Salt fog testing of RTV coated ceramic insulators and comparison with HTV silicone rubber insulators

In the Gulf of Mexico, climatic conditions with strong wind (more than 40 km/h maintained) are known as “Norths”. A “North” generates an abrupt change in the pollution level on the external insulation of power lines in a few hours. So, the pollution level is influenced strongly by climatic conditions and its behavior is very dynamic. In these conditions, the main pollutant deposited on the insulation is salt from the sea. RTV coated external insulation is frequently used in transmission lines and in substations to control pollution problems; however, the cost of this alternative for polluted areas is more expensive than the use of non ceramic insulators. Reliability for RTV coated insulators is certainly different than that for non ceramic insulators; this is the reason why some utilities prefer RTV coated insulators despite their high cost. In this work salt fog testing was done to evaluate the performance of RTV coated insulators and silicone rubber insulators during continuous conditions of high voltage and salt fog. Strings of six insulators were installed into the salt fog chamber and energized to 69 kV phase to ground, the NaCl concentration was 10 kg/m³. Insulator strings with short leakage distance either with or without RTV coating presented flashovers before 40 testing hours, while on insulator strings with large leakage distance and with RTV coating, the three flashovers developed between 400 and 500 hours. One silicone rubber insulator did not reach the flashover and the developed leakage current was lower than the measured in the coated insulator strings. It seems to be that the RTV coating is saturated with pollutant in a short time during continuous dry band arcing, consequently, hydrophobicity is lost after some hours causing flashover. The obtained results are discussed in this paper.