Static VAR Compensator (SVC)

A static VAR compensator is a shunt-connected device that regulates voltage on transmission and distribution networks by rapidly adjusting the reactive power it supplies or absorbs. It combines capacitor banks and reactors that are switched or controlled by thyristors, so its control path has no moving parts. SVCs belong to the family of flexible AC transmission systems (FACTS) and have been in commercial service since the 1970s.

The usual building blocks are thyristor-switched capacitors, which add capacitive support in discrete steps, and thyristor-controlled reactors, which absorb reactive power continuously as the thyristor firing angle changes. Harmonic filters tuned to the reactor's characteristic harmonics supply part of the capacitive output. A voltage regulator measures the bus voltage and adjusts the combined susceptance within a few cycles to hold it at the set point.

Because an SVC behaves as a variable impedance, its reactive output falls with the square of the voltage. That is its main limitation compared with a STATCOM, which keeps supplying close to its rated current as voltage drops. In return, SVCs are well proven, cost-effective at high ratings and familiar to most transmission operators, which is why they remain common on long transmission corridors, at major load centers and next to fluctuating industrial loads such as electric arc furnaces.

Planners now weigh SVCs against STATCOMs and synchronous condensers depending on what the network needs. Where the main issue is steady-state voltage control along a heavily loaded corridor, an SVC is often enough. Where the grid is weak, short-circuit levels are falling or fast recovery after faults is critical, operators increasingly choose STATCOMs or synchronous condensers, and many existing SVCs are being refurbished or replaced as their valves and controls reach end of life.