Application of pre-coated RTV insulators in UHV OHLs and steep-front impulse voltage test

In order to solve the problem of pollution flashover in the strain insulator-strings on transmission line in China, a large number of insulators with RTV silicone coatings are used for the transmission line. This paper summarize the experience of using RTV coatings for disc insulators on transmission lines in China, the main existing problems of RTV operation, such as poor adhesion, hydrophobicity decline, powder and product pollution, are pointed out. Some problems are related to the way of spraying on the tower in jobsite, because the detection methods are not effective and the quality of spraying could not be controlled, at the same time. So, the pre-coated RTV insulators have been widely used in UHV projects. Experimental studies show that pre-coated RTV insulators have a clear technological advantage in the quality of appearance, thickness uniformity, adhesion and so on. But it also founds that the combination of disc insulator with RTV coating leads to the sharp decline of steep-front impulse breakdown performance of insulator. Most of breakdown happened at the negative voltage polarity moment. Under the negative polarity voltage, the discharge channel tends to select the RTV coatings surface.