High impedance fault detection implementation issues
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Faults on distribution circuits are normally detected by simple overcurrent relays. Faults through a high impedance such as dry earth or asphalt do not have sufficient current to operate overcurrent relays and must be cleared manually. Such high impedance faults often occur as downed conductors and may pose a hazard to the public. Equipment is now commercially available to detect a high percentage of high impedance faults, but use of this equipment introduces operational issues which affect applications. This paper discusses technical as well as non-technical issues associated with applying high impedance fault detectors. Recognizing all the issues is important to the effective implementation of these new fault detecting techniques. Research on high impedance faults has focused on the development of a sensitive, substationbased detector to detect and clear such faults. This technology has demonstrated the ability to accurately and securely detect a large percentage of high impedance faults including some with current of 1-5 A. During this research it became apparent that there were significant issues which would affect the application of the technology. The complexities of applying these devices must be considered to fully realize the benefits and to minimize any adverse effects of casual application. The development of a suitable high impedance fault detector (HIFD) involved a significant shift in understanding for utilities in four key areas: identifying which faults and hazards could be detected; willingness to accept substantial improvement instead of having a "perfect" detector, the substantial differences between overcurrent protection and high impedance fault detection; and acceptance of a probabilistic basis for fault detection. The major issues guiding the application of high impedance fault detection are as follows: Accuracy and Security - Utilities expect indications from a detector to be accurate and secure, with a significant bias toward service continuity. Effects of Outages - While one may hope to improve safety by clearing suspected downed conductor faults, unnecessary outages may cause other unsafe conditions such as outages of traffic signals. Careful tradeoffs are required to collectively minimize all hazards and minimize nuisance detections. Identification of Imminent Hazards-Utilities prefer that a detector identify only those faults which are immediate hazards. As it is impossible to accurately measure how "safe" a condition is, one can only identify electrical conditions which are likely to be unsafe but can also be accurately measured. Emotional Issues - An emotional environment surrounds the discussion of high impedance faults among utilities because downed conductor accidents are concerned with public safety. Any solution must truly reduce exposure to all hazards (from downed conductors and also from outages) at a reasonable cost, and not complicate matters further. Several issues arise regarding how such a detector would be implemented in practice, In introducing high impedance fault detection technology at a utility, it is important to define a program for testing, evaluation, and application. Utilities can identify various feeder characteristics which would make them more prone to having hazards from high impedance faults. To coordinate with overcurrent protection, one must delay action of an (HIFD), especially tripping, until overcurrent devices have an opportunity to operate, to avoid tripping an entire feeder un less necessary. Typically, it is appropriate to wait 30 seconds or longer. The HIFD technology can be most effectively implemented with monitoring functions including waveform recording. To maximize effectiveness, it is important to implement an HIFD with remote communications, either with supervisory contro. and data acquisition (SCADA) or a direct link such as a telephone line to the HIFD. Perhaps the most critical considerations involving high impedance fault detection concern what actions to take if a detector identifies a fault. There is no one correct method for determining which control action a utility should take. Each utility will have to make decisions based upon the service area and their individual needs. However, it will be important to carefully consider these options and document decisions and reasonings. Utilities can choose to use a fault detector to trip or alarm. Each utility should establish a response procedure before placing high impedance fault detection in service. When used to alarm, an operator can consider additional information, such as data retrieved from a detector or a customer call which confirms a hazardous situation, and then perform a supervisory trip, if appropriate. The detector developed at Texas A&M provides two decision outputs: arcing detected and downed conductor detected. The downed conductor detected output is set when there is substantial evidence that a downed conductor is present. While there are common considerations which guide the actions taken, different utilities may reach vastly different conclusions on the responses taken. Fault location becomes challenging with a high impedance fault especially if an entire circuit is tripped. For fault location, some advantages can be gained by creative use of data recorded in HlFDs and with knowledge about the circuit and equipment on the feeder. The HIFD technology provides for the detection of a high percentage of high impedance faults while maintaining the highest levels of security. Based upon prior field experience, it can be expected that approximately 80 percent of presently uncleared downed conductor faults would be detected by HlFDs. There will always be faults which cannot be detected. Likewise, there will be accidents which cannot be avoided. Utilities can only attempt to reduce exposure, improve detection, and minimize response times while not adversely affecting customer service. Improvements in several aspects of utility operations can reduce h
