Rain and contamination tests on HVDC wall bushings with and without RTV coatings

Results of tests made to determine the ability of room temperature vulcanized (RTV) silicone rubber coatings to improve the performance of HVDC wall bushings are described. The behavior of uncoated full-scale ±500 kV wall bushings is first determined in wetting conditions consisting of nonuniform rain and fog with various amounts of predeposited surface contamination. Parameters affecting flashover performance, such as polarity, rain conductivity, and contamination severity, are discussed. Results of nonuniform rain tests on an RTV coated wall bushing are reported. Fog tests and in-service experience have demonstrated that RTV coatings improve the HVDC contamination flashover performance of station post insulators; no flashovers occur in laboratory clean fog tests at rated voltage with ESDD levels up to 0.1 mg/cm². The application of RTV, silicone grease, or oily coatings makes the surface of a wall bushing sufficiently hydrophobic to prevent flashovers during nonuniform rain tests without a pre-deposited layer of contamination