Investigation into the performance of outdoor insulators under high humidity conditions.

Abstract

performance of high voltage outdoor insulators can be greatly affected by weather conditions. The weather conditions under investigation are cold temperatures coupled with high humidity levels. Weather data from a number of coastal stations around South Africa was analyzed to determine whether surface condensation was likely on outdoor insulators under these weather conditions. A heat transfer equation was used to determine the response of the insulator surface temperature to the environmental temperature. It was found that surface condensation would occur on the insulator surfaces which would lead to sudden, heavy wetting of the surface. Outdoor insulators in coastal environments are often heavily polluted, due to salt spray, and when wet, a conductive layer can form on the insulator surface. This conductive layer can result in appreciable leakage currents flowing on the insulator surface, often leading to premature failure. The finite element method program, Maxwcll, was used to simulate the outdoor insulators both under these polluted, wet conditions and under unpolluted conditions. Both cases were simulated for a silicone rubber, glass cap-and-pin and two EPDM outdoor insulators. The polluted insulators were simulated with varying pollution severities. The results of the simulations are analyzed and the surface resistances of the wet polluted insulators were calculated. An experiment was' set up to mask the environmental weather conditions found which would lead to surface condensation. The insulators under test were placed in a chilled weather chamber which introduced a steam fog to simulate the humidity. The leakage current was measured and recorded for comparison with the simulation results. The results of the weather chamber test showed that surface condensation resulted in more severe wetting than manual wetting. The weather chamber surface resistances calculated were much lower than those calculated by the Maxwcll simulations. This was due to the difference it humid particle temperature in the condensation rate equations used for the Maxwell simulations, and the humid particle temperature of the steam fog used in the weather chamber. Polluted coastal outdoor insulators exposed to the above weather conditions will experience larger than normal leakage currents which will lead to premature failure of the units.