Thyristor

This picture is explain about how thyristor work. In power plant, the thyristor is using for control field current from exciter to generator.


The different thing between diode and thyrisor is the dioade only make the AC current to DC current, but the thyristor can control how big value of the current by set the voltage trigger as picture below.



The picture below tells how much current must be supply by thyristor to exciter. The thyristor have input from excitation transformer which read the voltage that should be maintain. If the voltage drop, the AVR will add more.



The picture below tells The digital AVR consists of EXC controllers, phase control unit, pulse amplifier and transducers.

The Excitation controllers perform the gate control on the Thyristor rectifier to regulate the excitation so that the generator voltage will match the voltage setting.

The primary control system for regulating the generator voltage consists mainly of a transducer unit for receiving analog quantities from PT and CT, and a master controller for performing the regulating control.

The controller is duplicated as EXC controller - A and B so that a fault in the main system will result in an automatic switchover to a stand-by system.

The gate pulse of the standby system is locked at the output stage of the phase control unit (PHC) in the master controller, so that only the gate pulse of the main system is output to the Thyristor rectifier through the pulse amplifier.