Thyristor-controlled reactor

In an electric power transmission system, a thyristor-controlled reactor (TCR) is a reactance connected in series with a bidirectional thyristor valve.

The thyristor valve is phase-controlled, which allows the value of delivered reactive power to be adjusted to meet varying system conditions.

Thyristor-controlled reactors can be used for limiting voltage rises on lightly loaded transmission lines.

A thyristor controlled reactor is usually a three-phase assembly, normally connected in a delta arrangement to provide partial cancellation of harmonics.

This protects the vulnerable thyristor valve from damage due to flashovers, lightning strikes etc.

The current in the TCR is varied from maximum (determined by the connection voltage and the inductance of the reactor) to almost zero by varying the "Firing Delay Angle", α. α is defined as the delay angle from the point at which the voltage becomes positive to the point at which the thyristor valve is turned on and current starts to flow.

Where: Vsvc is the rms value of the line-to-line busbar voltage to which the SVC is connected Ltcr is the total TCR inductance per phase The current lags 90° behind the voltage in accordance with classical AC circuit theory.

As α increases above 90°, up to a maximum of 180°, the current decreases and becomes discontinuous and non-sinusoidal.

The inverse-parallel connection is needed because most commercially available thyristors can conduct current in only one direction.

By connecting the TCR in delta, the harmonic currents of order 3n ("triplen harmonics") flow only around the delta and do not escape into the connected AC system.

must be filtered in order to prevent excessive voltage distortion on the AC network.