Comparison of a Distance Relay Behavior in a Transmission System With SEN Transformer or UFPC

A "Sen" transformer uses transformer and tap changers to regulate the voltage at a point in the transmission line as a voltage regulator does and additionally provides an independent and bidirectional active and reactive power flow control in the transmission line as a voltage-sourced converter (VSC)-based unified power flow controller (UPFC) does. Sen Transformer provides an area of controllability in the P-Q plane similar to a UPFC. When a fault occurs on a transmission line, the fast acting controllers of UPFC operate and change the pre-fault operating point, leading to different apparent impedance seen by distance relay. "Sen" transformer response is not as fast as UPFC, but its supply voltages and currents are changed during a fault, causing the apparent impedance seen by distance relay to be varied accordingly. In this paper the performance of a distance relay in a transmission line with UPFC is compared with the performance of a distance relay in a line with "Sen" transformer. The simulation results are compared for different operating points and different "Sen" transformer taps.