Protecting distribution feeders for simultaneous faults

Overhead distribution systems may experience faults involving more than one feeder. During simultaneous faults, the transformer low-voltage-side overcurrent relay measures a current greater than the current measured by faulted feeder relays. Therefore, the transformer relay may trip faster than faulted feeder relays. Transformer relay misoperation affects service availability in circuits not involved with the fault. In this paper, we describe the causes of simultaneous faults on distribution feeders and discuss overcurrent protection coordination problems caused by these faults. We then propose low-cost protection schemes using multifunction relays with communications and logic programming abilities. We summarize the operation experience of 19 simultaneous fault protection schemes installed in several substations of two Comisio¿n Federal de Electricidad (CFE) distribution divisions in Mexico. Finally, we analyze the operation of a protection scheme for an actual simultaneous fault.