operator by

displaying ‘O’ (ie inphase sequence OK). For the improper phase sequence, ie

R.B.Y., the auto synchronizer displays ‘n’(NOT OK)and ‘Halt instruction gets

executed to stop entire operation.

The frequency of incoming machine which depends on the speed of the alternator,

ie prime mover (dc shunt motor)is measured and adjusted to bring the difference in

frequency with in the tolerance limit.

To achieve the equality of voltages, the exciter voltage or circuit resistance was

adjusted by auto synchronizer. After obtaining proper phase sequence, equality of

frequency and voltage, the auto synchronizer has to carry out synchronization

CONCLUSION







The microprocessor based system of automatic synchronizer can be used more

effectively compared to conventional methods of synchronization such as dark lamp

method, bright lamp method and synchronization using sychroscope this because of

the fact that the conventional, method calls for of the operator and accuracy is less

and it depends on the sense of correct judgment of the operator. Moreover the

microprocessor based alternator synchronizer is user friendly and requires less

maintenance. It also exploits the advantage of superior performance of the

microprocessor like accuracy speed and reliability.

REFERENCES







1 JOURNAL OF INSTITUTION OF ENGINEERS (INDIA)

VOLUME-80, NOVEMBER1999

2 THEORY OF ALTERNATING CURRENT AND MACHINERY

ALEXANDER.S.LANGSDORF

3 FUNDAMENTALS OF MICROPROCESSORS AND

MICROCONTROLLERS B. RAM






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Automatic Alternator Synch Ron is at Ion

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