Automatic Voltage Regulator

The automatic voltage regulator is used to regulate the voltage. It takes the fluctuate voltage and changes them into a constant voltage. The fluctuation in the voltage mainly occurs due to the variation in load on the supply system. The variation in voltage damages the equipment of the power system. The variation in the voltage can be controlled by installing the voltage control equipment at several places likes near the transformers, generator, feeders, etc., The voltage regulator is provided in more than one point in the power system for controlling the voltage variations.

In DC supply system the voltage can be controlled by using over compound generators in case of feeders of equal length, but in the case of feeders of different lengths the voltage at the end of each feeder is kept constant using feeder booster. In AC system the voltage can be controlled by using the various methods likes booster transformers, induction regulators, shunt condensers, etc.,

Working Principle of Voltage Regulator

It works on the principle of detection of errors. The output voltage of an AC generator obtained through a potential transformer and then it is rectified, filtered and compared with a reference. The difference between the actual voltage and the reference voltage is known as the error voltage. This error voltage is amplified by an amplifier and then supplied to the main exciter or pilot exciter.


Thus, the amplified error signals control the excitation of the main or pilot exciter through a buck or a boost action (i.e. controls the fluctuation of the voltage). Exciter output control leads to the controls of the main alternator terminal voltage.

Application of the Automatic Voltage Regulator

The main functions of an AVR are as follows.

  1. It controls the voltage of the system and has the operation of the machine nearer to the steady state stability.
  2. It divides the reactive load between the alternators operating in parallel.
  3. The automatic voltage regulators reduce the overvoltages which occur because of the sudden loss of load on the system.
  4. It increases the excitation of the system under fault conditions so that the maximum synchronising power exists at the time of clearance of the fault.

When there is a sudden change in load in the alternator, there should be a change in the excitation system to provide the same voltage under the new load condition. This can be done by the help of the automatic voltage regulator. The automatic voltage regulator equipment operates in the exciter field and changes the exciter output voltage, and the field current. During the violent fluctuation, the ARV does not give a quick response.

For getting the quick response, the quick acting voltage regulators based on the overshooting the mark principle are used. In overshoot mark principle, when the load increase the excitation of the system also increase. Before the voltage increase to the value corresponding to the increased excitation, the regulator reduces the excitation of the proper value.

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