05_Dissado_model_tree - IEEE Transactions on Dielectrics...

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IEEE Transactions on Dielectrics and Electrical Insulation Vol. 12, No. 4; August 2005 1070-9878/05/$20.00 © 2005 IEEE 655 Manuscript received on 26 January 2005, in final form 23 April 2005. 1 Now at Central Research Institute of Electric Power Industry, 2-6-1 Nagasaka, Yokosuka 240-0196, Japan Model for Electrical Tree Initiation in Epoxy Resin Kai Wu 1 and L. A. Dissado Department of Engineering University of Leicester Leicester LE1 7RH, UK ABSTRACT A model for electrical tree initiation in epoxy resins is presented in which the process is driven by the generation of new charge traps as a result of energy transferred to the polymer via charge recombination processes. The electroluminescence intensity expected from the model is computed and shown to be in agreement with the experimental data. In particular it is shown how the initial emission due to recombination can change to an emission arising from impact excitation in a natural way when the trap density reaches a level sufficient for the high electric field of the tree initiation region to connect shallow traps to form conducting filamentary paths in the form of percolation clusters. This result allows the electroluminescence behavior to be correlated with the observed onset of filamentary damage. The model is also shown to be able to explain the decrease in emission intensity observed at the time of transition between the two mechanisms for luminescence. Index Terms Aging, electric breakdown, electroluminescence, charge carrier processes, percolation 1 INTRODUCTION The initiation stage of electrical trees can be defined as the period between the application of a potential to the initiating site and the creation of a gas-filled cavity (the first tubule or channel) capable of supporting gas discharges. During this time energy is transferred to the polymer from injected charges, and utilized to form the discharge-supporting cavity. It is possible to envisage a number of ways whereby this can occur. For example, the kinetic energy gained by the charges may be used to excite and/or ionize molecules that thereafter react, breaking valence bonds and forming low-density regions. The recombination of charges of opposite polarity may also release energy to the molecular centre where recombination takes place, and hence promote the breaking of valence bonds [1]. Even the trapping of mobile charges to a local bound energy level can be expected to release energy that may be used for local chemical and morphological changes. Electroluminescence (EL) has proved to be a good method for monitoring the processes involved in the electrical tree initiation stage [2-4], since it demonstrates the existence of excited molecular states and yields some information as to the type of process involved [5]. This is particularly the case with epoxy resins [4,6] where electroluminescence has been followed during the tree initiation process in a point-plane geometry, for a range of applied stresses. At the lowest ac fields able to initiate electrical trees, two distinct stages of
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05_Dissado_model_tree - IEEE Transactions on Dielectrics...

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