A static var compensator (or SVC) is an electrical device for providing fast-acting reactive power on high-voltage electricity transmission networks.[1][2] SVCs are part of the Flexible AC transmission system device family, regulating voltage and stabilising the system. The term "static" refers to the fact that the SVC has no moving parts (other than circuit breakers and disconnects, which do not move under normal SVC operation), unlike the Synchronous condenser, which is a rotating electrical machine. Prior to the invention of the SVC, power factor compensation was the preserve of large rotating machines such as synchronous condensers.[3]
The SVC is an automated impedance matching device, designed to bring the system closer to unity power factor. If the power system's reactive load is capacitive (leading), the SVC will use reactors (usually in the form of Thyristor-Controlled Reactors) to consume vars from the system, lowering the system voltage. Under inductive (lagging) conditions, the capacitor banks are automatically switched in, thus providing a higher system voltage. They also may be placed near high and rapidly varying loads, such as arc furnaces, where they can smooth flicker voltage.[1][4]
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Typically, an SVC comprises a bank of individually switched capacitors in conjunction with a thyristor-controlled air- or iron-core 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.[2] In this configuration, coarse voltage control is provided by the capacitors; the thyristor-controlled reactor is to provide smooth control. Smoother control and more flexibility can be provided with thyristor-controlled capacitor switching.[5]
The thyristors are electronically controlled. Thyristors, like all semiconductors, generate heat and deionized water is commonly used to cool them.[3] Chopping reactive load 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 export Mvars to the power system.
More complex arrangements are practical where precise voltage regulation is required. Voltage regulation is provided by means of a closed-loop controller.[5] Remote supervisory control and manual adjustment of the voltage set-point are also common.
Generally, static var compensation is not done at line voltage; a bank of transformers steps the transmission voltage (for example, 230 kV) down to a much lower level (for example, 9.5 kV).[3] This reduces the size and number of components needed in the SVC, although the conductors must be very large to handle the high currents associated with the lower voltage.
The dynamic nature of the SVC lies in the use of thyristors (also called valves). The disc-shaped semiconductors, usually several inches in diameter, are commonly located indoors in a "valve house".
The main advantage of SVCs over simple mechanically-switched compensation schemes is their near-instantaneous response to changes in the system voltage.[5] For this reason they are often operated at close to their zero-point in order to maximise the reactive power correction they can rapidly provide when required.
They are, in general, cheaper, higher-capacity, faster and more reliable than dynamic compensation schemes such as synchronous condensers.[5]
Similar devices include the static synchronous compensator (STATCOM) and Unified Power Flow Controller (UPFC).