Automatic Voltage Regulators

 Automatic Voltage Regulators 

Functions

✓To control system voltage within limits

✓To regulate the sharing of reactive load between machines operating is parallel.

✓To maintain voltage under system fault conditions to ensure rapid operation of protection

systems.

✓To keep the machine under synchronism.

Direct acting voltage regulator

✓ Adjustment of variable resistance

✓Voltage transformer to the operating coil thus the torque to operate the drum

✓Movement of pivot (P,P) causes sectors (S) to move

✓Clockwise movement increases the resistance

Magnetic Amplifier Regulator

✓ Controls the flow of current by means of saturation of magnetic core.

There are two windings:

1 Control/DC winding: controls the mmf o/p.

2 AC winding through which power is delivered to load.

✓The operation is based on reduction of impedance of the core as DC magnetization is increased.

✓When the DC is so high that full saturation of the core occurs, the impedance becomes minimum and load current is maximum.

Construction

1) Self excited main exciter

2) Motor operated field rheostat

3) Boost and buck control fields

4) Static components

5) Compensation

Operation

✓Deviation of the alternator terminal voltage

✓Difference between reference and compensated output

✓Amplified by input magnetic amplifier and power amplifier

✓High frequency AC supply ensures that delay in regulating system is negligible compared to  delay in exciter and alternator.

Solid State Electronic Regulator

1) The sensing circuit responds to average 3 phase voltage output connected from PT to sensing

element.

2) To regulate output, we need to sense voltage; for this we use potential transformer (step

down transformer), from CT through compensate to limit sensing for current measurement.

3) Compensator works as amplifier. It stabilizes CT/PT outputs because output is always

fluctuating.

4) Three phase voltage from PT is rectified, filtered and compared with a constant reference.

5) The output of the amplifier is the voltage error signal.

6) Single phase CT is used for reactive current compensations.

7) Minimum excitation limiter prevents the regulator from decreasing regulation below the set

characteristics value.

8) The signal mixer amplifier output controls the firing circuit of thyristor which supplies

excitation to the machine.

9) The power and control elements can be taken out for repair.

10) From voltage reference to voltage adjust.