Microprocessor Based System of Automatic Synchronizer Report

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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

generator 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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1. 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.

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2. 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.

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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 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.

3 AUTOMATIC SYCHRONIZATION

Synchronization 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

synchronization are satisfied, closing of the main switch of the incoming

machine is done by the automatic synchronizer

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4 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

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5 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.

5.1 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.

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5.2 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.

5.3 Potential Transformer Unit

This unit consist of a bank of four shell type

transformer(P.Ts).Fig.(2) shows the connection diagram. 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

5.3 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).

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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

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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, 340KHz and

3.4KHz 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

5.5 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.

5.6 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

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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.

6 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

4. Synchronizing at zero phase difference condition

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The following subroutines are developed and called in the main program

1 IN PHASE : The subroutines checks the in phase instant of Ralt

and Rbus

where Ralt refers to the phase R of the incoming

alternator and

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 either in clockwise or

anticlockwise

direction.

4 KCLOSE : Checks the closure of the key to be 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 register pair with the

contents of DE 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’

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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 motor 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

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8 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

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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.

9 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.

10 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 .

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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.

11 SYNCHRONIZING

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

consecutive inphase instants of Rus 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) wich will enable to switch on the

circuit exactly at the next inphase instant.

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

phase sequence is R.Y.B the auto ‘synchroniser’ gives a prompt to the

operator by displaying ‘O’ (ie inphase sequence OK). For the improper

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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

13 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 synchronoscope 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.

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14BIBLIOGRAPHY

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