07_voltagepolarity_spacechg - 44 IEEE Transacfionson...

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44 IEEE Transacfions on Dielecfrics and Electrical Insulation Vol. 4 No. 1, February 1997 Dynamics of Voltage Polarity Reversal as the Controlling Factor in Space-charge Induced Breakdown Insulating Polymers M. Mammeri, C. Laurent and M. Nedjar Laboratoire de Genie Electrique, Universitk Paul Sabatier, Toulouse, France ABSTRACT Our investigation is focused on the understanding of high-field phenomena in polymers. In highly insulating materials such as polyethylene, space charge has a strong influence on both the short and long term breakdown strengths. The aim of this report is to demon- strate and discuss the importance of the dynamics of voltage polarity reversal at the in- jecting needle electrode on space charge induced breakdown. Evidence of space charge injection is provided by the observation of local breakdown generated by grounding of the sample after dc polarization (so-called 'grounding' tree). The effect which is polarity dependent, can be observed only when the sample is short-circuited immediately after charging. Field computation is performed versus injected charge and a critical charge density is derived in order to explain polymer breakdown upon grounding. The time dependence of the effect is explained by space charge relaxation. Polymer degradation under ac voltage is discussed on the basis of this interpretation. 1. INTRODUCTION MPROVEMENT of the electrical properties of polymers needs a bet- ter understanding of the processes occurring in the material un- der high electrical stresses. For this reason much work has been per- formed to clarify degradation and breakdown mechanisms [l]. These phenomena are complex and might be due to various causes. Howev- er, the formation of space charge regions in the insulation affects the dielectric strength and seems to play a key role in the deterioration process. Measurement techniques are now available to study the spa- tial distribution of space charge in materials [2j. Unfortunately, their poor time and spatial resolutions do not allow for the study of fast space charge destabilization in the vicinity of electrodes, which can be the cause of breakdown. Charge injection from field-enhancing defects was shown to be the first step in a series of prebreakdown events, including electrical tree- ing [3-51. In order to simulate points of electrical stress enhancement, needle electrodes molded into the polymer are usually used. Space charge is formed due to the injection of electrons and/or holes from the electrode. It has been shown that under ac voltage, charge carri- ers are injected and extracted during the positive and negative voltage half cycles. This repeated process of charge injection and extraction leads to polymer aging through processes which are still controversial In this report we investigate local breakdown phenomena caused by space charge accumulation. Breakdown is triggered by grounding of the needle electrode after charging with a dc voltage much lower than the voltage of tree initiation. It is shown that the dynamics of charge relaxation plays a fundamental role in this phenomenon. Un-
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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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07_voltagepolarity_spacechg - 44 IEEE Transacfionson...

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