PURPOSE: To provide an overvoltage ground relay(OVGR) with an appropriate backup protective function by calculating a ground resistance digitally based on a zero-phase voltage detected through a ground relay for grounded instrument and a normal ground voltage of grounded phase and then operating the OVGR with a desired detection sensitivity based on the calculated ground resistance thereby bringing the detection sensitivities of a digital ground relay (DGR) and the OVGR close to each other.

CONSTITUTION: Tertiary windings of a CPT is connected in star and the neutral is grounded. Current limiting resistors 3rn are connected across three tertiary windings. The ground resistance is calculated digitally based on the zero-phase voltage and the normal ground voltage of grounded phase upon occurrence of ground fault. The ground fault is detected with a predetermined sensitivity and the OVGR is operated rationally. Consequently, the OVGR responds positively to a ground fault occurring on any feeder in one system and the DGR exhibits the backup protective function effectively. Since the function of DGR is enhanced including the optimization of ground fault detection sensitivity thereof and the detection sensitivities of the OVGR and DGR are brought close to each other, protective function of system can be improved drastically.