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Location of faults in power cables by fault-generated surges Hudak, Nicholas Edward

Abstract

The object of this research is to develop a satisfactory method for locating high-impedance faults in underground cables. Most methods of locating faults require that the fault-impedance be reduced to a low value before the measurement can be made. After a careful investigation of the available literature, it was decided that the most desirable method would be one utilizing the traveling-wave. Of the traveling-wave methods, the fault-generated surge method appeared to have the greatest possibilities; yet, according to the author's knowledge, this method has not been applied to power cables. In this method, the cable itself may be considered as the network that generates the required surges. The cable is initially charged to a voltage sufficiently high to establish an arc at the fault. The sudden collapse of the high voltage at the fault generates a surge which travels along the cable to the monitoring end, where it Initiates a timing device and is reflected back along the cable toward the fault. The arc which is still conducting reflects the surge back to the station. The time interval between the first and second arrival of the fault-surge at the station is recorded by the timing device and is proportional to the distance to the fault. In mathematically analyzing the surge phenomena in cables, the La Place operational method of analysis is used. The calculations for the transient produced by the discharge of a distortionless cable are worked out in full detail. The wave-form calculated is plotted and substantiated with experimental results. The transient produced is a rectangular wave that is exponentially attenuated and whose period is 4δ, where δ is the one-way transmission time of the cable in seconds. It is this wave-form generated by the cable itself that is used to locate the fault. Basically, the fault-locator developed consists of, a high-voltage low-current power pack, a triggering unit, a timing-pip generator, two uniform delay lines, and a double-beam oscilloscope. The block diagram of the fault-locator and the circuit diagrams of the triggering unit and timing-pip generator are given. The operation of the circuits and the procedure for measuring cable faults are fully explained. The fault-locator was tested on coaxial cable only, since no power cables were available. The results obtained were very satisfactory. The oscilloscope traces obtained were photographed and the experimental results discussed. It is concluded that the fault-locator can be used without modification for locating low and medium-impedance faults, as well as high-impedance faults in power cables. If the timing-pip interval is increased, the fault-locator can also be used for locating faults on overhead transmission lines.

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