The Effect of Water Saturation on Silicone Rubber Surface Discharge Behaviour

Abstract

In many practical outdoor insulation applications, silicone rubber materials have outperformed others due to its inherent water repellence or hydrophobicity. In some cases, however, it has been found that the leakage current of such insulators exceed that of glass, especially under prolonged water exposure, while still showing a lower pollution flashover probability. In this paper, the effect of water saturation on silicone rubber material is studied with particular interest in the surface discharge behaviour. We will show that the presence of moisture may reduce the severity of surface discharges under some conditions. The polar nature of water and its dipole relaxation time will change the surface charge distribution thereby hindering the growth of anodic (+) surface discharges. Experiments are performed below 10 kV on both dry and moisture saturated silicone-rubber test samples. The moisture-saturated samples were aged in a climatic chamber and in hot water respectively. The test arrangement consists of a sharp point against a plane electrode for the initial tests and a flat washer vs. plane for the remaining trials; the electrodes are separated (interposed) by a 2 mm thick material specimen. This arrangement yields an electric field with a strong tangential component, which is favourable for obtaining surface discharges. Electrical measurements are performed using standard partial discharge detection equipment to obtain phase resolved partial discharge patterns. A camera with an intensified charge coupled device, or ICCD, is used to obtain time resolved images. Tests show that surface moisture affects both the magnitude and position of the discharge pulses as well as the physical shape of the electrical discharges, whereas prolonged moisture exposure appears to have a minimal effect on filler free silicone rubber materials, which were wiped dry.