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Testing for silicone rubber
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2019-01-12
Test methods designed for ceramic insulators have not shown good correlation with actual service experience when applied to polymeric materials. Existing standards have been designed primarily for testing insulators made from porcelain, a static surface that has no interaction with contaminants; it will readily wet out. To improve the performance of insulators made from such materials, manufacturers simply increase creepage distances.
For this reason, they can be tested by continuous arc exposure, which is designed to estimate long-term performance. Unfortunately, the procedure has little resemblance to actual service conditions with silicone rubber units. Silicone sheds control leakage currents to help prevent high-energy dry band arcing from occurring. Similarly, some tests call for a massive build-up of contamination to be applied to simulate long service, followed by energized exposure to moisture and salt fog. Silicone rubber, however, has low surface energy and causes water to bead up. Its hydrophobic nature helps prevent contaminant buildup, and also allows a rain storm to help clean the surface. Applying a large dose of contamination is an unrealistic condition, one that silicone formulations are designed to avoid. Further, the natural encapsulation of particulates cannot occur if many years’ worth of contaminants are applied all at once for a short-term test.
Developing new test methods and equipment specifically for polymeric materials is currently a priority for utilities, universities and research organizations. In one advancement, the Electric Power Research Institute (EPRI) High-Voltage Transmission Research Center in the USA has developed a fog chamber method that allows testing with variable contamination levels. A specific insulator design can be subjected to a number of stresses that closely resemble the actual service environment. Despite the test refinements, much of the industry still relies on procedures developed decades ago for porcelain. While some manufacturers have adopted specific techniques to evaluate polymer compounds, they can vary widely from one company to another. Differences in process conditions among fabricators also complicate the development of standard compounds for molding HV insulators. As a result, some silicone suppliers have opted for a custom formulation approach. The industry is rapidly accepting tests that appear to correlate better to field service
For this reason, they can be tested by continuous arc exposure, which is designed to estimate long-term performance. Unfortunately, the procedure has little resemblance to actual service conditions with silicone rubber units. Silicone sheds control leakage currents to help prevent high-energy dry band arcing from occurring. Similarly, some tests call for a massive build-up of contamination to be applied to simulate long service, followed by energized exposure to moisture and salt fog. Silicone rubber, however, has low surface energy and causes water to bead up. Its hydrophobic nature helps prevent contaminant buildup, and also allows a rain storm to help clean the surface. Applying a large dose of contamination is an unrealistic condition, one that silicone formulations are designed to avoid. Further, the natural encapsulation of particulates cannot occur if many years’ worth of contaminants are applied all at once for a short-term test.
Developing new test methods and equipment specifically for polymeric materials is currently a priority for utilities, universities and research organizations. In one advancement, the Electric Power Research Institute (EPRI) High-Voltage Transmission Research Center in the USA has developed a fog chamber method that allows testing with variable contamination levels. A specific insulator design can be subjected to a number of stresses that closely resemble the actual service environment. Despite the test refinements, much of the industry still relies on procedures developed decades ago for porcelain. While some manufacturers have adopted specific techniques to evaluate polymer compounds, they can vary widely from one company to another. Differences in process conditions among fabricators also complicate the development of standard compounds for molding HV insulators. As a result, some silicone suppliers have opted for a custom formulation approach. The industry is rapidly accepting tests that appear to correlate better to field service
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