What is an automatic voltage regulator?

The AVR would stabilize the voltage level in an electrical device so that a proper voltage level can be achieved. This device does the regulation of the voltage by using an electromechanical mechanism, or by using passive or active electronic components. This device can be used to regulate AC or DC voltage. The major purpose of a voltage regulator is to ensure the safety of electrical and electronic devices. The operating voltage in which the electrical and electronic devices would operate at good efficiency is called the nominal operating voltage. If there is a variation in the nominal voltage level then it would affect the performance of the devices. So the AVR would keep the voltage level at the nominal operating range.

What are the major functions of a voltage regulator?

  • It would help to achieve the reliable operation of the power system
  • The voltage at the terminals of all the equipment of the system would be in the acceptable limits
  • The AVR can regulate the generator terminal voltage, the terminal voltage will be detected by the AVR, and then it would be compared with the setter.
  • It would adjust the reactive power by doing the voltage regulation in the generator
  • The stability of the power system will be improved, transient stability and dynamic stability can be achieved with the help of AVR
  • It can suppress the overvoltage on the load rejection
  • It would support controlling the frequency for large interconnections
  • Shut down of overexcitation
  • AVR would maintain the generator’s output power
  • It would regulate the power factor

What is voltage regulation?

The output voltage of a power supply would change according to the load and it would also vary according to the input. There are two types of voltage regulation and they are

Load regulation

There would be a change in the output voltage in case if there is a change in the load current and this must be regulated. The load regulation can also be described as the load voltage percentage change.

Line regulation

This is the input regulation, this is the percentage of the output voltage change with the input.

What is field forcing?

In case if the generator load is disconnected or thrown off due to some reason and this would cause a sudden increase in the voltage level. So the AVR would detect this increase in the voltage level, so this problem must be solved quickly and the AVR operation is not quick enough for this and if this fault is not cleared quickly then the generator could become unstable. So in order to decrease the excitation voltage very quickly, a negative voltage can be applied to the DC voltage.

If the generator operates in light load and if an additional load is applied to the generator then there would be a quick drop of the generator’s terminal voltage, so during this situation, our major choice is to apply voltage in addition to the existing field voltage.

What are the technical requirements for an AVR?

  • It should be able to apply the negative sealing voltage across the generator field winding for a quick de-excitation of the generator
  • It must be able to apply positive ceiling voltage across the generator field winding for a quick increase in the terminal voltage of the generator
  • It should be able to do the transformer drop compensation
  • It should provide the stator and rotor current limiters in order to utilize the capability of the generator properly
  • Drop characteristic for reactive load showing among the generators operating in parallel

What are the factors that must be considered while selecting a voltage regulator?

  • Band center
  • Bandwidth
  • Time delay
  • Line drop compensation
  • Operational modes
  • Over and under-voltage limits
  • Load capacity
  • Voltage correction
  • Voltage accuracy

Why should we use AVR in industrial plants?

The AVR would keep the generator voltage constant, by using an AVR in the generator the output of the generator will be stable and it won’t be affected by the variation in the load. This is the major requirement in most industries so there won’t be any interference in the production process. In an industrial power system, we should control reactive power. The automatic voltage regulator would keep the terminal voltage of the system at the required value by the feedback control.

What are the major components of an automatic voltage regulator?

The major components of the AVR are the two closed-loop control systems and both of them have separate gate control, thyristor set, and DC excitation equipment. The limiter is used to control the excitation.

What is an excitation system?

It is the process of strengthening the generator’s magnetic field by passing DC through the field winding.

How does an automatic voltage regulator operate?

As we discussed earlier the electrical or electronic equipment would operate at great efficiency at a particular voltage and this voltage is called the nominal voltage. So if the output of the generator is below the nominal voltage then the AVR would regulate the amplification current in the exciter, the current strengthening in the exciter would be increased by the AVR. If the output voltage of the generator exceeds the nominal voltage of the generator then the AVR would reduce the amplification current in the exciter. So by this, we can determine that if there is a change in the generator output voltage then the AVR would automatically detect and stabilize it. So thus the AVR would prevent the damage to the devices which would be caused by the voltage instability. The AVR would operate by using the power from a permanent magnet generator.

What are the types of automatic voltage regulators?

Single-channel AVR

In this type of AVR there would be two controllers and one of the controllers is automatic and the other one is manual and both these controllers utilize the same power supply. The circuit parameters would be determined by the AVR with the help of the current and the voltage transformer and a control pulse will be provided to start the control action and this pulse will be transferred to the circuit components. In order to change the firing angle, there is a gate controller and by this, we could control the field current for the excitation. In case if the automatic voltage regulator doesn’t operate then we can switch the control into manual mode.

Dual-channel AVR

In this type of AVR, there are also two controllers automatic and manual. But here the power supplies, gate control, and pulse amplifier unit for both the controllers are different. The reliability of the dual-channel AVR will be more than the single-channel, because if there is a fault in the supply or gate control unit then it would affect the whole system in the case of a single-channel AVR. In case if there are any of the above-mentioned problems in the dual-channel AVR then it can be prevented by switching to another channel.

Twin channel AVR

This is almost the same as the dual-channel AVR, the only difference is that in this type there are two automatic voltage regulators, instead of a manual and an automatic. The advantage of the dual-channel system is that if there is any fault in the automatic section then we can use the manual system. It can’t be done for the twin channel AVR.

What are the applications of voltage regulators?

  • Voltage control before synchronization during the no-load condition
  • It would control the reactive power output after synchronization
  • Isolated operation
  • Line charging


post-graduate in Electronics & communication.

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