Static VAR compensator

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 AC transmission system (FACTS) family of devices.

SVCs are used both on bulk power transmission circuits to regulate voltage and contribute to steady-state stability; they also are useful when placed near high and rapidly varying loads, such as arc furnaces, where they can smooth flicker voltage.

The SVC is an automated impedance matching device. If the power system's reactive load is capacitive (leading), the SVC will use reactors to consume VARs from the system, bringing the system closer to unity power factor and lowering the system voltage. A similar process is carried out with an inductive (lagging) condition and capacitor banks, thus providing a power factor closer to unity and, consequently, a higher system voltage.

The term "static" refers to the fact that the SVC has no moving parts other than circuit breakers and disconnects. Traditionally, power factor correction has been done with synchronous condensers, large synchronous motors whose excitation determines whether they absorb or supply reactive power to the system.

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). 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), disc-shaped semiconductors several inches in diameter. The thyristors, usually located indoors in a "valve house," can switch capacitors or inductors in and out of the circuit on a per-cycle basis, allowing for very rapid fine control of system voltage.

Principle

Typically, a 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. The thyristors are electronically controlled. Thyristors, like all semiconductors, generate heat, and deionized water is commonly used to cool them.

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 export MVArs to the power system.

More complex arrangements including banks of thyristor-switched reactors and thyristor-switched capacitors are practical where more precise regulation is required.

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.

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 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 compensators (condensers).

Similar devices include the STATCOM and UPFC.