| 摘要 |
A protection relay for preventing an over-current caused by a harmonic wave is provided to protect a system according to a characteristic of the harmonic wave by calculating currents of R, S, T, and N phases. Current detection units(20) are connected to the R, S, T, and N phases passing through a circuit breaker(10) and detect currents of R, S, T, and N phases respectively. A harmonic wave analyzing unit(40) receives the currents of the R, S, T, and N phases and detects fundamental wave currents of the phases and harmonic wave currents by orders. An A/D conversion unit(50) converts signals of the fundamental wave currents of the phases and the harmonic wave currents by each of the orders into digital signals. A microprocessor(60) outputs a control signal to generate a trip signal or an alarm signal to trip the circuit breaker when an operated value exceeds a predetermined value. The operated value is obtained by giving weights in proportion to frequencies of corresponding fundamental waves and harmonic waves by each of the orders to the fundamental wave currents and the harmonic wave currents by each of the orders. An input and output unit(70) transmits the predetermined value to the microprocessor and outputs the control signal from the microprocessor. A trip output unit(80) outputs the trip signal to trip the circuit breaker according to the control signal of the microprocessor.
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