DRY BAND FORMATION AND ITS EFFECT ON PERFORMANCE OF UHV SURGE ARRESTERS

The metal oxide varistors are used as voltage dependent switching devices to protect high voltage equipments and also low voltage electronic devices against transient over-voltages. The performance of Zno Surge Arrester (SA) is determined by the electrical and, thermal properties of metal oxide elements, insulating materials used and design features of SA. The primary consideration in applying Metal Oxide SA is the thermal consideration of environment. Ambient temperature and environmental conditions effect directly on the performance of, internal current drawn by and energy input to the Metal Oxide SA. The interaction between the polluted porcelain housing and the inner varistor columns due to capacitive coupling is responsible for temperature rise of varistor element. Metal Oxide SAs are exposed to heavy thermal and electrical stresses if the porcelain housings of the units are polluted by industrial dusts, coastal salts and etc. The first part of this paper proposes an electrical model of SA that is computer simulated and the second part presents the results of pollution tests and the effect of dry band on the performance of S.