| 摘要 |
A distance relay, for indicating single phase to ground faults on a polyphase electric power transmission system, having guard zone comparators (17,18,19) generating a first guard zone quadrilateral characteristic (21,23,25,27) in respect of each phase of the system and additional comparators (43,45,47) cooperating with the guard zone comparators (17,18,19) for generating a second main quadrilateral characteristic (23,25,27,49) in respect of each phase of the system, and a logic circuit arrangement (31 to 41) whereby the relay indicates the presence of a fault on a single phase to ground within the reach of the main characteristic relating to the relevant phase only if the corresponding guard zone characteristic, and no other, indicates a fault. The slope of the reactance line (21) of each guard zone characteristic is arranged to change its slope under two-phase phase to earth fault conditions by a lesser amount than the reactance line (49) of the corresponding main characteristic. By using the comparators false operation of the relay due to changes in slope of the reactance lines of the characteristics under two-phase to ground fault conditions is avoided.
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