Eskom, the local South African electricity supplier, did a recent study on insulator maintenance. For additional information I just want to add the different techniques, but according to the study, it seems as if long-term experiences of these coatings (original question) are very limited.
Insulator Maintenance normally takes one or more of the following forms:
Periodic hand wiping on the de-energized installation,
Periodic coating with grease compounds,
Periodic washing while either energised or de-energised,
Application of a hydrophobic coating.
Greasing
Grease compounds used for coating insulators are mainly silicone or hydrocarbon products. The action of the grease is to provide the insulator with a hydrophobic surface film and to surround the individual contaminating particles with grease. In this way, wetting of the particles are prevented and a conducting path is not formed along the insulator surface. This action will continue until the grease is saturated with contaminant. The thickness of the layer depends on the type of grease and on the degree of pollution. Generally, a thickness of 1mm is adequate for silicone products, whereas for hydrocarbon grease a thickness of 2 to 3mm may be necessary.
Once grease has been applied, it is necessary to clean and re-grease the insulators at intervals which depend on the degree of contamination and weather conditions. Intervals between cleaning and re-greasing will range from some months to some years. Greasing is an expensive procedure and should only be done if other remedies prove to be uneconomical.
Washing
Insulators can be washed with fixed or hand-held equipment. The hand-held equipment can be operated from the ground, from a platform or from a helicopter. For effective washing, care should be taken with the design of the spray nozzles and in the case of live washing, the conductivity of the water is also extremely important. Manual washing requires a sharply-focused jet, and cleaning is commenced at the bottom of the insulator or string. With automatic systems the spray must be such that the full length of the insulator is uniformly wetted. Water conductivity must be very low. The maximum conductivity which can be tolerated will depend on nozzle design and the water flow rate.
Washing as an anti-pollution measure requires a high capital investment, but generally the annual cost is less than the cost of greasing.
Application of a tough hydrophobic coating
The performance of glass or porcelain insulator can be significantly improved by applying a coating which bonds with the insulator surface and has hydrophobic properties. These coatings are usually silicone based and can be applied by brush or spray-gun. It gives a remarkable improvement in performance. Long time experience of these coatings is very limited and while it is expected that they will have a long live, inspections at intervals of two to three years are recommended.
