Digital protection of power transmission lines in the presence of series connected FACTS devices

The presence of series connected FACTS (flexible AC transmission system) devices like TCSC (thyristor controlled series capacitor), TCPST (thyristor controlled phase shifting transformer) and UPFC (unified power flow controller) etc. can drastically effect the performance of a distance relay in a two-terminal system connected by a double-circuit transmission line. The control characteristics of the series connected FACTS devices, their locations on the transmission line, the fault resistance especially the higher ones make this problem more severe and complicated. The fault location with respect to the position of the FACTS devices also greatly influences the trip boundaries of the distance relay. The paper presents apparent impedance calculations for relaying of double-circuit transmission system with varying parameters of the FACTS devices and location. The study reveals the adaptive nature of the protection scheme that necessities the use of an ANN based procedure for the generation of trip boundaries during fault conditions.