Deterministic High-Impedance Fault Detection and Phase Selection on Ungrounded Distribution Systems

Downed conductors, tree branches touching conductors, and failing insulators often cause high-impedance faults in overhead distribution systems. The fault currents of these faults are much smaller than detection thresholds of traditional ground fault detection devices, so reliable detection of these high-impedance faults is challenging. Although fault currents can be much smaller in ungrounded systems than fault currents in multi-grounded systems given similar fault conditions, fault detection for ungrounded systems is nevertheless easier. This paper contrasts the differences between high-impedance fault detections for ungrounded and multigrounded systems. The paper explains fault detection of ungrounded distribution systems and the issue of fault detection sensitivity. The paper also introduces a recent advance in faulted phase selection on these ungrounded systems and demonstrates this advance through a staged fault test example from a utility