High impedance fault detection on rural electric distribution systems

A high impedance fault (HIF) results when an energized primary conductor comes in contact with a quasi-insulating object such as a tree, structure or equipment, or falls to the ground. The significance of these previously undetectable faults is that they represent a serious public safety hazard as well as a risk of arcing ignition of fires. A high impedance fault is characterized by having impedance sufficiently high that it is not detected by conventional overcurrent protection, such as fuses and overcurrent relays. Unlike low impedance short circuits, which involve relatively large fault currents and are readily detectable by conventional overcurrent protection, these HIFs represent little threat of damage to power system equipment. High impedance faults produce current levels in the 0 to 50 ampere range. Typically, an HIF exhibits arcing and flashing at the point of contact. Throughout the utility industry, there have been differences of opinion on how often HIFs occur. Normally, utilities do not keep good records on the number of down conductor instances. It is seldom recorded on trouble reports unless it results in a fuse or breaker operation. While it is likely that only a few percent (5-20%) of all distribution faults are high impedance faults, means exist to detect a high percentage of HIFs. The detection of high impedance faults on rural electrical distribution systems has been one of the most persistent and difficult problems facing the electric utility industry. Advances in digital technology have enabled practical solutions for the detection of a high percentage of these previously undetectable faults. This paper will review several methods of detecting high impedance faults. The issues and application of this technology will also be discussed. Field experience using digital technology will be also reviewed.