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87_laurent_thermalstimuation - IEEE Transactions on...

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IEEE Transactions on Electrical Insulation Vol. EI-22 No.l February 1987 AN INVESTIGATION OF AC ELECTRICAL AGING PHENOMENA IN POLYETHYLENE BY THERMALLY STIMULATED DISCHARGE CURRENT MEASUREMENT S. Noel University College of North Wales Bangor United Kingdom C. Laurent and C. Mayoux Laboratoire de G&nie Electrique Associe& au C.N.R.S. Universite Paul Sabatier Toulouse France ABSTRACT In previous work, the authors have shown that electrical aging of polyethylene under ac voltage was due to the build-up of a space charge which was accompanied by light emission. Study of the electroluminescence allowed us to estimate the quantity of charges trapped in the material when ac voltage was applied to the polymer. The present paper reports an investigation on space charge in polyethy- lene stressed by an ac electric field by the thermally stimulated discharge current (TSDC) technique. Measurements were made on the same samples, i.e., the same polyethylene and the same point-plane electrode set-up as for the light emission study. This gives us a direct comparison of the results obtained using both techniques. It is shown that both the quantity of oxygen dissolved in the material and the amplitude, type and duration of electrical polarization have an influence on the observed peaks. We suggest that the applied electric field modifies the structure of the polymer. INTRODUCTION Study of the mechanisms ruling the electrical aging of insulators is of major interest. The understanding of these mechanisms might explain such problems as the long-term breakdown of polymers submitted to electric fields one or two orders of magnitude smaller than the material dielectric strength EB. Fig. 1 schematically represents the time evolution of polymer degradation when submitted to an electric field Elc0-1 to 10 2 EB. The mechanisms occurring during the first or incuba- tion phase are very different from those observed during the second phase during which both electric discharges and treeing can occur in the bulk of the material. The processes governing this second phase are characteristic of an advanced state of degradation of the mate'rial. They usually occur when breakdown is approached. Previous work concerning the first phase showed that electroluminescent phenomena occur in polyethylene samples submitted to a 50 Hz ac electric field [1,2]. We established that the electroluminescent intensity diminished when the quantity of oxygen dissolved in the amorphous part of the polymer was increased. Corres- pondingly, the degradation of the material was shown to be faster. The interpretation we proposed is the radia- tive recombination of injected carriers with ionized deep traps in the polymer. The characterization of the electroluminescense strongly suggested the existence of a space charge in the material. We, therefore, proposed to investigate 0018-9367/87/0200-0063$01.00 @ 1987 IEEE 63s
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IEEE Transactions on Electrical Insulation Vol. EI-22 No.1, February 1987 State of dearadation of the material BREAKDOWN I tB Fig.
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