An Investigation of Two Concurrent Relaying System Failures

Protection system failures are sometimes hard to detect, especially in the case of electromechanical relays with no inherent self-testing or alarming. Many times it 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 TXU Electric Delivery system in which a critical customer, normally served by four 138 kV 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. Data was available that, if analyzed properly, would have detected the failures and thus avoided the entire situation. The event, relaying failures, contributing factors, and how the situation could have been avoided are discussed, and recommendations for future changes are presented