02_Disado_understanding_tree - IEEE Transactions on...

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IEEE Transactions on Dielectrics and Electrical Insulation Val. 9, No. 4; August 2002 483 REVIEW Understanding Electrical Trees in Solids: From Gperiment to Theory L. A. Dissado Department of Engineering University of Leicester Leicester LE1 7RH, U.K. ABSTRACT A review of recent developments made in the understanding of the electrical tree mechanism is presented. The life of the tree is covered from initiation, through propagation, to long-term changes in shape. The initiation process is examined in terms of the injection of space charge and its ability to transfer energy to the poly- mer to create damage. Theoretical models for the processes involved are assessed in terms of the experimental data and an outline for the sequence of events in tree initiation developed. The inter-relationship between tree discharges, tree propaga- tion, and tree shape is discussed. Theoretical models for these processes are evalu- ated in terms of their ability to reproduce experimental data, especially tree shapes and discharge sequences in time and space. The chaotic nature of the tree propa- gation mechanism is discussed both through experimental data and the results of a completely deterministic theoretical model. Some special features of electrical trees such as the existence of conducting trees, acceleration at long times and slow growth in thick insulation are briefly touched upon. Finally a summary of the state of the art is presented. 1 INTRODUCTION T the first' International Conference on Conduction A and Breakdown in Solid Dielectrics (ICSD) in Toulouse (1983) Eric Forster said that to understand elec- trical tree formation in solids we had to "understand how low density regions are produced by the electric field",see for example [l]. Obviously this process requires a transfer of energy from the field to the solid material. Of course the same could be said of a deterministi: breakdown mechanism [2]. Electrical trees can, however, be distin- guished from deterministic breakdown because the trans- fer of energy and hence the growth of the region of dam- age does not accelerate in time [31, Figure 1,'Again other pre-breakdown mechanisms such as water treeing [4] and electrical aging [51 also involve the generation of low-den- sity regions. Electrical trees differ from these processes in the generation of a connected system of tubules that can support gas discharges. Our task is to understand the mechanism whereby they are formed and to develop a theoretical model for their description. ____ Manwc~pr receiued on 2 March 2001, in final form 11 March 2002. Electrical trees are initiated by the high divergent ac field of a stress enhancing geometrical feature, such as a point electrode, or by partial discharges taking place in a gas-filled cavity. Alternatively they may be produced dur- ing a short circuit or polarity reversal of dc fields. The generation of an electrical tree, however, is not simple. In general there will he at least two stages of development and there may he more before the tree causes an electri- cal failure. First there will be an inception stage before
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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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02_Disado_understanding_tree - IEEE Transactions on...

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