Thermal stress on ZnO surge arresters in polluted conditions. I. Laboratory test methods

Abstract

S. V i t e t E l e c t r l c i t b de FranceNon-member Clamart. FranceAhstract -The thermal performance of ZnO surge arresters in polluted conditions were studied in the laboratory and the field. Part I of the paper deals with laboratory test methods such as the salt fog, solid layer, partial wetting, and slurry method. The thermal stress imposed on the arresters by the different test methods in terms of pollution severity and temperature rise, is discussed.Several procedures for verifying the performance of surge arresters operating under polluted conditions have been developed during the years. Laboratory test methods exist for both the older gap-type S i c arresters as well as the more recent gapless ZnO surge arrester. Despite more than one decade with ZnO surge arresters on the market, however, the testing of ZnO arresters with regard to pollution performance is not yet covered by international standards. s Rackprnund Historically, it can be noted that older types of S i c arresters were rather sensitive to external surface pollution. Changes in thevoltage grading of the spark gaps due to pollution on the insulator surface could lead to sparkover of the arrester. Each sparkover resulted in a follow current that stressed the gaps thermally and reduced the ability of the arrester to extinguish against selvice voltage. Thus, repeated sparkovers could eventually lead to an arrester failure. Later S i c arrester types, however, showed an improved pollution performance due to the introduction of grading capacitors and low+hmic nonlinear grading resistors.The main purpose of pollution tests on S i c surge arresters was to verify that the arrester did not spark-over under polluted conditions. When the first ZnO surge arresters were introduced, the same pollution test procedures were adopted as for the S i c arresters. Soon it was found that these tests had limited or no effect on the ZnO arresters, simply due to the lack of spark gaps. The only measurable effect on these arresters was a possible increase in varistor temperature, emanating from the uneven surface voltage distribution created by the wet pollution layer.A considerable discussion on relevant test methods for ZnO arresters has therefore been going on during recent years [l-71. The main problem seems to be that the appropriate test procedures for verifying the pollution performance of ZnO arresters are not necessarily the same as for SIC arresters or high-voltage insulators. WM 013-3 PWRD by the IEEE