What can we learn from a failed transformer
In the spring of 2016, a fault occurred in a large single phase auto-transformer on the transmission system of a major US electric utility. The fault was cleared in a time that met standard operating speeds of both relaying and the circuit breaker but a tank failure occurred that caused the spill of a large amount of oil. Significant damage to the core and coils of the transformer also occurred. As a part of ongoing efforts to improve the performance of the overall protection system it was decided to replay the transformer fault into a relay with a different operating algorithm than the two that were used to protect the transformer. Results showed that the different relay operated from 8 - 20 ms faster than either of the two installed relays. Analysis of the fault energy shows that with that faster speed there would have been 15 - 30% less energy dissapated into the fault. Inspection of the ruptured tank and the physical damage to the transformer suggest that with that reduction in energy there may have been significantly less damage, resulting in lower cleanup and repair costs. The paper discusses the fault, the playback, and the operating principles of the relay under investigation. Transformer damage and standards are discussed. The relationship between speed and security for different operating principles is evaluated and economic and engineering tradeoffs are considered to assist engineers in making protection decisions.