Automatic Voltage Regulator (AVR) - Introduction, working


#1

What is AVR?

Automatic Voltage Regulator (AVR) is an automatic control device that is used to keep the terminal generator voltage constant. The Generator terminal voltage is fed to the AVR which compares it with a reference value and an error signal is generated. The function of AVR is as follows:

  1. The AVR compares the Generator terminal voltage with a preset reference voltage. How this reference voltage is obtained.

  2. If the Generator terminal voltage is less than the reference voltage, the AVR increases D.C. voltage across the Generator field.

There are two conditions occurred in regulating the voltage, which is listed below:

Under Excitation:

Under excitation condition is a condition of the generator whose gain is less (less reinforced) so that it will supply the current to the system. This can be reviewed according to the GGM / flux of the anchor reaction. When the generator is less amplified, it must provide an overdue current to the system because the overdue current will produce an anchor flux that will strengthen the rotor flux.

Over Excitation:

The condition of overexcitation is a condition of the generator whose gain is too large (too amplified) so that it will supply a lagging current to the system. When the generator is too amplified, it must provide a lagging current to the system because the lagging current will produce an anchor flux that will counteract the rotor flux to reduce reinforcement that is too large.

AVR working Model:

An AVR works by involving several parts of a generator. A simple model of an AVR on a generator whose amplification system uses a shunt type DC generator is displayed as shown below:

AVR on the generator serves to regulate, control and monitor the voltage that comes out of the main stator based on the principle of feedback.

The operating system of the AVR Unit (Automatic Voltage Regulator) functions to keep the generator voltage constant in other words the generator will still issue a voltage that is always stable not affected by changes in load that are always changing, because the load greatly affects the generator output voltage.

The working principle of the AVR is to regulate the strengthening current in the exciter. If the generator output voltage is below the nominal voltage of the generator, the AVR will increase the amplification of the exciter. And also vice versa if the output voltage of the generator exceeds the nominal voltage of the generator, the AVR will reduce the strengthening current in the exciter. Thus, if there is a change in voltage the output of the generator will be stabilized by the AVR automatically because it is equipped with equipment such as a device used for limiting minimum amplifier or maximum that works automatically.

Main parts of AVR:

Sensing circuit

The three-phase generator voltage is supplied to the sensing circuit passing PT and 90R first, and the three-phase output voltage of 90R is lowered then transmitted by diode circuit, and leveled by capacitor circuit and resistor and this voltage can be adjusted with VR (Variable Resistant). The advantage of the sensing circuit is to have a rapid response to the generator output voltage.

Comparative amplifier

The comparative amplifier circuit is used as a comparison between sensing circuits and set voltage. The amount of sensing voltage with set voltage does not have the same value so that the difference in the voltage range. The voltage difference is called an error voltage. This will be removed by installing VR (variable resistance) on set voltage and sensing voltage.

Automatic manual change over and mixer circuit

This circuit is arranged in an auto-manual connection switch and a circuit to control the voltage amplifying the generator. The automatic change over and mixer circuit in manual operation of generator field gain voltage regulation, and when the automatic manual change over and mixer circuits operate manually, the AVR (automatic voltage regulator) cannot operate.

Limited circuit

Limited circuit is for determining the limitation of more and less reinforcement (excitation) for setting output voltage on the excitacy system.

Phase synchronizing circuit

The thyristor unit is used to control the output voltage of the thyristor by using a control signal given to the thyristor gate by changing the magnitude of the signal angle at the thyristor gate. The synchronization phase circuit serves to change the angle of the thyristor gate which corresponds to the output voltage of the synchronization limit and also the control signal given to the thyristor below has a synchronization image.

Thyristor firing circuit:

This circuit is a complement to the thyristor to provide control signals at the thyristor gate.

Dumping circuit:

The dumping circuit will give the sensor the magnitude of the voltage amplification of the AC excite.