04_montanari - 554 Laurent et al.: Time-resoled Space...

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Laurent et al.: Time-resol © ed Space Charge and Electroluminescence Measurements in Polyethylene Under ac Stress 554 Time-resolved Space Charge and Electroluminescence Measurements in Polyethylene Under ac Stress C. Laurent, G. Teyssedre Laboratoire de Genie Electrique, CNRS UMR 5003 ´ Universite Paul Sabatier, Toulouse, France ´ and G. C. Montanari University of Bologna, Department of Electrical Engineering Bologna, Italy ABSTRACT () Time-resolved space charge SC and electroluminescence EL measurements are carried out on polyethylene films stressed under ac voltage at industrial frequency to probe injection, trapping and recombination of charge. SC is measured by pulsed electro-acoustic PEA technique at each zero voltage cross-over point of the ac voltage, thereby getting rid of the capacitive charge. EL is detected using photon counting techniques with a time resolution down to 50 m s. Both SC and EL exhibit a threshold response as a function of the applied voltage. Numerical values of these thresholds are in good correspondence for SC and EL measurements in every tested material. Observations are consistent with a model of bipolar injection with an un - balanced situation in positive and negative charge behavior. It is also shown that different polyethylene materials behave differently relative to the field above which the charge is detected, providing thereby a way to compare the space charge behav - ior of those materials under ac field. Index Terms — Insulating materials, space charge, electroluminescence, ac stress, electrical aging. 1 INTRODUCTION Ž. HE effect of space charge SC on insulation de- T gradation under ac electrical field at industrial fre- quency is still a debated topic in spite of the fact that the interest of researchers in this field was aroused a long time ago. Charge injection and extraction at a metal-dielectric contact has been recognized for long as a key factor in wx electrical treeing initiation 1, 2 . It has been shown that the charge injected at the tip of a needle electrode under time varying stress can be detected using electrical tech- niques 3, 4 . The field-limiting space charge FLSC con- cept has been put forward to account for the behavior of a SC cloud at the needle tip. This model suggests the exis- tence of a step-increase in the carrier mobility above a material-dependent field E , called critical field, at which c material degradation becomes effective 5, 6 . Charge transfer is accompanied by emission of light through elec- troluminescence EL process. EL is initiated by mobile carriers during the injection process or by charge recombi- nation upon voltage polarity reversal 4, 6, 7, 8 . This Manuscript recei ® ed on 13 February 2003, in final form 27 January 2004. emission of light has been associated with degradation re- actions 9 . Values of the electric field at which these pro- cesses have been detected in divergent field configura- tions lie in the range of 100 kV r mm or more, suggesting that electrical ageing would be initiated only under a very high field. Although this model is able to describe SC be-
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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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04_montanari - 554 Laurent et al.: Time-resoled Space...

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