Positive corona progression over profiled insulator surfaces near the breakdown condition

Experimental observations are presented of the development and propagation of positive impulse corona near the breakdown condition in a point plane gap in air at atmospheric pressure in the presence or absence of insulators having simple cylindrical and profiled surfaces. Optical techniques have been used and the effects of the insulator materials have been demonstrated. All the results with corona were obtained using an applied peak impulse voltage of 0.9U₅₀ in each case, where U₅₀ is the 50% breakdown voltage of the gap. Comparisons were made between air, polytetrafluoroethylene (PTFE) and porcelain surfaces. It has been demonstrated that corona development in the gap is dependent on the relative permittivity of the insulator material, the profiles and the proximity of the insulator to the high voltage point electrode. Simultaneous photomultiplier records and photographs taken on fast film with Quartz optics show details of corona development using the UV region of the spectrum.