Active power based method to prevent zone-3 maloperation during voltage instability

Voltage instability is a system stressed phenomena during which distance relay reach issues arises. Zone-3 unit of distance relay is highly affected by such phenomena. Protective relay unable to discriminate the fault situation from voltage unstable condition and issues a trip signal. As, three phase fault and voltage instability are balanced phenomena, zone-3 unit is unable to make a clear discrimination between them. In this work, a three phase active power based approach is proposed to solve the above problem. The rate of change of active power is used as a paradigm to detect the fault and then the magnitude of 5 period average rate of change of active power (ARCAP) is computed to discriminate the three phase fault from voltage instability. It is observed that during three phase fault magnitude of ARCAP is more than the threshold setting, while less than that during voltage stress conditions. The main advantages of the proposed technique are that it relies on local end information and within half power cycle time period fault detection is possible. The system is simulated on EMTDC/PSCAD environment and results are presented to highlight the effectiveness of the proposed technique.