Detection of partial discharges in 11KV cable termination

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Detection of partial discharges in 11KV cable termination

 

Author: Ip, Kam-wa Ricky
Title: Detection of partial discharges in 11KV cable termination
Degree: M.Sc.
Year: 1997
Subject: Electric discharges -- Detection
Electric power distribution
Hong Kong Polytechnic University -- Dissertations
Department: Multi-disciplinary Studies
Pages: vi, [122] leaves : ill. ; 30 cm
Language: English
InnoPac Record: http://library.polyu.edu.hk/record=b1398644
URI: http://theses.lib.polyu.edu.hk/handle/200/898
Abstract: Distribution networks are generally reliable but occasional breakdowns occur: apart from those due to contractor error, most occur at the crotch position of the high voltage cable termination. These do not normally occur abruptly but are the cumulation of a slow deterioration process caused by the effects of localised electrical discharges, usual termed 'partial discharges' (PD's). Such discharges emit ultrasonic radiation whose detection may allow the deterioration to be detected before failure occurs and allow appropriate remedial measures to be taken in good time. It is common practice for utilities to use an ultrasonic detector to 'listen' for PD's at terminations but the results are disappointing: breakdowns occur without the prior detection of PD's and apparent detection of PD's is not always followed by breakdown. A model simulating the actual 11kV-cable termination system, which includes a cable end box with 3 bushings, a short length of 11kV 3-core belt-type paper insulated lead sheath cable (PILC) with the 11kV heat shrinkable termination installed and a 30kVa.c. testing power supply, has been built in order to investigate the effectiveness of ultrasonic detection (UD) for determining the presence of PD within the cable crotch. Test specimens were made up with damage at the trifurcation to simulate PD sites. It was found that the positioning of the PD detector's microphone in the termination box's inspection slot was critical. Furthermore, faults facing away from the inspection slot were found to be hard or impossible to detect. As for comparison purpose, the conventional electrical measurement was also conducted. Although the elliptical display is more presentable to show the PD activity, it rarely uses for online application, as it requires making direct connection with the testing specimens. With the same equipment in use, PD measurement using ultra high frequency (UHF) method was conducted. When a partial discharge occurs it sets up an electromagnetic 'ringing' in the termination box, as a resonant cavity. The wavelengths will be equal or less than twice the internal dimensions of the box: the longest internal dimension, top to bottom, is about 750mm so the lowest resonant peak should be at a frequency of about 200 MHz. Using an ultra high frequency receiver and tuning to one of such resonance , 344 MHz, the amplitude (at that frequency) could be displayed versus the time on a 20 ms time-scale (i.e., one cycle of mains frequency). This could easily detect the artificially damaged insulation of the trifurcation whether facing the front or back of the box, as might be expected from the resonant cavity nature of the effect.

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