Flexible AC Transmission System

A flexible alternating current transmission system, or FACTS, is a family of power electronic devices installed on an AC transmission system to improve and control power flow and provide dynamic voltage support, often serving as a cheaper or faster alternative to building new transmission lines or substations. FACTS devices work through shunt compensation of voltage, series compensation of line impedance, or phase angle compensation, replacing what capacitors, inductors or phase shifting transformers once did, and because their power electronics can switch within a fraction of an AC cycle rather than over seconds or minutes, they can respond to grid conditions far faster than older mechanical equipment. The technology grew out of thyristor based static VAR compensators developed in the mid twentieth century, generally recognized as the first FACTS devices, and later expanded to include STATCOM voltage source converters, thyristor controlled series capacitors, and unified power flow controllers that can control voltage, impedance and phase angle together.