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The results of breakdown voltages in liquid silicone rubber (LSiR) insulation under stepped polarity reversal voltages at a uniform temperature in a needle-plane electrode system are presented in this article. In the presence of defects such as sharp conductor projections or protrusions in the cable insulation stressed under polarity reversal direct current (dc) voltages, the electric field distribution is far more complex due to nonlinear insulation conductivity and time-dependent field distribution. The electric field distribution and space-charge accumulation are intricate functions of factors such as geometry and nonlinear conduction. In this article, a two-dimensional (2-D) axisymmetric model of a needle-plane geometry is developed using the finite-element method to compute the time-dependent field, space charge, and current density distribution under polarity reversal dc, which apparently did not get much attention until now. Interesting results on the effect of polarity transition time and applied polarity time step on the abovementioned distributions are addressed and presented. The pronounced effects of nonlinear conductivity on the time-dependent field distribution are also demonstrated. Furthermore, from the breakdown experiments conducted on tip-plane electrode system for different tip radii and applied time steps, the results put forward a rational and practical estimate of the breakdown field using the proposed model. [ABSTRACT FROM AUTHOR]

In this paper, electric field and space charge accumulation have been investigated for needle-plane system (defect) under DC voltage conditions. Prolate spheroidal electrode system, believed to be closer to needle-plane system, has been used for solving the governing differential equations, numerically, for space charge and electric field distributions. Unlike past works in which space charge at needle tip was assumed either qualitatively or quantitatively, in this work, space charge formation is estimated using nonlinearity of material properties alone. A comparison with previous methods based on concentric spherical electrode approximations reveals that the results are different for prolate spheroidal system at different nonlinearities. Interesting results on the role of nonlinear conductivity on the DC electric field and space charge accumulation at needle-tip are presented. Furthermore, needle tip-plane breakdown experiments are conducted and the results suggest a reasonable and realistic estimate of tip-field at breakdown, using the proposed model. [ABSTRACT FROM AUTHOR]