| 摘要 |
PURPOSE:To detect surely even a one-phase ground-fault for a low frequency by allowing N-number of zero-phase overvoltage detectors different in set value to correspond to N-number of frequency divisions of a power system due to (N-1)- numbers of frequency detectors different in set value. CONSTITUTION:When electric power from a variable voltage and variable frequency power source 2 passing through an output transformer 3 is supplied to a load AC motor 4 of a variable voltage and variable frequency power system, a pilot generator 7 is interlocked with the motor 4. A ground-fault trouble decision device 11 decides a frequency division corresponding to the frequency of the power system out of three frequency divisions through two frequency detectors 9A and 9B different in set value. One of three zero-phase overvoltage detectors 10A-10C which are different in set value and are connected to an open tertiary winding 31 of the transformer to detect zero-phase overvoltages due to the power system ground-fault is selected in accordance with this frequency division. Consequently, different zero-phase overvoltages are detected with a high sensitivity in accordance with frequencies, and even the one-phase ground-fault of the power system for a low frequency is detected surely.
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