Positive corona characteristics on RTV SIR coated glass insulating surface under ramp high voltages

Positive corona discharge activity along glass surfaces, in pure form and coated with room temperature vulcanized silicone rubber (RTV SIR), bridging a short needle-plane gap is investigated under positive ramp high voltages. The case or air alone was considered as reference. At applied voltages just sufficient for discharge inception up to values causing flashover, the discharge current and the associated temperature rise of the insulating surfaces were measured; thus, I-U characteristics were obtained. In the presence of an insulating surface the glow discharge is suppressed but streamer formation is favoured. Surface corona inception, stable streamer formation and flashover occur at significantly lower voltages than those obtained in the case of air alone. Surface temperature starts to increase above the ambient value when the stable streamer discharge is formed. During the stable streamer regime the insulating surface temperature is closely related to the streamer current. Both the maximum temperature reached immediately before flashover and the cumulative energy required for flashover are higher for the RTV SIR coated than pure glass surface. The temperature rise on the RTV SIR coated glass surface never exceeded ~10 ^oC, thus it cannot be considered as crucial in terms of thermal degradation of the material.

I. Introduction

Pollution flashover of insulators is one of the main causes of power system outages, affecting the reliability of power networks [1]. The use of polymeric coatings characterized by hydrophobic properties is commonly employed by the power utilities to improve the performance of ceramic insulators, especially under severe polluted environments [1], [2]. However, in service, the hydrophobicity of polymeric materials can be significantly deteriorated, as a result of the interactive effects of several uncontrolled ageing stresses; significant contributions can be found in, for example, [3], [4] and the citations therein.