-I will talk today with you about an important equipment used in power system utilities, it is the Automatic Voltage Regulator (AVR). From its name it is a regulator which regulates the output voltage at a nominal constant voltage level.
Role of AVR
AVR (Automatic voltage regulator) has following roles.
1- To regulate generator terminal voltage.
Mainly generator under no-load condition, AVR regulates the generator voltage to voltage setter (90R).
*AVR detects terminal voltage and compare with voltage setter (90R).
*AVR regulates field current via the Exciter.
*Generator terminal voltage is regulated by field current.
Vt < 90R _ Field current will be increase
Vt > 90R _ Field current will be decrease
2-To adjust MVars (Reactive power).
When the generator connected to power grid, AVR adjust reactive power by regulate generator voltage.
MVar (Reactive power: Q) is regulated by generator terminal voltage. Therefore AVR can regulate MVars.
Vt is increased _ MVars will be increase
Vt is decreased _ MVars will be decrease
Hence;
To increase MVars _ 90R raise
To decrease MVars _ 90R lower
3-To improve the power system stability.
There are two stability
-Transient stability …… Improved by AVR
-Dynamic stability ……. Improved by PSS (power system stabilizer)
* Improve the Transient stability
Transient stability is improved by high initial response characteristic. In the fault condition, Field voltage is increased to keep the generator voltage constantly. If the excitation response is slow, it will not able to keep voltage and the generator cannot keep synchronizing.
* Improve the Dynamic state stability
Dynamic stability is improved by Power System Stabilizer (PSS). PSS is provided in order to improve the power system dynamic stability. PSS will control the excitation to reduce the power swing rapidly.
4-To suppress the over-voltage on load rejection.
When the load rejection, field current and field voltage should be reduced rapidly to keep terminal voltage constantly and prevent overvoltage.
This information source is MELCO CO. for CEPC generator
