Unified Power Flow Controller (UPFC)
Unified Power Flow Controller (UPFC) is a combination of Static Synchronous Compensator (STATCOM) and Static Synchronous Series Compensator (SSSC) which are coupled via the DC link. The UPFC independently controls active and reactive power flows in the transmission line.
Fig. 1 shows the schematic diagram of the UPFC. The VSC-1 is connected in parallel with the transmission line (i.e., STATCOM) and the VSC-2 is connected in series with the line (i.e., SSSC) through an interface transformer. The DC link is interconnecting STATCOM and SSSC.
The VSC-2 controls the active and reactive power flows in the transmission line by injecting the AC voltage in series with the line which is similar to the SSSC operation. The VSC-1 supplies or absorbs the real power demanded by the VSC-2 at the DC link to exchange the active power. Also, the VSC-1 provides AC voltage regulation at the connected bus by providing independent shunt reactive power compensation which is similar to the STATCOM operation.
The main applications of the UPFC are active and reactive power control, voltage control, VAR compensation, dynamic and transient stability, and damping oscillations.
Fig. 1. Schematic diagram of UPFC.
References
Mathur, R. Mohan, and Rajiv K. Varma, Thyristor-based FACTS controllers for electrical transmission systems, 2002.
Narain G. Hingorani, Laszlo Gyugyi, “Understanding FACTS concepts and technology of flexible AC transmission systems”, 1999.