D24 Protection Guide - Ground Overcurrent Relaying
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PROTECTION GUIDES ESO/System Protection & Analysis June 2006 Guide # 01 ---------------------------------------------------------------- Guide Topic: Ground Overcurrent Relays Settings Author: GLK ________________ __________________ _________________ ____________________________ PAGE 1 of 13 PROTECTION Guides are papers to provide guidance in setting protective relays. They are produced by System Protection & Analysis. A related set of documents are the PROTECTION NOTES, technical information papers on a variety of power system and protection topics. Approval Signature: Russell Patterson Approval Date: 9/27/07 Revision Level: R4 Reference: C37.112-1996 IEEE Standard Inverse-Time Characteristic Equations for Overcurrent Relays This guide addresses general issues to be considered when setting the pickup and time lever for ground overcurrent relays and for setting instantaneous overcurrent elements. This should not be considered a “cookbook” procedure and cannot cover every possible transmission line and system configuration, and should not be used as a substitute for thorough system knowledge and experience, but simply covers certain general guidelines. Topic link list: Ground overcurrents in looped system Ground overcurrents in radial system Ground overcurrents in looped system These guidelines are applicable to the networked (looped or non-radial) transmission system at any voltage level. When setting a ground time-overcurrent relay, the following items should be considered: 1. The time-current characteristic should be chosen as very inverse. 2. The relay pickup should be set to pickup for: • A remote bolted bus fault with all-ties-closed with 1000% sensitivity (e.g., remote bus fault ATC draws 1000 A, set ground overcurrent relay pickup on 100 A). Note that lower pickups that provide higher sensitivity may result in miscoordination with other ground overcurrent relays connected to the remote bus. This is due to the very inverse time-current characteristic. (See example below) • A remote line-end open fault with 40 ohms of fault resistance. The pickup should be at least half the fault current under this condition (200% sensitivity). • The pickup should be no less than 10% of the maximum line capability, to avoid operating on unbalance.
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Protection Guide
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