Electrode roughness effects on the breakdown of air-insulated apparatus

The dielectric breakdown of air insulating systems is believed to be sensitive to local irregularity of the electric field which may result from the presence of defects such as contaminants adhering to electrode surfaces and surface roughness. Normally metal machining methods are used to eliminate such electrode surfaces irregularities. However, system aging and harsh operating conditions create and sustain such rough surface conditions which may, in turn, lead to the failure of insulation under the resulting enhanced electric stresses. Electrode surface roughness causes a large reduction in the breakdown strengths of gas insulated apparatus. Surface roughness leads to the existence of localized microscopic regions with local field intensities larger than the average field in the gas near the electrodes. This paper models the insulation breakdown mechanism in the presence of such surface roughness, or protrusions, taking into account their random nature which lends the problem to probabilistic treatment. In order to generalize the surface roughness effect on the dielectric withstand of air-insulated systems, surface roughness is simulated by using a random event generator. The perturbations which these protrusions inflict on the field distribution in a nearly-uniform field gap are assessed. The corresponding breakdown voltages are estimated for different patterns of surface roughness. The results are statistically formulated.