| 摘要 |
PURPOSE:To make a ground-fault detection sensitivity constant approximately independently of a ground-fault frequency by allowing overcurrent detectors different in set values to correspond to frequency divisions of plural frequency detectors different in set value. CONSTITUTION:Electric power is supplied from a variable voltage and variable frequency power supply device 2 to a load AC motor 4 of a variable voltage and variable frequency power system through an output transformer 3. Frequency divisions of the power system due to a pilot generator 8 interlocking with this motor 4 are detected by frequency detectors 10A and 10B different in set value. A ground-fault decision device 12 is connected to a neutral point-grounded resistance of the transformer 3 in accordance with detection results to select overcurrent detectors 11A-11C which detect zero-phase overcurrents due to ground-fault and are different in set value. Consequently, the ground-fault is detected with a high sensitivity even for a low frequency by the most suitable one of detectors 11A-11C whose outputs are changed in accordance with the frequency, and the ground-fault is detected with an approximately certain sensitivity independently of the frequency.
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