To prevent unnecessary operations from being operated caused, even if magnetic saturation phenomenon occurs in current transformers installed at both ends outside the output terminals of a generator and a differential current is produced.
A differential relay RY is provided with a differential element, which obtains a restraining current Ires from the secondary current of current transformers CT1, CT2 installed at both ends outside the output terminals of a generator G, further derives an operating current Iop from a differential current Id obtained by differentially connecting the current transformers CT, CT2, and if the magnitude of the operating current Iop is larger than that of the restraining current Ires, produces on operating output. The differential relay RY is provided with a current transformer saturation detecting means comprising a phase difference detecting means θ, which detects the phase differences between the secondary currents of the current transformers CT1, CT2, and the differential current ID and a determining means TDD, which if the phase difference detected by the phase difference detecting means θ exceeds a specified value, determines that a magnetic saturation phenomenon has occurred in the current transformers CT1, CT2. Operating output from the differential element is locked by detecting an output from the current transformer saturation detecting means.
SHIMADA HIDEYUKI
SATO CHIKAO
