Effect of long-term corona on non-ceramic outdoor insulator housing materials

The influence of long-term corona on the degradation of non-ceramic composi te insulator (NCI) housing materials was studied. Polymer samples were obta ined from full-scale NCI, employing three different housing materials, name ly silicone rubber (SIR), ethylene propylene diene monomer (EPDM) rubber an d a blend of EPDM with silicone rubber. The effect of relative humidity and externally applied mechanical stress as acceleration factors was assessed. Several modes of degradation were observed, namely, cracking, roughening, and discoloration. In general the severity of degradation increased with th e relative humidity level and application of mechanical stress. Periodic mo nitoring of corona discharge magnitude and pulse repetition rate allowed fo r a simplistic computation of the energy needed for initiation of surface c hanges. A comparison of these calculated values with expected conditions in the field was made. It is postulated that even under severe operating cond itions over a period of many years, degradation of the housing materials ev aluated due to water droplet corona is highly unlikely.