Why do we need an Automatic voltage regulator and what are it's functions

What is automatic Voltage Regulator (AVR)

The automatic voltage regulator can maintain the generator terminal voltage at a constant value. It is done by changing the excitation current to the generator field. The AVR gives the required DC supply to the generator field depending on the load, power factor, terminal voltage the input of the AVR is a 3-phase AC supply

Why do we need an AVR

  • It is needed to regulate the voltage of the power system at a constant value otherwise it will affect the performance of the machinery
  • If a new load is given to the AC generator the voltage drop will be more substantial so such voltage drop must be controlled
  • The voltage drop can be controlled by an AVR
  • The real purpose of an AVR is to deal with the voltage drop when an extra load is connected, it can restore the rated voltage without undue overshooting and regeneration
  • When an extra load is connected there would be two stages which follow it they are Transient stage and steady-state condition

Functions of an AVR

  • AVR can compare the generator voltage with the preset reference voltage
  • If the terminal voltage of the generator is less than the reference voltage the AVR increases the DC voltage across the generator field
  • In case if the generator terminal voltage is more than the reference voltage the AVR reduces the DC voltage across the generator field
  • If the generated is working in rated load and rated power factor, if the load is removed then the voltage will increase suddenly and the AVR can sense this increase in terminal voltage and the DC voltage will be increased across the generator field
  • In case of removing the load the AVR response time will take some seconds at this time the generator could be unstable and in case of this the excitation voltage must be reduced quickly and it could be done by applying a negative voltage to the already applied DC voltage
  • In case of adding additional load to the generator the terminal voltage will fall down, in this case, additional voltage is applied to the already existing field voltage

How does it work

There are three basic parts for an AVR they are

  • Error detection element
  • Correcting element
  • Stabilizing element

AVR can sense the DC output voltage and it can act to filter the field current to maintain the output voltage at the rated value

The voltage sensing unit rectifies and smoothes the generator output voltage and it produces DC signal proportional to the actual generator voltage

The signal is compared at the comparator with the set value the error signal which is from the comparator is used to control the excitation current through ERROR AMP and controller

There are two independent control system

  • Auto control
  • Manual control

Control is affected on the three-phase output of the pilot exciter and provides a variable DC input to the main exciter the voltage feedback and the control output of the generator is fed to the AVR in there it is compared with the set point generator volts