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

The influence of long-term corona on the degradation of non-ceramic composite insulator (NCI) housing materials was studied. Polymer samples were obtained from full-scale NCI, employing three different housing materials, namely silicone rubber (SIR), ethylene propylene diene monomer (EPDM) rubber and 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 the relative humidity level and application of mechanical stress. Periodic monitoring of corona discharge magnitude and pulse repetition rate allowed for a simplistic computation of the energy needed for initiation of surface changes. A comparison of these calculated values with expected conditions in the field was made. It is postulated that even under severe operating conditions over a period of many years, degradation of the housing materials evaluated due to water droplet corona is highly unlikely.