Thyristor Controlled Reactor
Thyristor controlled reactor consist reactor L placed in series with the thyristor valve. This reactor is the controlled element of the TCR, and it controls the thyristor valve. TCR consists two opposite poled thyristor which conducts every alternate half cycles of the supply.
In TCR the duration of current flowing through the reactor is controlled by the firing angle of the thyristor. For every half cycle, the thyristor is given a triggering pulse by the controlled circuit. It is used in EHV (Extra High Voltage) lines for providing lagging VARs during the low load or load rejection.
Thyristor Switched Capacitor
The thyristor switched capacitor is used in EHV lines for providing leading VARs during heavy loads. The current through the capacitor can be varied by controlling the firing angles of back to back thyristor connected in series with the capacitor.
When the voltage at a bus reduces below the reference value, the static VAR compensator used TSC for injected capacitive volt-amperes and when the voltage at the bus rise above the reference value, inductive VAR are to be injected to lower the bus voltage by using TCR.