Impact of corona on the long-term performance of nonceramic insulators

A majority of nonceramic insulator (NCI) application is under relatively clean conditions where leakage current may be nonexistent. Occurrence of corona for long periods of time on NCI operating under relatively clean conditions is possible due to localized high electric field (E-field) brought about by design and manufacturing deficiencies. Therefore, resistance to corona-induced degradation of NCI housing materials is an important factor that impacts NCI long-term reliability, and is the focal point of this paper. Two sources of corona have been investigated: corona from a metal electrode, and corona from water droplets. Field inspections were performed on 230 and 500 kV insulators to demonstrate the existence of corona even under relatively clean and dry conditions. Three commonly used NCI housing materials, namely high temperature vulcanized (HTV) silicone rubber (SIR), ethylene propylene diene monomer (EPDM) and a blend of EPDM and silicone polymer were evaluated in the laboratory. In addition, housing materials from several insulators removed from the field were analyzed. Data gathered from electrical and physicochemical analyses show that corona induced degradation is dominantly due to a combination of electro-chemical reactions rather than thermal effects.