05_T.J.Lewis_fieldinduced_morpho

05_T.J.Lewis_fieldinduced_morpho - IEEE Transactions on...

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IEEE Transactions on Dielectrics and Electrical Insulation Vol. 12, No. 5; October 2005 1070-9878/05/$20.00 © 2005 IEEE 951 The Contribution of Field-Induced Morphological Change to the Electrical Aging and Breakdown of Polyethylene J. P. Jones, J. P. Llewellyn and T. J. Lewis School of Informatics, University of Wales, Bangor Dean Street, Bangor Gwynedd, LL57 1UT, U K ABSTRACT A brief review of the early literature is given which provides evidence that electrically-induced mechanical stresses make an important contribution to the electrical breakdown of solid dielectrics. Special attention is given to polyethylene and the manner in which this semi-crystalline polymer yields under mechanical stress by microvoid, crack and craze development in the amorphous phase between the lamellar crystallites. The nature of the forces induced by an electrical field is considered and it is shown that a significant component of tensile stress is generated in a direction orthogonal to the field and can become large as breakdown is approached. This suggests a correlation between the responses of the polymer to mechanical and electrical stresses and consequently the importance of morphology in determining the latter. The likely effect of field-induced morphological change on charge transport and electrode processes is described and its underlying contribution to possible aging markers for polyethylene, such as high field conduction, electroluminescence, space charge and charge packets is considered. Index Terms - Electromechanical forces, electric breakdown, trees, conduction, aging, space charge, charge transport, electroluminescence, polyethylene. 1 INTRODUCTION POLYETHYLENE (PE) is an insulating solid of primary interest for a number of reasons. It is a material of choice in a wide variety of high voltage applications because of its stability, relative ease of processing and high insulating and low loss properties at normal temperatures. Many of its features make it equally attractive as a material for mechanical application. In spite of these attractions it is nevertheless a material of some complexity with a semi-crystalline morphology dependent on processing and subsequent environmental conditions. The considerable research literature concerning its behavior under high electrical stress contains a number of puzzling features which reflect this complexity. In spite of this, polyethylene continues to be considered in many situations as a relatively homogenous amorphous material without microstructural features which would seriously affect its electrical properties under high stress. The objective of the present paper is to revise this traditional view by emphasising the complex molecular structure of PE and the close link between this structure and the electrical properties, some of which have been considered in the earlier literature. The morphological response of the polymer to a high electrical field then becomes a fundamental aspect of the electrical aging process which precedes electrical breakdown. The primary step is an electromechanical one in
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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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05_T.J.Lewis_fieldinduced_morpho - IEEE Transactions on...

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