What is a synchronous generator Automatic Voltage Regulator (AVR)?

Vili Matošević

Control and AVR, Synchronous Generator

February 2, 2022

What is a synchronous generator Automatic Voltage Regulator (AVR)? ​

An automatic voltage regulator (AVR) is an electronic device whose function is to automatically maintain a synchronous generator output voltage at a set value.

How does an AVR work? ​

It controls the output by sensing the voltage from the generator terminals and comparing it to a set reference. AVR regulates the field current by increasing or decreasing the field current concerning the measured voltage value at the generator terminals and the setpoint reference value.

Basic connections of AVR with synchronous generator ​

Every regulator or generator manufacturer has its recommendations on connecting an AVR to a generator.

The basic principle is given in the following figure.

 

Namely, the AVR needs the necessary power to power supply the generator excitation. Self-excitation is often used where power is used by the generator itself. Then the remanent voltage of the generator itself must be large enough to allow the electronic part of the AVR to work. The required remanent voltage for AVR is often 6V or 12V.

 

In some cases, an external source is used for the generator’s power supply.

 

AVR supply excitation winding with the rectified DC Voltage.

The AVR regulates the value of DC excitation current depending on the voltage value at the terminals. For the AVR to measure the voltage value at the terminals, measurement feedback is conducted from the generator terminals to the AVR.

 

Data to be checked when replacing the AVR

Sometimes, replacing an existing AVR with another standard type regulator is necessary. However, it is first required to read and coordinate the connection in more detail according to the instructions of the new AVR, the old AVR, and the synchronous generator.

Some elements that need to be checked and matched between the AVR and the generator are:

  • Sensing input
    • 1 phase or 3 -phase connection
    • Sensing input Voltage range. If not fit, install a transformer
    • Nominal frequency

  • Power output

    • output DC current
    • output DCvoltage
    • minimum field resistance
  • Power input
    • 1phase or 3 phase connection
    • Frequency
    • Power input voltage range. If not fit, install a transformer
  • Buil up voltage

How to setting up AVR in practice?

After checking that all parameters between the AVR and the generator are harmonized, and after checking the AVR connection according to the manufacturer’s instructions and the obligatory checking of the correctness of all protection measures, we can approach the AVR setting.

When adjusting, it is necessary to use a measuring instrument (voltmeter) and measure the voltage and frequency at the generator terminals. Then proceed to the AVR setting according to the manufacturer’s instructions.

 

The AVR setup basic procedure is similar to the following steps:

  • The generator rotates at a rated speed, and we should adjust the frequency with the speed of the drive machine.
  • Use the potentiometer to adjust the voltage reference on the AVR to adjust the reference voltage on the generator.
  • Adjust the U / f characteristic elbow with the underfrequency potentiometer according to the manufacturer’s instructions. Often the knee is tuned to 3 Hz below the rated frequency. In some cases, the AVR is already adjusted for underfrequency protection. You only need to use a jumper to determine if the rated frequency is 50Hz or 60Hz. Namely, the purpose of this protection is that when the speed of the drive machine is reduced and thus the AVR frequency, the excitation and voltage on the generator are automatically reduced. If we did not adjust it, a significantly higher excitation current would flow in the excitation and possibly thermally damage the excitation winding.
  • Use the stability potentiometer to adjust the dynamic response of the generator voltage to load on and off. For this setting, it is best to connect an oscilloscope and observe the transient voltage response at the generator terminals and the measured excitation current shape.

The AVR must be carried out by professional staff.

 

Recent articles

Data to be checked when replacing the AVR