A Selective Ground Fault Detection Relay for Ungrounded DC Power Systems

Ungrounded d.c. distribution systems, although not as common as a.c. systems, nevertheless exist in installations like steel mills and mines. Detection of a ground fault on a d.c. system is simple and straight-forward by measuring the bus-to-ground voltages, but selective ground fault detection to identify the faulty feeder within the system is a more complex problem and equipment for the purpose has not been available. This paper describes a method and product by which this objective is achieved. An a.c. generator is used to superimpose an a.c. signal of predetermined frequency onto the positive and negative bus via the ground fault resistance upon detection of a ground fault in the system. Measurement of the magnitude of the a.c. signal by a current sensor and a ground fault relay on each feeder will identify the fault location as well as the severity of the ground fault. The components used are standard zero sequence current sensors and ground fault relays, together with an a.c. generator of a special design and laboratory tests have shown that the system operates according to its design parameters. A working system will be installed in a steel mill in 1985 for full scale testing.