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The echo ranger : a fault locator for power cables Naylor, Thomas Kipling

Abstract

The location of faults in low-attenuation coaxial cables and open-wire lines by the use of the echo-ranging techniques of radar prompted this investigation of a method to accurately locate faults in underground power cables. As the propagation-velocity of disturbances on a smooth line or cable is constant, the time delay between the transmission of a pulse into a cable and the reception of an echo from an internal discontinuity is proportional to the distance to the discontinuity. The low inductance and high dielectric losses in power cables attenuate and distort the pulses. This distortion limits the accuracy and range of equipment which must measure time intervals to the nearest 3 x 10⁻⁸ seconds. Basically, the Echo Ranger consists of a portable low-voltage impulse generator combined with a timing Oscillator and a delayed high-speed sweep on a commercial split-beam oscilloscope. A high-power hydrogen thyratron delivers 0.1-microsecond pulses of five kilowatts (peak) to the cable. Although the range of the apparatus now constructed is only two miles on power cable, faults at least five miles away should be visible. The minimum resistance of a detectable series fault is about five ohms and the maximum resistance of a detectable shunt fault is about 2000 ohms. Without modification, the Echo Ranger can be used on overhead lines up to four miles long. On a 1044-foot piece of RG8U polyethylene cable, two 100-ohm shunt faults 20.8 feet apart were located within 0.63%. On a three-conductor oil-filled lead-sheathed power cable 1389 feet long, a transformer tap 424 feet away and a Joint 320 feet away were located within 1.2%. The apparatus can be readily modified to deliver 16-kilovolt 5-megawatt (peak) pulses to initiate and locate an arc at incipient faults. The power to hold the arc must come from a superimposed power supply such as a kenotron set or the normal line voltage. Further refinements which increase the accuracy and range without sacrificing simplicity of operation could be applied to advantage.

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