Negative-sequence time-overcurrent relay

A negative-sequence time-overcurrent relay for use in a polyphase synchronous rotary electric machine having a main-circuit breaker and an exciting circuit, said relay comprising:

• first extracting means for extracting a fundamental-wave component and a harmonic component from a predetermined electric parameter for each phase given to said polyphase synchronous rotary electric machine;
• second extracting means for extracting a fundamental-wave negative sequence current for each phase from a fundamental wave for each phase extracted from said first extracting means;
• first calculating means for squaring said fundamental-wave negative sequence current for each phase, extracted by said second extracting means, for each phase;
• second calculating means for squaring said harmonic component for each phase, extracted by said first extracting means, for each phase;
• first adding means for adding phase-by-phase operational outputs from said second calculating means;
• multiplying means for multiplying an output from said first adding means by a predetermined constant;
• second adding means for adding a phase-by-phase output from said first calculating means to an output from said multiplying means;
• and
• determining means for determining that said polyphase synchronous rotary electric machine should be protected when a sum of added values for individual phases obtained from said second adding means exceeds a predetermined value.