NEGATIVE AND ZERO SEQUENCE DIRECTIONAL OVERCURRENTUNIT FOR AC POWER TRANSMISSION LINE PROTECTIONABSTRACT OF THE DISCLOSUREA protective relay for detecting faults within apredetermined protected zone of a three phase alternatingcurrent electrical power distribution system, of the type whichutilizes at least one restraint signal in conjunction with atleast one operate signal to generate a net operate signal,generates an operate quantity which is related to a zerosequence component of current flowing in the power distributionsystem and a second operate quantity which is related to anegative sequence component of the current flowing in the powerdistribution system. The operate quantities are summed to forman operate signal. An adaptive restraint, which is proportionalto a negative sequence component of a phase voltage of the powerdistribution system, is generated and utilized to provideincreased sensitivity to higher resistance ground faults. Aportion of a positive sequence component of the current flowingin the power distribution signal is utilized as a restraintsignal to prevent the unit from operating on negative sequencecurrents present during normal load conditions. Adaptiverestraint enables the directional overcurrent unit to operatecorrectly should a second fault occur on the transmission lineduring the period that one pole is open.
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