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This paper presents the electrical and material characteristics of field-aged silicone rubber composite insulators, which have been deployed for 15 years on a 400 kV transmission line in a coastal region of the UK. There were no indications of reduced performance in service. Observations indicate non-uniform aging of the insulators on the different surfaces of the insulator sheds and core, along the insulator string length and in the different compass orientations. A uniquely large number of contact angle measurements, made on each of the insulators' different surfaces, confirm the visual aspect of nonuniform aging. Electrical investigations of the insulators have been performed in terms of leakage current analysis, ac flashover / withstand and switching-surge impulse flashover. A correlation is seen between the ac leakage current and the hydrophobicity measurements. Energy dispersive X-ray (EDX) and Fourier transform infrared spectroscopy (FTIR) analysis of the changes of the materials' surface chemistry is presented. The role of solar radiation and organic growth appears critical to the observed non-uniform aging and must be considered if the longer term operation of the insulator is to be forecast. It is proposed that a reduction of the difference between positive and negative wet-flashover voltages may be a good indicator of early insulator aging. [ABSTRACT FROM AUTHOR]

The practice of backfihling existing paper/mineral oil insulated cables with dodecylbenzene (DDB) in the UK has produced systems where the fluid phase is, in reality, a mixture of these two oil types. This investigation therefore set out to study the ageing behavior of a range of DDB/minerat oil blend systems. Samples ranging in composition from 100% mineral oil to 100% DDB were aged in air (oxidising) and nitrogen (inert) atmospheres and the resulting systems were analyzed using a range of physical and chemical techniques. All approaches produced a consistent picture, demonstrating that the addition of between 10 and 25 % of mineral oil to DDB significantly improves its resistance to ageing under the various conditions imposed in this study. Such blend systems exhibit a combination of properties that renders them superior to either mineral oil or DDB atone. [ABSTRACT FROM AUTHOR]