A PRACTICAL PROTECTIVE RELAY FOR DOWN-CONDUCTOR FAULTS

A practical relay for detecting down-conductor high impedance ground faults (HIGF) is described, with supporting theory and data from live line tests. The use of the third harmonic current magnitude and the phase relation to the system voltage is shown to provide viable information for detecting this type of fault. By suppressing ambient harmonics and utilizing only the change in the harmonics, fault current sensitivity of one per cent of feeder rating is shown feasible. Fault detection concepts include harmonic phasors and harmonic impedances, plus harmonic phase sequence components in the three phase unit. Relays for both single phase laterals and three phase circuits are described. Redundant measuring techniques in both versions are shown to enhance the reliability of the system and contribute to the feasibility of the noted sensitivity.