Effect of high dielectric protrusions on the breakdown phenomena of large electrodes under positive switching impulses

To improve the insulation strength, larger electrodes are widely used on high voltage apparatus, bus terminals and interconnections. In practical applications, such large electrode may often exposed to natural conditions like rain, insects and dust, among others. In order to take into account the influence of these high dielectric irregularities on the behavior of the fifty percent breakdown voltage, experimental and theoretical studies are required. This paper presents and analyses the results from a laboratory study on the effects of high dielectric protrusions, such as insects on the dielectric strength of a large electrode in an air gap. The test results indicate a drastic reduction of the dielectric strength due to the high dielectric protrusion located on the large electrode. It is theoretically demonstrated that the inception voltage of streamer is detrimentally reduced due to the local enhancement of electric field produced by the high dielectric protrusion. In addition, the unstable leader inception voltage was calculated and preliminary results indicate that the inception of unstable leader depends on the relation between the mean electric field of the set-up and the corresponding local electric field where inception of streamers takes place.