AUTOMATIC VOLTAGE REGULATOR

Automatic voltage regulator (AVR) maintains the Generator terminal voltage at a given valve 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.

The input to the AVR is a 3 phase A.C supply

The A.V.R. consists of a i) Auto chennel and ii) A Manual channel

The Auto channel regulates the Generator terminal voltage from 90% to 110% of the rated voltage. This is a closed loop voltage regulation.

The manual channel regulate the Generator terminal voltage from 0-100% of the rated voltage. This is called excitation current regulation.

The Generator terminal voltage is fed to the AVR which compares it with a reference valve and an error signal is generated.

The function of AVR is as follows :

1. The AVR compares the Generator terminal voltage with a preset reference voltage.

How this reference voltage is obtained. 2. If the Generator terminal voltage is less than the reference voltage, the AVR

increases D.C. voltage across the Generator field.

If the Generator terminal voltage is more than the reference voltage, then the AVR reduces the D.C. voltage across the Generator field.

Field forcing :

Suppose the Generator is operating at rated load and rated power factor. Suddenly the load is thrown off then the voltage suddenly increased.

Then AVR senses the increase in terminal voltage and increases the D.C. voltage across the Generator field.

This process takes some seconds if the AVR acts in a normal way. During this period the Generator may be unstable and may pull out of step.

During such operating conditions, it is necessary to reduce the excitation voltage very quickly.

This can be achieved by applying a negative voltage to the already available D.C. voltage.

In another case if the Generator is running on a light load and suddenly additional load is applied, then the Generator terminal voltage suddenly falls down.

In such cases, it is necessary to apply voltage in addition to the already existing field voltage.

This is called field forcing.

Manual regulation

This is field current control.

Automatic regulation

Generator is brought to rated speed and suppose the rated voltage is excitation is applied and gradually increase is built up on No load (Ia=0) by increasing the excitation to if that the Generator is gradually loaded to 25%, 50%, 75% and 100% of Armature current then the voltage drops as shown in the fig. 2.

Fig. 2 is a graph drawn between terminal voltage V and load current (armature current) I a

on x-axis. This graph is called LOAD CHARACTERISTIC OF Generator.

But it is necessary to maintain terminal voltage constant always. To maintain the terminal constant, we have to increase the excitation.

At 25% armature current, we have to increase the excitation marginally. At 50% armature current we have to increase the excitation further. Like that at 75% IA and at 100% IA we have to increase the excitation. Suppose the machine is operating at 100% IA and rated voltage, if the armature current reduces to 50% then the terminal voltage increases beyond rated voltage (over voltage). So we have to reduce the excitation.

That means for every change in load current, we have to change the excitation. It is very difficult to change manually. Hence automatic voltage regulators are employed to maintain terminal voltage constant always.

The schematic diagram of AVR is shown in figure.

The Generator terminal voltage is always compared with a reference voltage and the difference between them (error signal) is used to increase/decrease the excitation. This is a closed loop voltage control.

The AVR has some additional features other than automatic voltage control.

They are

Change over from manual to auto channel

We have seen here, that the T.G. set is started and excited in manual channel and the AVR will automatically take over when the Generator voltage exceeds 90% of rated voltage. When the terminal voltage is 100%, the machine is synchronized and loaded. The auto channel will maintain the voltage constant for different load conditions.

1) If the Generator terminal voltage is less than 90% or more than 110%, the AVR changes from auto channel to manual channel.

2) In case of P.T. fuse failure also, the AVR changes from auto channel to manual channel.

The main parts of auto channel are :

i) Auto channel-Iii) Auto channel-IIiii) Manual channeliv) Follow up circuitv) Field forcingvi) Series compoundingvii) Field flashingviii) Field breakerix) Limiters

The following are technical requirement of AVR.

a) Possibility of applying negative sealing voltage across generator field winding for fast de-excitation of the Generator.

b) Possibility of applying a positive ceiling voltage across generator field winding for fast increase in Generator terminal voltage.

c) Transformer drop compensation.d) Provision stator current and rotor current limiters for optimum

utilization of Generator capability.e) Drop characteristic for reactive load showing among the

Generators operating in parallel.

Manual regulation

Manual regulation is used during

i) Commissioningii) Maintenanceiii) When the auto channel is disturbed

Automatic follow up circuit

Automatic follow up circuit is provided to ensure a smooth change over from auto regulation to manual regulation in case of problems in auto channel or P.T. fuse failure.

AVR for B.L. Excitation

1. Exciter field – excitation is controlled by AVR to regulate the Generator terminal voltage.

2. Required D.C. supply for excitation of exciter field is taken from 3. output of PMG after rectification.

Exciter field requirementPMG output

Rectifier – Thyristers

Inter locking between field breaker and generator circuit breaker

High voltage is induced in Field winding when field breaker is opened.

e= Ldi

dt

When the field breaker is opened :

The terminals of Generator field winding will disengage from DC source only after making contact with a discharge resistor.

The discharge resister will dissipate the energy stored in the Field winding and there by prevents the induction of high voltages in field winding. The discharge resister shall be designed such that the voltage induced in field winding should not exceed 1 KV.

Auto channel – with closed loop voltage regulation.Manual channel – current regulation.

Auto channel

Range of voltage regulation 90 – 110% En

Manual regulation

Range of regulation of generator field current – 20 – 110%

Application of Positive ceiling voltage – 150% of rated voltage. Negative ceiling voltage

Limiters Setting range Stator current limiter 100 – 130% of stator current Rotor current limiter 100 – 130% of rotor current Rotor angle limiter 60-120

Series compounding

Series compound transformer and diode bridge is required to provide excitation to generator field winding to maintain 2.0 per stator current during short circuit of Gen. Terminals. This is required in the AVRs of static excitation systems. Refer figure-5.4