High impedance fault

High Impedance Fault Detection Technology
Report of PSRC Working Group D15
John Tengdin, Chairman, Ron Westfall, Vice Chairman, Kevin Stephan, Secretary
Members: M. Adamiak, J. Angel, J. Benton, S. Borlase, S. Boutilier, M. Carpenter, P. Carroll, A. Darlington, D. Dawson, P. Drum, G. Fenner, F. Galvan, M. Gordon, R. Hoad, D. Hemming, J. Huddleston, D. Jamison, T. Kendrew, E. Krizauskas, J.Linders, J. McConnell, M. McDonald, J. Murphy, G. Nail, T. Napikoski, R. Patterson, M. Pratap, R. Reedy, D. Russell, E. Sage, D. Shroff, D. Staszesky, W. Strang, C. Sufana, B. Tyska, J. Waldron, C. Wester,T. Wiedman, P. Winston, J. Zipp

Abstract
High impedance faults (HIFs) on distribution systems create unique challenges for the protection engineer. HIFs that occur do not produce enoughfault current to be detectable by conventional overcurrent relays or fuses. This report presents a brief synopsis of what has transpired to date. It is based heavily on the history and application of today's technology, and discusses the results seen to date. It also presents some of the implementation strategies that are being used when applying this technology.
Introduction
This is a statusreport to the Line Protection Sub-Committee of the PSRC on the applications of high impedance fault detection technology. Not all unsafe conditions involve a HIF, i.e. a sagging conductor. This paper does not address the detection of those abnormal conditions where a conductor breaks and does not contact either another conductor or a grounded element.
A high impedance fault (HIF) does not have toinvolve a path to ground and, in fact, whether a ground is involved does not matter to the HIF detector. A high impedance fault can exist between two phase conductors (a tree limb lying across two phase conductors). The majority of HIFs do involve ground. In this discussion, high impedance faults will be referred to as HIFs whether or not ground is involved.
A high impedance ground fault resultswhen a primary conductor makes unwanted electrical contact with a road surface, sidewalk, sod, tree limb, or with some other surface, or object which restricts the flow of fault current to a level below that reliably detectable by conventional overcurrent devices. Often this leaves a conductor energized on the ground surface posing a danger to the public. The nature of HIFs has been studied sincethe early 1970's with the hope of finding some characteristic in the current or voltage waveform that would make detection possible and practical.

History
A fault on a distribution feeder is an abnormal circuit condition which results in energy being dissipated in a manner other than the serving of the intended load. Also known as a "short circuit", a fault may result in damage to theelectrical system, loss of power to customers, and/or possible unsafe conditions. The traditional method of detecting and isolating such abnormal conditions is overcurrent protection. The primary purpose of overcurrent protection is to protect the electrical system, and is based on the electrical properties of the circuit. Excessive current for too long will damage or interfere with normal operation ofthe system. The parameters (current and time) are easy to measure. Devices to detect an overcurrent event are well established, with decades of field experience. In practice, the removal of safety hazards caused by the fault is an additional motivation for overcurrent protection. See References 15 & 16 for further data on distribution feeder faults.
Most of the faults on power systems result ina substantial increase in current flow towards the fault point. Over the years, conventional overcurrent based protection schemes have been successfully used to detect and protect against these "low impedance" faults. However, for HIFs on distribution systems, the high impedance of the fault does not result in a substantial increase in current. Thus they can not be reliably detected using...