In this post, we will discuss What is AVR (Automatic Voltage Regulator) its use and working principle.
What is AVR and AVR Panel
AVR, as the name indicates AVR is a device that is designed to automatically control, adjust or maintain a constant voltage level that means AVR is used to regulate or control the voltage level automatically.
Now, we will see where AVR is used and where AVR Panels are used.
AVRs are used in the LT Generators of rating up to 1 MW without synchronization with the grid.
AVR panels are used for
Synchronization with grid and
For the Generators of rating more than 1 MW or
For HT Generators
The main function of AVR Panels:
In this heading, it will be more clear what is AVR and its working.
Automatic Voltage regulator (AVR) works on a closed control loop principle. In a closed loop control system, a feedback is compared with a reference value and an error signal is generated.
AVR takes alternator/generator output voltage as feedback and compares this output to the set point and generates an error signal that is used to adjust the excitation of the alternator/generator and maintain terminal voltage constant from no load to full load.
This feedback voltage is different for different type of generators. For LT Generators feedback voltage is 415 volts and for HT Generators feedback voltage is 110 volts.
The AVR Panels operate on the following modes.
Manual mode or current feedback mode
PF or VAR mode.
The generator consists of rotor winding and stator winding. The output voltage is taken from the stator winding.
Operating principle of generator and AVR in Auto mode:
The turbine is coupled with the rotor of the generator when the turbine rotates at the rated RPM the rotor also rotates at its rated RPM. And AC exciter winding is also rotates at rated RPM. When a small DC voltage is provided from the AVR panel to the field of the exciter, as a result, the magnetic field induces surrounding the field and hence magnetic flux is generated.
The AC exciter cuts this magnetic flux hence AC voltage is induced in the exciter. This AC voltage is given to the rotating diode rectifier assembly and this three-phase AC voltage is converted to DC voltage.
This DC voltage is provided to the main field of the generator, by the result of this, the flux is induced surrounding the main field, and also the main field is rotating. So stator winding will cut this flux and terminal voltage will produce and this terminal voltage is given as a feedback to AVR panel. Once half cycle of AC supply is available at the terminal voltage, the external DC supply will be removed. AVR panel will convert AC supply from the Generator to the DC supply. this DC Supply will be supplied to the field of the exciter.
This terminal Voltage depends upon the load. So, on increasing or decreasing the load, terminal voltage will also change and the AVR panel will control the change in terminal voltage automatically.
The manual mode of operation or constant current mode:
As it is clear from the name itself, in this mode AVR will not control the terminal voltage automatically on the variation of the load. This mode of operation is used in the failure of auto mode or in the failure of sensing PT. This mode of operation is used while testing the generator. In this mode of operation the AVR senses the field current of the exciter, so if the field current varies AVR will correct only the field current not the terminal voltage. This mode of operation cannot be used for the continuous running operation.
PF / VAR Mode of operation:
When the supply is given to the grid, the power factor will vary according to the grid voltage variation. In this case, the auto power factor controller mode is used. Auto power factor controller mode will sense the power factor of the generator and will correct the excitation in order to maintain the constant power factor.
Trip points are also provided for AVR panels for the safe operation of the AVR panel and other electrical equipment.
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