Experience with a composite insulator testing instrument based on the electric field method

Two large electric utilities, located on the US West Coast, use a considerable number of composite insulators on their power transmission lines. Some of these insulators may be prone to premature insulation degradation caused by poor design, manufacturing defects, pollution or construction mishandling, or gunshot vandalism. This may result in flashover and power outages, unless the state of the insulators can be evaluated periodically and the defective ones replaced in time. Also for reasons of economics, replacement of defective composite insulators is often done using bare-hand live line maintenance methods. Maintenance personnel safety can be improved if an appropriate testing method is available to determine if sufficient insulation remains in the insulator. This paper describes laboratory and field evaluation tests that were performed with a new type of insulator tester to determine if it is suitable to assess the remaining insulation level. These tests have demonstrated that conductive defects, sometimes as short as 2.5 cm (1"), and conductive pollution deposits are detectable with this type of instrument.