High impedance fault detector

Details High impedance fault detector High impedance fault detector

1995-03-16

1997-02-11

Gaffin, Jeffrey A.

Electricity: electrical systems and devices

Safety and protection of systems and devices

With specific quantity comparison means

361 65, 361 89, 361 94, 324 7642, 324520, H02H 346

Patent

active

056027095

DESCRIPTION:

BRIEF SUMMARY
This invention relates to a method and means for detecting a high impedance fault or arcing phenomenon in an alternating current electrical distribution system.
For the purpose of supplying uninterrupted power to consumers, protection systems must isolate any type of electrical fault which may occur in the electrical distribution system under various conditions. In modern distribution systems, protective relay means have been developed to cope with most circumstances, and these systems are able to respond rapidly to sudden and sometimes violent changes in current and voltage caused by short circuits. However, the problem of high impedance faults presents a major concern since many relay systems are unable to respond quickly and accurately under such conditions. A high impedance fault is a type of electrical fault which commonly occurs in a distribution feeder when the fault current flows through an equivalent high impedance which is caused by an electric arc. Because high impedance faults are often associated with electric arcs they are sometimes called an "arcing fault".
A typical single phase high impedance fault occurs when a plant such as a tree branch contacts a high voltage distribution power line, which often results in an arc between the plant and the power line. From the point of view of a protective relay situated at the distribution point for the power line the arc between the plant and the power line will present a high impedance. Further, if the protective relay is not able to detect the fault the result may be quite catastrophic, for example resulting in a bush fire.
Protective relay systems are often not able to detect such high impedance faults because fault currents are often far too small to be recognized by conventional relay systems. There are often no rapid nor ample changes in voltage or current waveforms when high impedance faults occur because the impedance from the power line to ground is quite high.
It is common for low current, high impedance faults to occur on distribution feeders and other relatively low voltage power lines. A high impedance fault usually occurs when a distribution line conductor is contacted by a high impedance grounded object, such as a tree, or when a conductor breaks and falls onto a poorly conducting earth surface. A great deal of research in the past has concentrated on creating more sensitive ground fault detection devices, since many researchers believed that high impedance faults were often not detected because the protective relays were not sensitive enough. Consequently, much research has gone into the design of very sensitive over current relays, in particular zero sequence over current relays for distribution networks. Although this research has improved grounded fault protection systems, the problem of low current faults remaining undetected has remained, the reason for this being that these devices may not distinguish fault currents below normal load levels.
Recent research has investigated the characteristics of high impedance faults and the use of signals other than fundamental frequency signals to detect high impedance faults. It was found that one of the most important characteristics observed during arcing faults is that harmonic signals increase to a measurable value which may be detected during occurrence of a fault. For example, research at Texas A & M University has addressed the characterisation of fault currents associated with downed conductors. From this fundamental understanding of low current fault characteristics a technique was developed which utilised high frequency (two kilohertz to ten kilohertz) current components for fault detection. This technique is based on the observation that arcing was almost always associated with downed conductors resulting in a modulated current waveform rich in high frequency components. The high impedance fault detector therefore utilised high frequency components to detect the occurrence of a high impedance fault, and operated in a satisfactory manner under certain operational conditions. However

REFERENCES:
patent: 4466071 (1984-08-01), Russell, Jr.
patent: 5185684 (1993-02-01), Beihoff et al.
IEEE Transactions on Power Delivery, vol. 6, No. 2, issued Apr. 1991, W. H. Kwon et al., "High Impedance Fault Detection Utilizing Incremental Variance of Normalized Even Order Harmonic Power", pp. 557-563.
IEEE Transactions on Power Delivery, vol. 6, No. 2, issued Apr. 1991, D. I. Jeerings et al., "A Practical Protective Relay for Down-Conductor Faults", pp. 565-571.

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