Field Trials of 400 kV Silicone Rubber Composite Insulators in a Coastal Location

A number of silicone rubber composite insulators have been taken out of service after 15 years on a twin-circuit, 400 kV transmission line located in a coastal region in the south west of the UK. Visual observations and contact angle measurements are presented. The various surfaces of the insulators were degraded in a non-uniform manner, both in the rotational symmetry of the insulator and also along the length of the string. Leakage current and surface discharge measurements carried out in the laboratory are also presented. Deterioration of performance is identified, and this is found to vary along the length of the insulators.