What is Automatic Voltage Regulator ?

What is Automatic Voltage Regulator (AVR)?

An automatic voltage regulator is used to regulate the fluctuating voltage. Voltage stability is very important to note because it can affect the electrical system. In a large-scale interconnection system, a manual voltage stabilizer is never used and instead an automatic voltage stabilizer is installed called an AVR (Automatic Voltage Regulator) in each generator.

The function of AVR is as follows :

  1. The AVR compares the Generator terminal voltage with a preset reference voltage.
  2.  If the Generator terminal voltage is less than the reference voltage, the AVR increases D.C. voltage across the Generator field

Working principle of the Automatic voltage regulator:


The working principle of AVR is to regulate the excitement flow in the exciter. If the generator output voltage is below the nominal voltage of the generator voltage, the AVR will increase the excitement current in the exciter. And vice versa if the generator output voltage exceeds the nominal voltage of the generator then AVR will reduce the exciting current in the exciter. Thus if there is a change in the generator output voltage will be stabilized by the AVR automatically because it is equipped with equipment such as a device used for limiting the minimum or maximum amplifier that works automatically.

AVR is operated by obtaining a power supply from a permanent magnet generator (PMG) for example AVR with a voltage of 110V, 20A, 400Hz. And get sensors from potential transformers (PT) and current transformers (CT).

Sensing circuit:

The three-phase generator voltage is supplied to the sensing circuit passing PT and 90R first, and the three-phase output voltage of 90R is lowered then transmitted by diode circuit, and levelled by capacitor circuit and resistor and this voltage can be adjusted with VR (Variable Resistant). The advantage of the sensing circuit is to have a rapid response to the generator output voltage.

Comparative amplifier:

The comparative amplifier circuit is used as a comparison between the sensing circuit and the set voltage. Large sensing voltage with a set voltage does not have the same value so that the difference / large voltage range. The voltage difference is called error voltage.

Technical requirement of AVR:

  • The possibility of applying the negative sealing voltage across generator field winding for fast de-excitation of the Generator.
  • The possibility of applying a positive ceiling voltage across generator field winding for fast increase in Generator terminal voltage.
  • Transformer drop compensation.
  • Provision stator current and rotor current limiters for optimum utilization of Generator capability.
  • Drop characteristic for reactive load showing among the Generators operating in parallel.
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