Impact of weathering on flashover performance of nonceramic insulators

Nonceramic (composite) insulators that were in service on 69-230 kV transmission lines for periods up to 25 years were evaluated in the laboratory with the objective of assessing future performance of similar devices in the field. These insulators had housings made from different formulations of silicone rubber and ethylene propylene diene monomer (EPDM) rubber. They were removed from relatively clean locations where weathering was probably the most dominant aging factor. Some EPDM insulators had flashed over during service. Using a combination of scanning electron microscopy (SEM), electrical surface resistance under wet conditions, and a theoretical model for contamination flashover, it has been shown that it is possible to predict the flashover performance in service, based on the thickness of the degraded surface. A threshold value has been established beyond which flashover is likely to occur in the field. This information can be used for planning insulator replacement or maintenance.