Q6Power System 2

Working of STATCOM

A STATCOM (Static Synchronous Compensator) is a shunt-connected FACTS controller built around a voltage-source converter (VSC), which uses fast-switching power-electronic devices (typically IGBTs or GTO thyristors) to synthesize a controllable three-phase AC voltage waveform from a DC-side capacitor, connected to the power system bus through a coupling (step-up) transformer.

Reactive power compensation principle: the STATCOM operates by controlling the magnitude of its internally-generated AC output voltage (Vconv) relative to the system bus voltage (Vbus) to which it is connected, since the reactive power exchanged between the STATCOM and the system bus through the coupling transformer's reactance X is given approximately by Q = (Vbus*(Vbus-Vconv))/X. When the STATCOM's internally generated voltage Vconv is controlled to be higher in magnitude than the system bus voltage, the STATCOM supplies (injects) reactive power into the bus, behaving like a capacitor and helping to support/raise a sagging bus voltage; conversely, when Vconv is controlled to be lower than the bus voltage, the STATCOM absorbs reactive power from the bus, behaving like a reactor and helping to reduce an excessively high bus voltage — critically, this reactive power output is controlled purely by adjusting the converter's switching pattern (and hence its output voltage magnitude) at essentially electronic switching speed, without needing to physically switch discrete capacitor or reactor banks in or out, giving the STATCOM extremely fast, smooth, and continuously variable reactive power compensation capability.

Voltage source converter operation: the VSC uses its array of fast-switching semiconductor devices, controlled via a pulse-width-modulation (PWM) or similar switching strategy, to synthesize the desired AC output voltage waveform from the DC-side capacitor's voltage, with the DC capacitor itself continuously maintained at its required operating voltage by drawing a small amount of real power from the AC system (just enough to cover the converter's own internal switching and conduction losses), since the STATCOM in its basic reactive-compensation role does not otherwise exchange real power with the system.

Advantages over conventional (thyristor-switched or fixed) compensation: compared to conventional shunt capacitor banks, switched reactors, or even thyristor-controlled reactor (TCR)/thyristor-switched capacitor (TSC) type static VAR compensators, a STATCOM provides notably faster dynamic response (able to respond within a fraction of a power-frequency cycle), smoother, more continuous reactive power control without the discrete switching steps of capacitor-bank-based schemes, and importantly, a STATCOM's maximum reactive current output capability is largely independent of the system voltage level (unlike a fixed capacitor bank, whose reactive output falls off with the square of a sagging system voltage, precisely when reactive support is most urgently needed), making the STATCOM particularly effective at providing dynamic voltage support during and immediately following system disturbances, and a widely adopted modern FACTS solution for voltage stability enhancement, flicker mitigation, and dynamic reactive power compensation in transmission and distribution networks.