Real-time monitoring of zone 3 vulnerable distance relays to prevent maloperation under load encroachment condition

Undesired trip operation of zone 3 units of distance relays in power transmission network, during static and dynamic load encroachment can lead to cascading failures and even catastrophic power system blackouts. Stressed conditions such as power FLow redistribution and power swing can cause maloperation of the third zone of distance relays. Fast and dependable detection of the symmetrical faults, occurring during these conditions poses an additional challenge. One of the effective methods to avoid the zone 3 contribution in cascading failure propagation is the detection and monitoring of vulnerable relays which are exposed to maloperation in power system stressed conditions. In this study, a new relay ranking index is proposed for real-time identifying the zone 3 of distance relays, most vulnerable to incorrect operation due to load encroachment. A novel synchrophasor data-based monitoring index is also presented for supervision of critical distance relays operation. This index is extracted based on positive-sequence fault component phasors of the current and voltage signals obtained by modern PMUs or GPS based digital relays which have been reconfigured for this application. The proposed monitoring scheme is able to quickly distinguish the fault from load encroachment condition. The performance of the proposed supervisory scheme is evaluated and approved on a nine-bus WSCC system.