Corona-induced degradation of nonceramic insulator housing materials

Nonceramic insulators (NCI) can be subjected to corona for long periods of time even under relatively clean conditions due to localized high electric field brought about by design and/or manufacturing deficiencies. 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, ethylene propylene diene monomer (EPDM) and a blend of EPDM and silicone polymer have been evaluated. 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.