Pilot Protection System Failure - an Investigation

Protection system failures are sometimes hard to detect, especially in the case of electromechanical relays with no inherent self-testing or alarming. It often takes a misoperation of the protection system and the following investigation to discover the problem. At this point it is too late and the damage is already done. This paper examines an event on the Oncor Electric Delivery system in which a critical customer, normally served by four 138kV transmission lines, was left hanging on a single feed due to a fault and two relaying system failures. Subsequent investigation revealed that both relaying system failures had been present for extended periods of time. Recommendations for future changes are presented. More