Real-Time Secured Third Zone Relay Operation Under Dynamic Stressed Conditions

This paper proposes a new philosophy for detecting and predicting system conditions that are capable of endangering the integrity of the power system. Dynamic stressed conditions often lead to cascading outages. Distance relays are quite vulnerable to maloperations during these stressed conditions. Under such circumstances, the apparent impedance seen by the relays might encroach into the third zone and can cause maloperations. Moreover, under such conditions, generators experiencing stable power swings might also move to an out of step condition in subsequent stages. The objective of this paper is to develop a technique that not only differentiates the stressed conditions from faulted ones but further identifies if the system is approaching into an out of step condition. It suggests a Decision Tree-based logic that prevents the third zone of distance relay from maloperating and assists the system engineer to initiate remedial action schemes or restrain any unintentional islanding. Simulations were performed for two systems: WSCC 9 bus system and IEEE 39 bus system using the Power Factory-DIgSILENT simulation software tool. The performance of the proposed scheme has been further validated using real-time simulation tool (Opal-RT 5600). The simulation and test results demonstrate the applicability of the scheme in maintaining the security of the system.