Investigation, analysis & modelling of IEC 587 tracks

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Investigation, analysis & modelling of IEC 587 tracks

 

Author: Yim, Ho-tung Stephen
Title: Investigation, analysis & modelling of IEC 587 tracks
Degree: M.Sc.
Year: 1997
Subject: Breakdown (Electricity)
Electric insulators and insulation
Hong Kong Polytechnic University -- Dissertations
Department: Multi-disciplinary Studies
Pages: 236 leaves, [128] p. : ill. ; 30 cm
Language: English
InnoPac Record: http://library.polyu.edu.hk/record=b1398609
URI: http://theses.lib.polyu.edu.hk/handle/200/2290
Abstract: Surface tracking is one of the common electrical breakdown processes on the insulation surface of high voltage electrical equipment when the insulation surface is covered with contaminations under high humidity environment. The IEC 587 test is a standard test on the electrical strength of insulators against the surface breakdown in adverse conditions. This is actually an accelerated surface tracking process. Surface tracking is a complex breakdown process. During the breakdown, current flow and sparks generate a lot of heat that burns the insulation surface. After the breakdown, a black carbonized track remains on the insulation surface. The track pattern of different breakdown tests shows some kind of similarity: they are all self-avoiding zigzag curves similar to the lightning pattern familiar to most people. This project aims at investigating the track patterns with different analytical approaches and constructing models to simulate the tracks. The breakdown tracks on the insulation surface were scanned into a computer, saved as bit-map (BMP) files and then decoded to x-y coordinate form. Programs were then written to analyze the tracks and produce simulated tracks. The Most Significant Turning Point Detection Test was applied to all the 53 tracks obtained from the IEC 587 experiment. The distance between turning (segment length) concentrated on 20 - 30 pixel distance (p.d.), equivalent to 0.6 - 1.0 mm, with typical segment lengths of 55 p.d. (1.71). The turning angle distribution is symmetrical about the zero degree and concentrated in the range 25 to 35 degree and -25 to -35 degree. The fixed segment concept is thus introduced to explain the tracking process. Results obtained with the Fixed Segment Angle Analysis show that turning angles concentrated on the zero degree and other particular angles symmetrically. The angle distribution function obtained from both Relative Turning Angle and Absolute Reference Angle approaches show most angles concentrated on the zero degrees and many local maxima occurred at around +10, +15, +30 degree etc., especially in the Absolute Reference Fixed Segment Angle Analysis. From a physical point of view, this implies that in many cases, the tracks make zero turning or small turning because of the field enhancement effect. This is called "memory effect". When the tracks make a turning, they tend to turn at certain fixed angle more frequently due to the field enhancement effect at these particular angle ranges. Fractal analysis was also applied to all the tracks and the results show that the tracks are also fractal patterns. The fractal dimension of the tracks is in the range 1.051 to 1.065. The fractal dimension of the tracks was found to be only slightly dependent on the voltage and duration of the breakdown. Finally, different simulation models were developed based on the results obtained from the analysis. The Semi-random Fixed Segment Model is the best one being developed: the simulation track is constructed with a series of fixed length segments joined together with angles that partly depend on the previous angle (i.e., has a "memory" of the previous angle) and partly randomly generated following a centered triangular distribution function. The optimal values of the modelling parameters (i.e., segment length and memory factor) were found with the aid of fractal analysis. Different aforementioned analyses were also applied to the simulation tracks obtained to compare and verify with the real tracks.

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