A New Single-Phase-to-Ground Fault-Detecting Relay

In the application of differential relays the need for a supervising relay which will detect the existence of a single-phase-to-ground fault condition to the exclusion of all others has frequently arisen. Heretofore, the only scheme available has been to utilize a relay energized by zero-sequence quantities. Generally, the relay has been energized by a current transformer connected in the station ground. Such a relay, however, will also detect the existence of a two-phase-to-ground fault and only partially solves the problem. The new relay described in this paper derives its operating force from the zero-sequence voltage at the bus and is restrained by the negative-sequence voltage at the bus. The addition of properly proportioned negative-sequence restraint provides the relay with a means of recognizing a single-phase-to-ground fault only. It is applicable on those systems where the zero-sequence impedance of the system exceeds the negative-sequence impedance by a reasonable margin.