A New Approach to Supervise Vulnerable Third Zone Relay Operation for Power Transmission System

Distance relay third zone mal-operation has been recognized as one of the vital reasons for cascading failures in power system networks. The investigation on past power blackouts revealed that the prime reason for such maloperation was erroneous distance relay logical operation during stressed situations, such as power swings, voltage instabilities. This work focuses on enhancing the performance of zone-3 protection without compromising with security of protection operation. A relay ranking index named relay vulnerability factor (RVF) is proposed, which measures the vulnerable relay ranking according to the severity of stressed conditions. The most vulnerable relay (MVR) set is monitored by using one of the well-known data mining tool named decision tree to separate faults during stressed conditions. Extensive studies are carried out on WSCC 9 bus power system network to investigate different scenarios where third zone of distance relay fails. Our proposed scheme is highly accurate in identifying fault during such abnormalities and substantially improves the reliability of distance relay operation.