In-Service Surface Degradation of MV Composite Insulators under Severe Environmental Conditions and Low Electric Stress

Ethylene propylene diene monomer (EPDM) tension insulators, rated for 15 kV, have been recovered after approximately seven years in service. The samples were recovered from three different 11 kV systems all of which were located less than 3 km from the coast. Some differences have been identified between their environmental conditions, of which azimuth appears to be the most significant. Four characteristic regions are identified by orientation and physical appearance. The maximum local values of electric field have been determined using FEM, and were found to be 3.5 kV/m or 35 kV/m, depending on the circuit, which is a low intensity of electrical stress. High roughness and complete loss of hydrophobicity are readily observed consequences of ageing of the material surfaces. Electrical and physicochemical tests have been carried out to identify the changes to polymer properties, and the resulting impact on insulator performanc e. A small degree of recovery of hydrophobicity was observed after a few months out of service in some cases. Corrosion was found on end fittings of one of the evaluated circuits. I.