Readers of INMR are certainly aware that RTV coatings have achieved a permanent place in the diverse group of silicone elastomers used in outdoor power applications. These special materials were originally developed and applied to turn the hydrophilic surface of glazed porcelain or glass into a hydrophobic one and thus improve pollution withstand performance.
Most often, these coatings have been applied to electrical apparatus already in service but found to be exposed to more severe pollution than originally estimated. The material has also been used to extend the service life of deteriorated housings (see article by Sklenicka & Brückner in INMR Issue 110).
These days, after decades of good service experience and continued improvement, room-temperature-curing (RTV) methyl-vinyl-polymer material is no longer being applied only ‘after the fact’ to existing power installations. More and more OEMs have begun to transform virgin ceramic and glass insulators into a new type of product that they offer.
One can therefore regard the coating as a type of a composite insulating component even though it consists only of a thin water repellent layer added to glass or porcelain materials. Unlike conventional composite insulators, final shape is determined by the structure being coated. While not really a housing material, the coating still functions basically as a ‘property- modifying’ finish with respect to wetting behavior.
Looking into IEC 62039, TR ‘Selection guide for polymeric materials for outdoor use under HV stress’ it becomes clear that coatings are not being well covered by the recommendations given for the housing materials of composite insulators. Is it not time to add a standard for finishing materials?
Experience shows that coatings do their job as long as the surface along the creepage distance of the coated insulator remains water repellent to at least some extent. Thus, it is quite interesting that recent salt-fog testing of coated porcelain insulators show a fully water repellent surface, even after 1000 h.
In my view, there is still lack of a suitable document to define properties and minimum requirements for thin VMQ-coatings in their typical applications.
Dr. Jens Lambrecht