AUTOMATIC VOLTAGE REGULATOR(AVR)

Automatic voltage regulator (AVR) maintains the Generator terminal voltage at a given value automatically by changing the excitation current to the Generator field.

The AVR supplies the required D.C. to the Generator field depending on the load, power factor etc. to maintain a constant terminal voltage.

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CONTROL SYSTEMS OF AVR

1. Auto control

2. Manual control

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TYPES OF AVR

1.Single channel AVR

Two controllers one is automatic and the other is manual

Both the controllers are fed from the same supply

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2. Dual channel AVR system One automatic voltage controller

and one manual controller Different power supply, gate

control and pulse amplifier units for each of the controllers

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3.Twin channel AVR system

Two automatic voltage regulators

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Function of AVR compares the Generator terminal

voltage with a preset reference voltage.

If the Generator terminal voltage is less than the reference voltage, the AVR increases D.C. voltage across the Generator field.

Maintaining the constant voltage as per the setting.

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The automatic voltage control system

EXCITATION AS A CLOSED LOOP CONTROL SYSTEM

Grid

If Vg Synchronous

Generator

Excitation

System

2

3

4

7

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GAIN

INPUT OUTPUT

OP AMP

REF

FEED BACK

OPERATIONAL AMPLIFIER

Block diagram of AVR

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AVR inputs Generator voltage feedback signal Generator voltage reference AVR provides the following

functions and signals Summing and amplification of signals to

provide the exciter rectifier firing angle control signal.

A signal for manual follow up.

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AVR Action Two inputs are of opposite polarity. When the magnitudes are equal,

the net input is zero. When unequal-the error is

amplified and reversed to obtain a correction signal which goes to the

GPG(gate pulse generator)

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AVR CIRCUIT

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+-50 V SUPPLY To power the regulator amplifier as

well as the optional accessories such as manual follow up and power integral stabilizer

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Regulator auxiliary card 3 phase voltage from the generator

PT’s are stepped down and applied to the regulator auxiliary card which converts it to ripple free dc voltage suitable for the input of regulator amplifier.

Balance Voltmeter

A 30-0-30 V dc voltmeter is connected across the output of regulator amplifier to read the magnitude and polarity of the output voltage of OP-AMP at any instant.

AVR Voltage Setting

From the +-50 V source ,-50 V is applied and taken across a motorized potentiometer which can be operated from the control bench board. The variable output from the pot. is applied to the input of the regulator OP-AMP.

Manual Voltage Control In the absence of AVR the voltage

can be manually controlled. The stabilized 15 V from the MPS is

applied across a second potentiometer. The variable output goes to the GPG together with output of the regulator

OP-AMP.

MANUAL FOLLOW UP

When there is a noticeable output from the regulator OP-AMP, depending on its polarity, either of a pair of relays is automatically closed to drive the manual potentiometer motor in the required direction.