PURPOSE: To detect the ground fault of an isolated or high-resistance-grounded three-phase one-circuit transmission line with a phase voltage detector installed to one phase and, at the same time, to detect the ground-fault phase.
CONSTITUTION: A synthesizing circuit 2 fetches a voltage equal to a ground fault voltage by synthesizing a phase voltage outputted from a phase voltage detector 1 with the output voltage of a transformer 5 after the phase voltage is delayed by means of a delay circuit 3 which delays the phase voltage by an integer multiple of the period of the phase voltage and inverted by means of an inversion circuit 4 and a comparator circuit 5 compares the fetched voltage with a set value. When the fetched voltage is larger, the circuit 5 outputs an output signal (a). In addition, a ground-fault phase discriminating circuit 10 discriminates the ground-fault phase by fetching the outputs of the circuits 3 and 2 and a ground-fault phase display 11 displays the discriminated ground- fault phase.
FUJII MASAHIKO
KUMITA KOJI
