numerical_spacechg_G - 322 Numerical Solution of Space...

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322 Numerical Solution of Space Charge Distribution in High Field Dielectrics Jinbo Kuang Steven A. Bogs Department of Electrical Engineering University ofToronto 10 King's College Road Toronto, Ontario M5S 1A4 Department of Electrical Engineering University of Toronto and Electrical Insulation Research Center University of Connecticut Storr~, CT 06269-3 136 introduction Electrical insulation often fails as a result of defect-induced highly inho- mogeneous electric fields and the high field phenomenainduced thereby. An understanding of what goes on in the microscopic high field region surrounding a small defect is therefore of great interest. Experimental investigation is made difficult by the small size of the high field region, typically just a few pm. The region cannot be made larger for experimen- tal purposes, as the power densities involved in the high field phenomena would cause thermal runaway if dissipated throughout a larger volume. The difference between impulse and dc electric strength, the influence of prestressing and impulse risetime on breakdown voltages, and the polarity dependence of breakdown voltage in needle-plane geometry have all been explained by invoking high field-induced space charge Ill. The present contribution therefore focuses on the computation of such space charge in a two-dimensional system. Field Limiting Space Charge Model Zeller et al. 12.31 have published a field limiting space charge (FLSC) model for high field dielectrics. This model assumes that no space charge is generated for a field, F, which is below some critical field, F,. For a field above Fo space charge is generated to limit the field to Fe The FLSC concept is simple and facilitates computation of the space charge density from Poisson's equation. Given the spatial distribution of the space charge density, other parameters can be estimated, such as the peak force density and maximum mechanical stress within the dielectric. However, the method does not facilitate computation of waveforms, power densities, temperature rises, etc. Computation of these more de- tailed parameters requires solution of Poisson's equation in time with field-dependent conductivity and time-dependent applied voltage, the task undertaken in this study. Authorized licensed use limited to: INDIAN INSTITUTE OF TECHNOLOGY KANPUR. Downloaded on August 12,2010 at 11:43:59 UTC from IEEE Xplore. Restrictions apply.
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323 The FLSC model can be extended through some simple considerations which facilitate estimation of the limiting field, F, As shown in [41, the limiting field is given, approximately, by the condition that where o(E) is the field-dependent conductivity, E is the permittivity, and o is the radial frequency. o(E~&
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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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numerical_spacechg_G - 322 Numerical Solution of Space...

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