UPFC based on single-phase converters for voltage control

The conventional UPFC (Unified Power Flow Controller) is the most versatile equipment based on three-phase converters for controlling power flow in electric systems. This paper presents a UPFC with new and simple structures for voltage control (based on single-converters). It is shown that the voltage at a given point may be controlled in a simple way. In the first case, for each phase, one single-phase VSC (Voltage Source Converter) is connected in series with the system imposing the voltage, which has controlled magnitude and phase. This VSC is connected in “back-to-back” to another VSC, which is connected in parallel with the line. In the second case, the VSCs connected in series with the line are connected in back-to-back to three-phase VSC through a common DC link. Simulations were carried out using PSCAD/EMTDC to demonstrate that these UPFC configurations allow easy voltage control at a given point of the circuit using VSCs with smaller rating than using STATCOM.