Question

Describe how the AVR monitors out put and controls the excitation system

Sketch a circuit diagram for AVR and how AVR utilize a silicon controlled rectifier to control the excitation system for an alternator.

Figure a Static automatic voltage regulator {SA VR

Figure b. Error signal used to control thyristors in the excitation system

The direct current derived from the alternator output is applied to a bridge (Figure a) which has fixed resistances on two arms and variable resistances (zener diode voltage references) on the other two.

The zeners operate in the reverse breakdown mode, voltage of very low value.

As can be seen from the earlier description of zener diodes, voltage remains constant once breakdown has occurred despite change in current.

This implies, however, that changes in applied voltage, while not affecting voltage across the diode, will cause a change in resistance which permits change in current. As with a Wheatstone bridge, imbalance of the resistances changes the flow pattern and produces in the voltage measuring bridge an error signal.

The error signal can be amplified and used to control alternator excitation in a number of different ways. Thus it can control the firing angle of thyristors (Figure b) through a triggering circuit to give the desired voltage in the brushless alternator described. It can be used in the statically excited alternator to correct small errors through a magnetic amplifier arrangement. The error signal has also been amplified through transistors in series, for excitation control.

Sudden load current surges in a generator causes a voltage dip. Load shedding produces an overvoltage at bus bar. Such fluctuation is undesirable and AVR is required to rapidly correct such voltage changes. The AVR senses the generator out put voltage and acts to alter the field current to maintain voltage at the set value. Manual trimmer regulator may be fitted on the generator control panel to set voltage level.

The voltage sensing unit transforms down, rectifies, and smooth the generator output voltage

This produces a low voltage DC signal proportionate to AC generator voltage. In comparator unit it is compared with the set value [dc value produced by reference unit of zener diodes and reactance]

The correction is then amplified and through thyristor control is used to alter the alternator field current in order to reach the set voltage value.

Additional component and sub circuit are included in the AVR to ensure.

Rapid response time with voltage stability

Fair current sharing, when generator are running in parallel

Quick voltage build up during generator run up

Over under voltage alarm trip protection.