Previous Year Question

Correct answer: C — Thyristor-Controlled Reactor

TCR construction and operation

A Thyristor-Controlled Reactor (TCR) contains an inductor connected in series with two thyristors in antiparallel. One thyristor conducts during one current half-cycle and the other conducts during the opposite half-cycle. Changing their firing angle changes the interval for which reactor current flows, thereby controlling its fundamental-frequency component.

With the firing-angle reference convention normally used for a TCR, α = 90° gives full reactor conduction. Increasing α towards 180° reduces conduction, and at 180° the reactor is effectively blocked. The reactor therefore behaves as a controllable inductive shunt element.

Role in reactive-power compensation

A TCR absorbs inductive reactive power. In a Static Var Compensator, it is commonly combined with capacitor branches. The capacitor branches provide capacitive VARs, while the TCR adjusts inductive VAR absorption so that the complete compensator can regulate bus voltage and reactive-power exchange.

The word reactor identifies the inductive element; thyristor-controlled identifies how its current is varied. Phase-angle control produces harmonics at intermediate conduction angles, so practical installations use suitable connections and filtering.