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Protective Relay Frustrations

By Phill Feltham

Solving misoperation with line parameter measurements


Between 80 to 90 percent of all power system faults involve ground. Many protective relaying schemes depend on ground distance protection to accurately sense and locate ground faults on multi-terminal sub-transmission and transmission lines. In addition to the need of dependable ground fault detection, protective relay devices must provide adequate selectivity to avoid overtripping for faults outside of its zone of protection and other undesired consequences, such as undertripping or unintended automatic reclosing initiation.

The problem escalates due to recent major power system disturbances in North America, such as the Northeast blackout of 2003. Correct application and settings of protective devices, particularly distance relays, have become subject of heavy scrutiny lately.

Validation of accurate distance relay settings is now a major topic of discussion by electric utilities as well as professional technical committees such as the IEEE Power Systems Relaying Committee. It becomes apparent very quickly that the accuracy of line parameter values may affect many people.

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