The performance of high voltage insulators is strongly affected by environmental
pollution in contaminated regions. Extreme weather conditions, ocean spray, salt fog, cement dust, airborne agricultural dust and all other kinds of pollution can cause an insulator flashover.
In my experience, I know this is the equation you have to fear:
Deposition of contaminants + Water droplets dripping = Dry-band Arcing leads to Leakage Currents and end up in Flashovers
In order to avoid it, here are 4 known ways to mitigate insulator flashovers:
1. Water washing
Energized washing of substation, distribution and transmission insulators is often the fastest method to maximize system performance. It’s a short-term solution and has safety implications because a flashover might occur during energized washing. Additionally, it has extensive lifetime costs.
2. Increase the creepage level
Creepage extenders have been used to prevent pollution flashover on insulators for over 20 years. The extenders are sealed to porcelain or glass insulators, driving the high leakage currents found in polluted areas around the edge of the extender’s skirt. The downside: by increasing the surface path over an insulator it limits leakage current, although in less frequency, you still have to wash the insulators and, finally, flashovers may still occur during long periods of “No Rain” and high humidity.
3. Silicone grease
Since the 1960’s, utilities have used "silicone grease" with good results to prevent the effects of pollution on the insulators surface. Its water repellent and arc track resistant surface, it encapsulates pollutants, has a good performance and lower lifetime costs than water washing. The disadvantages: it gets saturated in a very short period of time (between 6 months and 2 years) and therefore, needs replacing.
4. RTV Coatings
Room Temperature Coatings are specifically formulated for high voltage insulators. They manage to suppress the development of leakage currents due to a dynamic and interactive surface which retards water film formation and contamination solubility.
How do they protect? Hydrophobic surface prevents water filming, then the surface oil coats the contaminant particles and renders them hydrophobic. This reduces the contaminants ability to combine with water to form an ionic solution. The RTV coating maintains these features over the long life of the installation. They are the most competitive solution over long term, by providing arc track resistance additives, unparalleled UV resistance, Superior adhesion, wide temperature stability and hydrophobicity.
If you’d ask me which of these 4 ways I would recommend to avoid insulator flashovers, I would definitely go for an RTV silicone high voltage insulator coating.
I cannot stress enough the fact that the initial cost of comprehensive RTV Coating does not compare to the long-term maintenance and repairs of not implementing coating. Still, any other mitigation approach is better than no mitigation at all.
Imad Kanouni
Electrical Engineer