Distribution High Impedance Fault Detection Utilizing High Frequency Current Components

This paper describes work performed by Texas A&M University on the detection of high impedance faults on distribution primary conductors. Some grounded distribution primary conductors may exhibit a very low fault current such that they may not be cleared by over-current protection. These faults may persist indefinitely, possibly causing a fire hazard or a hazard to humans by contact with an energized line. The paper begins with an examination of the high impedance fault problem from the perspective of system protection. The fault detection theory is presented next. The system utilizes a fault-generated increase in the 2-10 kHz component of feeder current for fault detection. EPRI funding enabled the verification and demonstration of this fault detection concept. Measurements were made on several faulted and unfaulted feeders to develop a representative data base of signals to which a relay would be subject. Analysis of these data provided a time and frequency domain signature of these faults. A microcomputer-based prototype relay was constructed and installed on a typical utility feeder. It successfully detected most staged faults while not indicating a false trip during a three month demonstration.