UPFC concept was proposed by GyuGyi in 1991. The UPFC was devised for real time control and dynamic compensation of ac transmission systems. It provides multifunctional flexibility to solve many of the issues facing the power delivery industries. UPFC is able to control synchronic or individually all the parameters (i.e. voltage, phase angle, and impedance) affecting power flow in the power system network. Thus this unique capability is announced by the adjective “unified” .the main reason behind the wide spreads of UPFC are its ability to power flow bi-directionally maintaining well regulated DC voltage, workability in the wide range of operating conditions.
This is the second or latest generation of FACTS technology. This FACTs device combines the two features of two other FACTS devices STATCOM (static synchronous compensator) and SSSC (the static synchronous series compensator). Basically these devices are voltage source converters (VSC’s) .the UPFC is a generally synchronous voltage source (SVS). The SVS usually exchange both reactive and real power with the transmission system. Frankly speaking an SVS is able to generate only reactive power exchanged; the real power must be supplied to it, or absorbed from it by a suitable power supply or link.