Effect of static var compensator on the performance of digital distance relay protection of transmission line

This paper presents modelling of distance relay with static Var compensator (SVC) and investigates its impact on the apparent impedance seen by the transmission line distance relay. The functional structure for SVC built with a Thyristor Controlled Reactor (TCR) and its model are described. The model is based on representation of the controller as variable susceptance (BSVC) that changes with the firing angle of the TCR. SVC has a remarkable effect on the apparent impedance seen by the distance relay. The impact of SVC is more pronounced on the apparent impedance seen by the phase to ground fault. The mal-operation of the distance protection for the transmission line with SVC at various locations is studied. It is also shown that the primary winding connection of the interfacing transformer of the SVC has extraordinary effect on the apparent impedance seen by the relay. The simulation results show that the under-reaching and over-reaching is more severe with SVC at mid-point of the transmission line. The impact of SVC on the relay tripping boundaries is also clearly demonstrated.