Unified Power Flow Controller

A unified power flow controller, or UPFC, is an electrical device that provides fast-acting reactive power compensation on high-voltage electricity transmission networks, using a pair of three-phase controllable bridges to inject current into a line through a series transformer and thereby control both its active and reactive power flow. Described in 1995 by L. Gyugyi of Westinghouse, it is considered the most comprehensive of the third generation of flexible AC transmission system, or FACTS, devices, combining a static synchronous compensator and a static synchronous series compensator coupled through a shared DC voltage link. In steady state it regulates a line's active and reactive power flow to raise its transmission capacity; during a transient event it delivers fast reactive power compensation that supports voltage at its connection point, improves system stability and can damp power system oscillations. Its solid-state devices give it functional flexibility beyond conventional thyristor-controlled systems, though it depends on a balanced sine-wave source and cannot function through a disturbance on the source side.