Backup Protection Scheme to Prevent Unintended Relay Operation During Voltage Stress and Load Encroachment

Third operating zone (zone-3) of distance relay is the primary cause behind the Indian power system blackout occurred in 2012. Due to the inherent settings of the third protective zone, it is severely exposed to system stress conditions such as voltage instability and load encroachment. The similar incident is also experienced when a three-phase fault occurs during system stress conditions. To address these issues and to limit propagation of cascaded events lead to a blackout, a supervisory technique based on superimposed positive sequence current (SPSC) is proposed in this paper. The proposed technique is activated only when the scope of third zone maloperation exists. The amplitude of SPSC is zero during any operating conditions of the power system and significantly high during a fault. This helps to detect a three-phase fault and discriminate it from system stress conditions. Simulations and performance of the proposed technique are carried out on the IEEE 39 bus test system by using EMTDC/PSCAD and MATLAB software.