ELECTRICAL PERFORMANCE OF NON-CERAMIC INSULATORS IN ARTIFICIAL CONTAMINATION TESTS - ROLE OF RESTING TIME

The paper presents the results of an investigation on the electrical p erformance of artificially contaminated non-ceramic insulators as a fu nction of resting time. Resting time is defined as the time interval b etween the application of contamination and start of testing. New (una ged), full scale, 69 kV non-ceramic insulators using different types o f silicone rubber and ethylene propylene diene monomer (EPDM) rubber a s weathersheds were studied. Their electrical performance was assessed using the clean fog technique. The applied voltage was kept constant throughout the test, while contamination severity and resting time wer e varied as parameters. The transfer of hydrophobicity through the con tamination layer was demonstrated by flashover tests. This was also vi sually depicted by analyzing samples in a scanning electron microscope . The results show that resting time exerts little influence on the el ectrical performance of EPDM insulators. However, for silicone rubber insulators, resting time is shown to drastically improve its electrica l performance, Experimental evidence indicates that the transfer or re covery process of hydrophobicity in silicone rubber insulators occurs as a progressive superposition of silicone oil layers with time. Thus, the net effect of resting such insulators before tests can be thought of as a gradual reduction of the effective contamination layer thickn ess. Such a reduction alters the way in which the contaminant layer in teracts with external stressing agents, which could impact the insulat or's electrical performance in service. In this work, an attempt has b een made to identify and quantify the electrically significant changes introduced by a reduction in the effective contamination thickness. I t is in the light of this reduced effective contamination layer that w e explain how a seemingly wettable silicone rubber insulator is still able to hold the applied voltage without flashover.