Static VAr compensator
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A Static Var Compensator (or SVC) is an electrical device for providing fast-acting reactive power compensation on high-voltage electricity transmission networks. SVCs are part of the Flexible Alternating Current Transmission FACTS family of devices. The SVC provides a system automated impedance matching device. If the power system's Reactive load is too 'Capacitive', the SVC will use reactors to reactively balance the impedance. A similar procedure is done with a too 'inductive' load, thus providing a power factor closer to unity.
[edit] Principle
Typically an SVC comprises a bank of individually switched capacitors in conjunction with a thyristor-controlled air- or iron-cored reactor. By means of phase angle modulation switched by the thyristors, the reactor may be variably switched into the circuit, and so provide a continuously variable Mvar injection (or absorption) to the electrical network.
Coarse voltage control is provided by the capacitors; the thyristor-controlled reactor is to provide smooth control. Chopping the reactor into the circuit in this manner injects undesirable odd-order harmonics, and so banks of high-power filters are usually provided to smooth the waveform. Since the filters themselves are capacitive, they also contribute to the net Mvar injection.
Other arrangements such as a thyristor-switched reactor and thyristor-switched capacitors are also practical.
Voltage regulation is provided by means of a closed-loop controller. Remote supervisory control and manual adjustment of the voltage set-point are also common.
[edit] Advantages
The main advantage of SVCs over simple mechanically-switched compensation schemes is their near-instantaneous response to changes in the system voltage. For this reason they are often operated at close to their zero-point in order to maximise the Mvar reserves they can rapidly provide when required.
They are in general cheaper and require lower maintenance than dynamic compensation schemes such as synchronous compensators.
Similar devices include a STATCOM.
