A unified power flow controller (or UPFC) is an electrical device for providing fast acting reactive power compensation in high voltage electricity transmission networks. It uses a pair of controllable three-phase jumpers to produce current that is injected into a transmission line using a serial transformer. The controller can control active and reactive power flows on a transmission line.
The UPFC uses solid state devices, which provide functional flexibility, generally not achievable by conventional systems controlled by thyristors. The UPFC is a combination of a static static compensator (STATCOM) and a static synchronous series compensator (SSSC) coupled through a common DC voltage link.
The main advantage of the UPFC is the control of active and reactive power flows in the transmission line. If there is any disturbance or fault on the source side, the UPFC will not work. The UPFC operates only under a balanced sine wave source. The controllable parameters of the UPFC are reactance in line, phase angle and voltage. The UPFC concept was described in 1995 by L. Gyugyi of Westinghouse. The UPFC allows a secondary but important function, such as stability control, to suppress oscillations of the power system, improving the transient stability of the power system.