The condition and performance of insulators on substations and overhead transmission and distribution lines contributes to the overall reliability of a power system. As these insulators are exposed directly to the environment it exists, thus pollution present and carried by air will deposit on insulator surface when wet. These deposits will produce conductive surfaces that may contribute to flashovers.
Insulation flashovers have caused multiple outages to some areas especially on highly contaminated ones. Several ways have been proposed in the industry on mitigating these flashovers, which were discussed on our previous blog. However, among the listed countermeasures, anti-pollution flashover coating, room temperature vulcanized silicone rubber as the highly recommended type, is the most effective way to prevent flashovers.
What is a room temperature vulcanized “RTV” silicone rubber?
Room temperature or RTV coating has been extensively studied and rapidly developed due to its excellent experience in the industry. This method was applied as early as 1970s and have had over 30 years and 13 years on alternating current (AC) and direct current (DC) systems, respectively.
The components of RTV silicone rubbers are the base silicone polymer (with the case of Midsun, polysiloxane base is used) with extending fillers, condensation catalyst, adhesion promoter, pigment, reinforcing filler and a cross-linking agent (some types exclude other components). These components are dispersed in a solvent which acts as a medium of transfer the RTV rubber to the applied insulator surface. The evaporation of the solvent, forming moisture in the air, will then trigger the vulcanization process which forms the solid rubber coating. This process depends on the following – type of solvent, cure system chemistry, temperature and humidity.
One of the characteristics of a RTV coated insulator that surpasses all other types of insulator is its hydrophobicity under outdoor weather and high voltage conditions. Hydrophobicity is a property of a surface that has the ability to repel water. This water-repellent property suppresses leakage current and improves flashover voltage, especially on highly polluted areas. An investigation was conducted as to the use of RTV coatings in heavily contaminated areas. Results of the said investigation showed that RTV coatings could suffice a long term protection against pollution flashovers even in heavily contaminated areas.
An interview with Farah AlShatti -an experienced engineer based in Kuwait- discussed the problems they faced in the electrical network especially on weather conditions. Chemical components, caused by nearby factories, build up on the surface of the insulator that cause electrical network problems. Air pollution as well contributes to the deposition of dusts and sand on the insulator surface. However, with the application of RTV coatings, it protects the insulator from flashover caused by the build up of chemical materials, dusts and sand. Another advantage of applying RTV coatings to their electrical components is the money and resources saved from the traditional washing.
Another study was conducted in Malaysia on the effects of RTV coating on the electrical performance of insulators under lightning conditions. An impulse lightning test was conducted at different conditions – dry, wet and polluted, to determine the breakdown voltage capacity of an insulator. Study shows that RTV coatings were more effective especially on a polluted condition, as it increased the insulator’s voltage withstand capacity by 50% under a lightning impulse.
An economical evaluation was conducted by Iran University of Science and Technology for using RTV coating compared to periodical washing and application of silicone grease. Annual cost for periodical washing will reach about 160.2 Million Rials, and to be considered as operating cost annually. Application of silicone grease will cost as high as twice the cost of periodical washing for about 322.6 Million Rials, and also to be considered as operating cost annually. However, RTV coating with its 10-year life span, based on this study, would definitely cost higher than the other two for about 526.3 Million Rials but the application will be done every after 10 years. In conclusion, RTV coating is very economical considering a thirty (30) year maintenance period as shown in Table 7 Comparison between maintenance method cost.
Several studies, investigations and evaluations including the abovementioned were conducted to support the effectiveness of RTV coating in terms of flashover protection and cost savings. RTV coatings are now made available globally by different suppliers and manufacturers. Midsun Group Inc., based in USA, specializes in providing a range of RTV silicone coatings in highly contaminated areas through its Midsun High Voltage Insulator RTV Coating or MIDSUN HVIC. This type of RTV coating is constantly tested globally by many high voltage laboratories in conformance with the international standards.
National Center for Biotechnology Information. Farah Adilah Jamaludin (November 14, 2017). Effects of RTV coating on the electrical performance of polymer insulator under lightning impulse voltage condition. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5685609/
Research Gate. Mohammad Ali Talebi (February 2005). Technical & Economical Evaluation of Using Silicone Rubber RTV Coating for HV Substation in Polluted Area. https://www.researchgate.net/publication/327755488_Silicone_Rubber_RTV_Coating_of_HV_Substations_and_Over_Head_Lines
IEEE Std. 1523-2018, IEEE Guide for the Application, Maintenance and Evaluation of Room-Temperature Vulcanizing (RTV) Silicone Rubber Coatings for Outdoor Ceramic Insulators