Silicone Rubber Insulators: Impacts of Material Formulation in Coastal Environment

Doctoral thesis, 2002

This thesis deals with influences of material formulation on the performance of silicone rubber high voltage insulators in a coastal environment. Samples and insulators with known differences in material compositions have been used in this task. In the first step, a screening test with cylindrical samples was initiated. The electrical performance was quantified by counting the number of peak leakage current values. In addition, the samples were visually inspected for erosion and hydropho-bicity during the test. Afterwards, the materials were analyzed for chemical changes and a dominating aging mechanism was identified. It was found that the thermal energy supplied by short pulsed discharges was the most probable origin for the observed surface changes. These discharges resulted in consumption of the added flame retardant filler and depolymerization of the polydimethylsiloxane matrix of the rubber. Moreover, no significant surface oxidation was found. The electrical performance was improved by increasing the ATH filler content and by adding extra silicone oil. Still, addition of extra silicone oil was judged as unnecessary as the low molar mass siloxane content did not decrease during the aging and that regeneration of such diffusible molecules had taken place. The results from the screening test were utilized for refining the test plan and selecting material formulations for simultaneous tests with cylindrical samples and real insulators having identical material formula-tions. An essential requirement, when the influences of material composition were evaluated in respect to insulator performance, was that the tested insulators had an identical geometry. Such insulators were designed and produced within this work. The highest electric field strength at the insulator surface was reduced to 0.54 kV/mm by means of specially selected terminations, to suppress degradation caused by water droplet corona. The electric field distribution along the insulator was simulated by finite element calculations. In addition, the rotating wheel dip test was used for accelerated aging. This method evaluated the early aging period of hydrophobic materials and both the applicability of this method as well as the performances of the tested materials are evaluated.