| Summary: | This thesis presents investigations of field-aged composite silicone rubber insulators, which have been deployed for 15 years on a 400 k V transmission line in a coastal region of the UK. Extensive measurements of their hydrophobicity, as detennined by contact angle, are given along with a description of their appearance. The hydrophobicity change varies from the low voltage end to the high voltage end with the lowest contact angles being found in the middle of the string. The sheds also aged differently around their circumference and this was reflected in discoloration differences on different sides of the insulator, in addition to hydrophobicity changes. Leakage current and surface discharge measurements carried out in the laboratory are also presented. Material analysis shows cracking and oxidation of the surface. Ageing effects were far from uniform over each shed, the greatest oxidation being on the top surfaces of the sheds on the south side: Microscopy, EDX and FTIR were found to be the most useful and effective tools for analysis of these polymeric insulators. The measurements of the various parameters over the whole surface of the insulators are found to be consistent with each other and can be interpreted in tenns of the chemistry of ageing and the longer term performance of the specific insulators in question. Two distinct types of ageing are identified. The role of solar radiation appears critical. It is suggested that wind direction, which is very consistent in the region concerned, may also be an important consideration. It is concluded that a possibility exists of an accelerated reduction in performance because of the non-uniformities observed. The asymmetry adds to the complexity of any model generated, and must be considered if the longer term operation of the insulator is to be forecast.
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