Disclosed is a negative-sequence time-overcurrentrelay for use in a polyphase synchronous rotary electricmachine having a main-circuit breaker and an excitingcircuit. The negative-sequence time-overcurrent relaycomprises first extracting unit for extracting afundamental-wave component and a harmonic component froma predetermined electric parameter for each phase givento the polyphase synchronous rotary electric machine,second extracting unit for extracting a fundamental-wavenegative sequence current for each phase from afundamental wave for each phase extracted from the firstextracting unit, first calculating unit for squaring thefundamental-wave negative sequence current for eachphase, extracted by the second extracting unit, for eachphase, second calculating unit for squaring the harmoniccomponent for each phase, extracted by the firstextracting unit, for each phase, first adding unit foradding phase-by-phase operational outputs from thesecond calculating unit, multiplying unit formultiplying an output from the first adding unit by apredetermined constant, second adding unit for adding aphase-by-phase output from the second calculating unitto an output from the multiplying unit, and determiningunit for determining that the polyphase synchronousrotary electric machine is protected when a sum of addedvalues for individual phases obtained from the secondadding unit exceeds a predetermined value.
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