Studies On Silicone Rubber Nanocomposites As Weathershed Material For HVDC Transmission Line Insulators

Abstract

Outdoor insulators are one of the most important parts of a power system. The reliability of a power system depends also on the reliability of the insulators. The main functions of an insulator used for outdoor applications are to give the necessary insulation, provide the necessary mechanical support to the transmission line conductor and also to resist the various environmental stresses like pollution, ultra violet rays etc. Traditionally porcelain and glass insulators have been used for outdoor insulator applications. They are good insulators under normal conditions and the cap and pin arrangement allows them to take up the mechanical load of the line. But owing to their large weight and brittle nature they are susceptible to vandalism and also they have increased cost of installation and commissioning. But the main problem of porcelain and glass insulators is its performance under polluted environmental condition. Under wet and polluted conditions, the porcelain insulators allow the formation of a conducting layer on the surface which results in setting up of leakage current, dry band arcing and power loss. This problem is further augmented under dc voltages where the stress is unidirectional and the contaminant deposition is higher as compared to ac. Polymeric insulators are a good alternative for porcelain and ceramic insulators for use especially under dc voltages because of their good pollution performance. The property of surface hydrophobicity resists the setting up of leakage currents and hence polymeric insulators help in reducing power loss. They are also light in weight and vandalism resistant and hence easier to install. But being polymeric, they form conductive tracks and erode when exposed to high temperatures which occur at the surface during dry band arcs and when exposed to corona discharges. The surface hydrophobicity is also temporarily lost when exposed to different electrical stresses. Silicone rubber is the most popular among the various choices of polymers for outdoor insulator applications. They have good surface hydrophobicity and tracking performance. But polymers in their pure form cannot be used as insulators because of their poor mechanical strength. Adding inorganic fillers into the polymer matrix not only improves its mechanical properties but also its erosion resistance. Micron sized Alumina Trihydrate (ATH) is used traditionally to improve the tracking and erosion resistance of polymeric insulators. A very high loading (up to 60%) is used. Adding such a high filler loading to the base polymer hampers its flexibility and the material processing. With the advent of nanotechnology, nano fillers have come into vogue. Studies conducted on nano filled polymers showed exciting results. A small amount of nano fillers in the polymer matrix showed significant improvement in the mechanical strength without hampering its flexibility. The electrical properties like tracking and erosion also improved with filler loading. Hence the use of nano filled silicone rubber is a good alternative for use as a high voltage insulator especially under dc voltages. Reports suggest that adding nano fillers into the silicone rubber matrix improves the tracking and erosion resistance and the corona degradation as compared to the unfilled samples under ac voltages. The literature on the dc performance of silicone rubber nano composites is scarce. So the present study aims to evaluate the performance of silicone rubber nano composites for tracking and erosion resistance and corona degradation under dc voltages. The tracking and erosion resistance under dc voltages was measured using the Inclined Plane Tracking and Erosion Resistance set up as per ASTM D2303 which was modified for dc voltage studies. The performance of nano Alumina and nano Silica fillers were evaluated under negative dc and the performance was compared with micron sized Alumina Trihydrate filled samples. The effect of filler loading was also studied. It was seen that the performance of the silicone rubber improved with filler loading. A small loading percentage of nano fillers were enough to give performance similar to silicone rubber filled with micron sized ATH filler. The silicone rubber performed better under negative dc as compared to ac and positive dc. The positive dc tests showed a migration of ions from the electrodes onto the sample surface. The increased surface conductivity resulted in very heavy erosion in the case of positive dc tested samples. The corona aging studies were also conducted on silicone rubber nano composites. Nano silica was used as filler in this case. Different filler loadings were employed to understand the effect of filler loading. The corona was generated using a needle plane electrode and samples were exposed to both positive and negative dc corona. The samples were exposed to corona for different time intervals – 25 and 50 hours to study the effect of exposure time. The hydrophobicity, crack width and surface roughness were measured after the tests. Adding nano fillers into the polymer matrix improved the corona performance. With filler loading, the performance improved. The samples exposed to positive dc corona performed better than those under negative dc corona. The loss of hydrophobicity, surface cracks and the surface roughness was less in the case of positive dc corona tested samples. With exposure time, the performance of silicone rubber became poorer for positive dc corona tested samples. For the negative dc corona tested samples, the performance seemed to improve with exposure time. The tracking and erosion resistance and the corona aging studies conducted showed that the performance of silicone rubber is improved by adding nano fillers into the polymer matrix. A small amount of nano filler loading was enough to perform similar to a heavily loaded micron filled sample. Hence nano fillers can be used as a good functional material to improve the performance of silicone rubber insulators especially under wet and polluted conditions.