PURPOSE: To improve reliability by using a zero-phase overvoltage relay of which input is the sub-winding output voltage as an operation condition in a protective relay unit of which input is the secondary and tertiary output voltage of a capacitor type potential transformer (PD).

CONSTITUTION: A tertiary coil 5' is set up in a transformer TB, and each inter- phase is connected to the block delta and system zero-phase voltage is led. By setting up a zero-phase over voltage relay OVG' of which input is this output voltage, the zero-phase overvoltage relay OVG' operates at system fault and cannot accurately be operated at the secondary and tertiary failure of a potential transformer 92