03_FEA3 - 176 Cisse et al.: Electric Field Calculationsfor...

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176 Cisse et al.: Electric Field Calculations for Needle-Plane Geometry and Space Charge in Polyethylene Electric Field Calculations for Needle-Plane Geometry . and Space Charge in Polyethylene L. Cisse, S. S. Bamji and A. T. Bulinski National Research Council of Canada Ottawa, Ontario KIA OR6 Canada ABSTRACT Electric fields in polymeric insulation are calculated with the Boundary Element Method (BEM) in the presence and absence of space charge. A 3-D version of the BEM software is employed to determine the profiles of divergent electric fields pro- duced hy metallic and semicon needles embedded in the polymeric insulation. It is shown that the value of the electric field in the insulation decreases rapidly and within 5 pm from the tip it is less than 50% of its initial value. The BEM can also provide 3D profiles of the electric field distributions in the presence of space charge. Index Terms - Electric field calculation, needle-plane, polymeric insulation, space charge, Boundary Element Method (BEM). 1 INTRODUCTION LECTRIC field calculations are required for the de- the insulating material in such devices is not subjected to electrical stresses above which deterioration can initiate. For simple parallel-plane geometry the electric field in the insulating material can be.easily calculated but for com- plex geometries fast computer and large memory are re- quired. Two distinct approaches are employed for electric field calculations. The domain type approach used by the finite difference and the finite element method (FEM) directly solves the governing differential equation for the potential [I]. The finite difference method involves an interactive process and the main disadvantages are the crude model- ing of problem geometry and the large number of un- knowns in open field problems. FEM uses a variation technique in which the potential is approximated by a se- quence of functions defined over the entire domain of the complex geometly. However, the derivative of potential could have discontinuities and it is not possible to model infinitely extending regions. An alternate approach to the solution of the boundaly value problem is the boundary element method (BEM), which is based on a formulation of the boundary integral equation [2]. The main advantages of this method over the direct approach are the elimination of differentiation and interpolation to calculate potential or its derivatives and it E. sign and operation of power devices to ensure that Manu~tip rrceiued on 1.7 May 2002, m final form 2 October 2002. has a good means for checking the accuracy of the solu- tion. Needle-plane geometries that produce divergent elec- tric fields are often used for electrical treeing tests [31. They are also employed to stimulate defects, such as pro- trusions and inclusions, which can be accidentally intro- duced into the polymeric insulation during the manufac- ture of underground power cables 141. Recently, the BEM was used to calculate the electric field profiles at needle tips embedded in polyethylene [51. Field distributions were
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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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03_FEA3 - 176 Cisse et al.: Electric Field Calculationsfor...

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