FAFA9280d01 - Nonstationary behaviour of partial discharge...

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Nonstationary behaviour of partial discharge during discharge induced ageing dielectrics R.J. Van Brunt P. von Glahn T. Las Indexing terms: Ageing, Epoxy, Partial discharge, Point-electric gaps, Stochastic behaviour, Surface conductivrry Abstract: Changes in the stochastic behaviour of pulsating partial discharge with time have been observed when an alternating voltage is applied to point-dielectric gaps in which the dielectric is a cast epoxy resin either with, or without, A1,0, filler. The changes in discharge behaviour are shown, with the help of a Monte-Carlo simula- tion, to be consistent with discharge induced increases in the surface conductivity of the epoxy. This ‘ageing’ effect is shown to be accelerated as the frequency of applied voltage is increased from 50 to 800 Hz. The implications of the results on accelerated ageing tests and definition of partial discharge ‘signatures’ for possible pattern recogni- tion are discussed. The relationship between average partial discharge current a:id partial dis- charge pulse height distributions is also discussed. 1 Introduction Investigations have been under way in a number of laboratories to relate the observed statistical character- istics of partial discharge (PD) to properties of the site at which the discharge occurs [l-61. There is an expectation that the statistical characteristics of PD can provide information needed for insulation defect site identifica- tion [7-12]. The success of PD identification schemes depends on finding a sufficiently complete set of defect site descriptors to allow assignment of unique, meaning- ful PD signatures to these sites. Special difficulties are encountered in cases where PD induced ageing occurs (i.e. where there are physical and chemical changes at the discharge site that, in turn, cause modifications in the sta- tistical characteristics of the PD). There is experimental evidence that the statistical behaviour of PD will change with time owing to dis- charge induced changes in the insulating characteristics of a dielectric [13-151. Of particular relevance in the present work is the case where the occurrence of PD causes a change in the conductivity of a dielectric surface which, in turn, changes the discharge characteristics. This case was first investigated in 1958 by Rogers [16] who showed that the ‘self-extinction’ of pulsating PD in a 6 IEE, 1995 Paper 1435A (S3), first received 3rd March and in revised form 7th June 1994 R.J. Van Brunt and P. von Glahn are with the National Institute of Standards and Technology, Gaithersburg, MD 20899, USA T. Las is with the Electrotechnical Institute, Warsaw, Poland IEE Proc.-Sri. Meas. Technot., Vol. 142, No. 1, January 1995 dielectric cavity can be attributed to a discharge induced increase in the conductivity of the cavity walls. More recently, Hudon and coworkers [I71 have been able to relate the transition of a pulsating discharge into a glow discharge in a gap formed by parallel epoxy resin planes to a discharge induced increase in the epoxy surface con- ductivity. Changes in discharge behaviour associated
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This note was uploaded on 06/08/2011 for the course ELECTRICAL 124 taught by Professor Ghjk during the Spring '11 term at Institute of Technology.

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FAFA9280d01 - Nonstationary behaviour of partial discharge...

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