98_capa_r_field - IEEE Transactions on Dielectrics and...

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IEEE Transactions on Dielectrics and Electrical Insulation Vol. 5 No. 2, April 1998 237 Capacitive-resistive Field Calculation on HV Bushings using the Boundary-element Method S. Chakravorti, Electrical Engineering Department, Jadavpur University, Calcutta, India and H. Steinbigler Institute of High Voltage Engineering and Electric Power Transmission, Technical University, Munich, Germany ABSTRACT Capacitive-resistive field distributions including surface and volume resistivities have been computed in and around porcelain and capacitor bushings. The boundary element method has been applied for field computation in two axi-symmetric bushing configurations including four dielectrics. Effects of uniform and non-uniform distributions of surface resistivity and vol- ume resistivities of different dielectric media are studied in detail. Special emphasis has been given to the stress distributions along the various dielectric-dielectric boundaries. Further, the values and locations of maximum stresses in different dielectrics are determined and reported in this paper. The plots of equipotential lines for a few typical cases field distributions are also presented here for both bushings. 1 INTRODUCTION V bushings are integral components of very important electric H power system equipment such as transformers, shunt reactors etc. Depending upon the operating voltage of the bushing, it may occur that an electrical discharge is initiated at a certain point in or around the bushing surface because the local electric field at that point has reached the critical value. Once initiated, the discharge will in general grow rapidly and unless interrupted by local conditions elsewhere, such as much lower electric field at some other location, the discharge may quickly develop into a complete breakdown. Thus the knowledge of electric field distribution in and around HV bushings are of great im- portance for reliable operation of major HV equipment. The distribution of electric field in and around an outdoor HV bush- ing is governed mainly by the bushing geometry, permittivity and vol- ume resistivity of the insulating materials and the surface resistivity due to surface contamination [l]. Of all these factors, the surface resistiv- ity is the most variable parameter as a result of the ambient environ- mental conditions. The well-known combination of surface pollution deposit and wetting by fog or condensation is considered to be a major factor responsible for failures of outdoor HV bushings [2]. However for HVDC wall bushings it has been suggested that uneven wetting from rain is also a decisive triggering mechanism for most of the observed flashovers [24]. Hence, the determination of capacitive-resistive field distributions in and around outdoor HV bushings including volume and surface resistivities is of significant practical importance.
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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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98_capa_r_field - IEEE Transactions on Dielectrics and...

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