Round robin testing of RTV silicone rubber coatings for outdoor insulation

This paper presents the results of round robin tests performed to evaluate the electrical performance of Room Temperature Vulcanizing (RTV) silicone rubber coatings used for improving the contamination performance of outdoor ceramic insulators. The tests, performed in six different laboratories, included several salt-fog chambers and a tracking wheel. Most test facilities were equipped with an on-line data acquisition system for measurement of leakage current activity. Four generic formulations of RTV silicone rubber coatings applied on ceramic rods were evaluated. The results indicate a general consensus in the ranking of the coatings' leakage current suppression capability, despite the variation in the experimental conditions employed. This suggests that guidelines for a laboratory test for evaluating the RTV coating material's electrical performance under contaminated conditions could be developed. The use of RTV silicone rubber coatings for improving the contamination performance of high voltage ceramic insulators has become a well established practice. There is enough evidence from service experience to show that RTV coatings offer long term cost and performance advantages over other methods for improving contamination performance, such as, grease coatings, water washing, cleaning and semi-conducting glaze coatings. The dominant factor responsible for the superior performance of RTV silicone rubber coatings is their long term ability to retain a high surface resistance even under wet and contaminated conditions, thereby minimizing leakage current to insignificant levels. This ability to maintain a high surface resistance is related to the property of surface hydrophobicity, which has been extensively studied. For economic reasons, utilities are interested in the useful life, of RTV coatings. As with all polymeric materials used for outdoor insulation, aging that occurs due to exposure of the material to natural elements and electrical discharges during service, is of concern. The aspect of evaluating the long term electrical performance of polymeric insulating materials via accelerated aging laboratory tests is extremely challenging and still not adequately resolved. Presently there is no single test method that can duplicate all aspects of aging that occurs during service. For a laboratory test to be useful for evaluating the contamination performance of RTV coated insulators, it is important that the test uses as samples, actual insulators coated in methods consistent with utility practice. The electric stress and its distribution in service should be reproduced. In addition, the type and quantity of contamination simulated should be representative of actual conditions, which in itself varies with geographic location. Thus there are a number of variables which complicate the test development. Nevertheless, progress can be achieved if the underlying mechanisms that cause degradation can be identified and duplicated in the laboratory. Recognizing this, the committee embarked on a round robin test program. The interaction between the surface, moisture and contamination can be more easily studied on simple geometries and does not need elaborate test facilities. An initial evaluation from a number of test arrangements that are popularly used in industry and a comparison with field data helps in identifying the merits and limitations of the test procedures. Test procedures and experimental conditions that lead to degradation inconsistent with field experience could be eliminated, and those that yield more meaningful results could be pursued for standardization for evaluating RTV coating materials. Another objective of the round robin test program was to determine the effect of test method on the ranking of the coatings performance. With variations in RTV coatings that are pos-sible, it would be of practical interest to establish a method for ranking the various coatings. Conclusion: The results indicate a general consensus in the ranking of the coatings' leakage current performance, despite the variation in the experimental conditions employed. This suggests that guidelines for a meaningful accelerated aging laboratory test for evaluating RTV coating materials could be developed.