PROTECTIVE RELAY

PURPOSE:To enable optimal protection corresponding to the number of operating transformers by switching the electrical amount to be inputted to a protective relay. CONSTITUTION:When two transformers are operating, an impedance 113 is separated and protective coordination takes place between the primary protective relay 104 of transformer, the secondary relays 107, 108 of transformer and a distribution line RY110. Upon switching to single TR operation, fault current decreases when compared with two transformer operation. Since the operating time of the primary relay 104 and the distribution line relay 110 lags while the operating time of the secondary relay 107 or 108 leads when compared with the case of two transformer operation, an impedance 114 is connected in parallel with the secondary relays 107, 108 so that the operating time of the secondary relay lags and protective coordination takes place between the primary, secondary of the transformer and the distribution line. When the electrical amount to be inputted to a protective relay is switched depending on opening or closing of the secondary circuitbreaker CB for transformer, protective coordination takes place optimally resulting in the improvement of power supply reliability.