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 multigrounded 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. Copyright © SEL 2005 (All rights reserved).
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