In the first half of the last century HV transmission lines had been insulated with insulator strings made of ceramic and glass material. High-grade electrical ceramic / glass of the proper composition free from laminations, holes and cooling stresses is the recognized dielectric for insulating high voltage lines. The perfection in manufacturing processes and specified tests have brought the product to a standard of uniformity. However, as discussed above, the main problem with ceramic and glass high voltage insulators are that water readily forms a continuous film on their surface. In the presence of contamination, leakage current then develops which may lead to flashover that could be followed by an outage of the power system.

Utilities have developed a number of techniques for coping with contamination. High-pressure washing, silicone dielectric greases and RTV silicone coatings have all been applied to ceramic and glass insulators. Each method has its drawbacks. Washing of insulators is quite costly and more over there may be certain areas where the contaminant is so adherent that it does not wash off and where the contaminant simply overwhelms any reasonable amount of insulation. Silicon greases offer a longer –lasting solution, as silicone fluid within the grease formulation encapsulates contaminants, rendering them nonconductive and retaining a hydrophobic surface. However, the grease will eventually be overwhelmed by contamination, at which time it must be removed and replaced (generally from three months to five years, depending on the severity of the conditions).

 

From: Internet