89_Bamji_degradation_photo - I EEE 'Ikansactions on...

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IEEE 'Ikansactions on Electrical Insulation Vol. 24 No. 1, February 1989 91 Degradation of Polymeric Insulation Due to Photoemission Caused by High Electric Fields S. S. Bamji, A. T. Bulinski, and R. J. Densley Division of Electrical Engineering, National Research Council of Canada, Ottawa, Ontario, Canada ABSTRACT The mechanism of electrical tree initiation in polyethylene sub- jected to ac, dc, and half-rectified ac voltages is studied by an optical technique which is at least two orders of magnitude more sensitive than partial discharge detection. For dc voltage up to 46 kV, no light emission or tree inception was detected in the polymer containing the normal and decreased concen- trations of air in its free volume. ac and half rectified ac voltages, tree inception only occurred if light, in the visible and ultraviolet ranges, was detected. Photodegradation of the polymeric insulation by ultraviolet radiation can occur when the voltage applied to the polymer exceeds the threshold volt- age of light inception, such as during switching and lightning overvoltages. This can account for the gradual degradation of the polymeric insulation which ultimately leads to the forma- tion of an electrical tree. By comparing the light emitted from the polymer subjected to ac voltage having half-cycles of dif- ferent amplitudes to dc superimposed on ac it is shown that the space charge injected into the polymer causes the sero- crossing of the local field in the polymer. This is required for light emission and subsequent tree inception. 0016-0367/89/0200-01$1.00 @ 1080 IEEE
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82 Baq'i et al.: Degradation of polymeric insulation due to high electric fields 1. INTRODUCTION LECTRICAL treeing which occurs in polymeric in- E sulation used in HV devices is a precursor to the failure of the equipment. The phenomenon of electri- cal treeing has been extensively studied over the past several years but the important aspect of tree initia- tion is still not adequately explained, particularly when the trees appear after long periods of voltage applica- tion. Under normal service conditions impurities, con- taminants or defects accidently introduced into the poly- meric insulation during synthesis and manufacturing pro- cesses can act as points of electrical stress enhancements at which electrical trees can initiate. It is recognized [1,2] that charge injection and electroluminescence from these stress concentration sites in the insulation play a central role during the initiation phase of an electrical tree. The aim of this study was to find the type or polar- ity of charge required for electroluminescence, in poly- meric insulation, that leads to tree initiation. In or- der to control the polarity of the charge injected into the polymer ac, dc and half rectified ac voltages were used. It is shown, in this paper, that injection of unipo- lar charge (electron or hole) into the polymer does not cause light emission or tree inception, over the range of voltages used in this study, and that the injection of both electrons and holes is required for tree inception.
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This note was uploaded on 06/11/2011 for the course ELECTRICAL 124 taught by Professor Ghjk during the Spring '11 term at Institute of Technology.

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89_Bamji_degradation_photo - I EEE 'Ikansactions on...

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