page_title: Thyristor-controlled reactor
section_title: Thyristor valve
hierarchical_section_title: Thyristor-controlled reactor / Main equipment / Thyristor valve
caption_attribution_description: English: Voltage and current waveforms for a Thyristor Valve for a Thyristor Controlled Reactor (TCR)
context_page_description: In an electric power transmission system, a thyristor-controlled reactor 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. Another device which used to be used for this purpose is a magnetically controlled reactor, a type of magnetic amplifier otherwise known as a transductor.
  In parallel with series connected reactance and thyristor valve, there may also be a capacitor bank, which may be permanently connected or which may use mechanical or thyristor switching. The combination is called a static VAR compensator.
context_section_description: The thyristor valve typically consists of 5-20 inverse-parallel-connected pairs of Thyristors connected in series. The inverse-parallel connection is needed because most commercially available thyristors can conduct current in only one direction. The series connection is needed because the maximum voltage rating of commercially available thyristors (up to approximately 8.5 kV) is insufficient for the voltage at which the TCR is connected. For some low-voltage applications, it may be possible to avoid the series-connection of thyristors; in such cases the thyristor valve is simply an inverse-parallel connection of two thyristors.
  In addition to the thyristors themselves, each inverse-parallel pair of thyristors has a Resistor - Capacitor circuit connected across it, to force the voltage across the valve to divide uniformly amongst the thyristors and to damp the "commutation overshoot" which occurs when the valve turns off.
caption: TCR Thyristor valve voltage and current waveforms
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