Negative and zero sequence directional overcurrent unit for ac power

Abstract

A protective relay for detecting faults within a predetermined protected zone of a three phase alternating current electrical power distribution system, of the type which utilizes at least one restraint signal in conjunction with at least one operate signal to generate a net operate signal, generates an operate quantity which is related to a zero sequence component of current flowing in the power distribution system and a second operate quantity which is related to a negative sequence component of the current flowing in the power distribution system. The operate quantities are summed to form an operate signal. An adaptive restraint, which is proportional to a negative sequence component and a zero sequence component of a phase voltage of the power distribution system, is generated and utilized to provide increased sensitivity to higher resistance ground faults. A portion of a positive sequence component of the current flowing in the power distribution signal is utilized as a restraint signal to prevent the unit from operating on negative sequence currents present during normal load conditions. Adaptive restraint enables the directional overcurrent unit to operate correctly should a second fault occur on the transmission line during the period that one pole is open.