99_Bamji_impulse_xlpe - 288 I EEE Transactionson...

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288 IEEE Transactions on Dielectrics and Electrical Insulation Vol. 6 No. 3, June 1999 Electroluminescence due to Impulse Voltage in Cable-grade XLPE S. S. Bamji, K. Tohyamal and A. T. Bulinski National Research Council of Canada, Ottawa, Ontario, Canada ABSTRACT Underground power cables in service are inadvertently subjected to impulses generated by lightning and switching surges that are superimposed on the ac voltage at which they operate. HV transients caused by lightning and switching operations not only radiate large electromag- netic fields but also impose additional stresses on the insulation and could initiate deteriora- tion which can continue under normal operating conditions. Electrical breakdown of ca- bles is a local phenomenon and electrical aging at local sites in the polymeric insulation occurs by molecular dissociation of the polymer and formation of new chemical bonds. This process usually involves the electronically excited states of the molecules that give rise to radiative phe- nomena, such as electroluminescence (EL). Analysis of EL can help to clarify the degradation mechanisms that occur at points of electrical stress enhancement and lead to cable breakdown. The EL technique is several orders of magnitude more sensitive than the commonly employed partial discharge detection and can provide a better understanding of the various time depen- dent mechanisms such as space charge injection, trapping and decay that can lead to insulation failure. The characteristics of in crosslinked PE subjected to impulse voltage, are described in this paper. Impulses having the same polarities as the half cycles of the ac voltage on which they are superimposed give rise to the largest number of pulses. Also, more pulses are emitted when impulses are applied at the peaks of the positive and negative half cycles than at the zero crossing of the acvoltage. This suggests that the amount of charge injected and trapped into the polymer plays a crucial role for emission. 1 INTRODUCTION OLYMERS are widely used as electrical insulating material in power P devices such as capacitors, cables and transformers. Modern HV devices operate at higher power densities and efficiencies than their pre- decessors and this has resulted in an increase of the electric stress ap- plied to the insulation. In the case of power cables, aesthetic concerns and difficulties in obtaining right-of-way have increased the pressure to place them underground, and the future will probably see a significant expansion in the use of underground systems. With the development of better insulating and semiconducting materials there is a tendency to increase further the operating stress of the polymeric insulation used in transmission and distribution class underground cables. Hence, in or- der to achieve high reliability, it is important to understand high field phenomena in cable insulation.
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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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99_Bamji_impulse_xlpe - 288 I EEE Transactionson...

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