Evaluation of sympathetic nuisance tripping involving instantaneous ground fault relays and general considerations for ground fault protection of industrial power systems

The behavior of ground fault relays in large industrial power systems is generally perceived to be well-understood. In reality, the behavior of these relays can be influenced by more than the magnitude of 60 Hz ground fault induced short circuit currents, current transformer ratios, and relay tap settings. This paper addresses issues that can lead to sympathetic nuisance tripping of ground fault relays and other ground fault protection concerns. Sympathetic nuisance tripping occurs if a feeder with no conventional ground fault current source (generator or utility tie transformer) downstream from the feeder circuit breaker trips when a ground fault occurs elsewhere on the power system. The advantages and disadvantages of various types of ground fault sensing relay configurations are also discussed. Specific protection application considerations relating to high resistance grounding are presented.