Negative-sequence time-overcurrent relay

Abstract

Disclosed is a negative-sequence time-overcurrent relay for use in a polyphase synchronous rotary electric machine having a main-circuit breaker and an exciting circuit. The negative-sequence time-overcurrent relay has a first unit for extracting a fundamental-wave component and a harmonic component from a predetermined electric parameter for each phase given to the polyphase synchronous rotary electric machine, a second unit for extracting a fundamental-wave negative sequence current for each phase from a fundamental wave, a first calculating unit for squaring the fundamental-wave negative sequence current for each phase, a second calculating unit for squaring the harmonic component for each phase, a first adding unit for adding phase-by-phase operational outputs from the second calculating unit, a multiplying unit for multiplying an output from the first adding unit by a predetermined constant, a second adding unit for adding a phase-by-phase output from the first second calculating unit to an output from the multiplying unit, and a determining unit for determining that the polyphase synchronous rotary electric machine is protected when a sum of added values for individual phases obtained from the second adding unit exceeds a predetermined value.