Electrical and material characterization of field-aged 400 kV silicone rubber composite insulators

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.