Working and Application of Voltage Stabilizer (Automatic Voltage Regulator)

Automatic voltage regulator regulates the voltage of the grid, generator, transformer or feeder. When an electrical load draws current from the grid, generator, or transformer, a voltage drop occurs in the transmission line or cable. The transmission medium has a certain resistance and the IR voltage drop in the transmission line results in a lower voltage at the receiving end. The voltage drop in the line depends on the power factor of the load. Loads with lower power factor consume more current, resulting in more voltage drop.

It is essential to maintain the voltage according to the rated voltage of the electrical load. If the supply voltage is not within the rated voltage range, the performance of the electrical equipment will deteriorate. A voltage regulator is a device that controls the output voltage and maintains it within the set limits.

1.There are various methods used to regulate the voltage of AC and DC supply:

The voltage of feeders of equal length in a DC system is regulated by compound generators. However, if the feeder lengths are unequal, then the voltage is regulated by feeder boosters. Step-up transformers, inductive voltage regulators, shunt capacitors, automatic voltage regulators (AVRs), etc. regulate the AC supply voltage. After understanding the necessity of voltage regulators, we will discuss how it maintains the required voltage.

2.Working principle of voltage regulator:

The controller of the automatic voltage regulator compares the actual voltage with the set point and outputs an error signal that controls the exciter power supply. The output is taken from the generator output after voltage reduction through the voltage transformer. The silicon controlled rectifier (SCR) controls the output voltage of the exciter.

When the generated voltage is lower than the set point, the controller outputs an amplified error signal, which reduces the firing angle of the SCR, resulting in an increase in the exciter voltage. The increase in exciter voltage leads to an increase in field current and field flux. Therefore, the generator voltage also increases until it reaches the set point of the automatic voltage regulator. This occurs when the load on the generator increases.

If the generator is running at light load or no load, its voltage may exceed the set value. To solve this problem, the controller outputs an amplified error signal, which increases the firing angle of the SCR, resulting in a decrease in the exciter voltage. The decrease in exciter voltage leads to a decrease in field current and field flux. The decrease in field flux, in turn, reduces the output voltage of the generator.

In order to maintain a consistent voltage output, the excitation system of the AC generator must adjust according to sudden changes in load. This is where the automatic voltage regulator (AVR) comes in. The AVR controls the exciter field current and modifies the output voltage. However, it is important to note that during severe fluctuations, the AVR may not respond as quickly as needed.

A fast-reacting voltage regulator based on the overshoot principle is used to obtain a fast response. According to this principle, when the load increases, the excitation voltage also increases. However, before the voltage reaches the value corresponding to the increased excitation, the voltage regulator reduces the excitation to the appropriate value. This ensures that the voltage remains within the required range and prevents any damage to the system.

3.Application of automatic voltage regulator:

It regulates the system voltage to bring the machine closer to stable operation.

When multiple alternators are operated in parallel, they share the reactive load between them.

The automatic voltage regulator reduces the overvoltage caused by sudden load loss in the system.

In fault conditions, it enhances the excitation of the system to ensure that maximum synchronous power is available during fault clearing.