THE BEHAVIOUR OF DIELECTRIC MATERIALS UNDER POLLUTION

This paper examines the suspension insulator design characteristics which affect their performance in service on transmission lines in polluted areas. In the first section the mechanism by which pollution can give rise to flashover is explained in four stages, namely deposit build-up, wetting, dry band formation and arcing, and increasing arc length leading to flashover. The example of a 132 kV line in Dubai is used to demonstrate the build-up of pollutant on the upper and lower surfaces of insulator shells of different shapes. The insulator shown in this study are the standard toughened glass shell profile and the desert application aeodynamic profile. The line in Dubai is subject, in some areas, to mixed industrial and coastal pollution, and in other areas to desert pollution. The build-up of both types of pollution is examined. A study at a test station in Saudi Arabia is presented, which compares the build-up of pollution on glass and porcelain fog-type insulator shells. These "in-service" examples are followed by a discussion of the limitations of the laboratory test methods used to imitate the different types of pollution. Shell design and inter-rib spacing are considered with respect to their effect on effective leakage distance. This is followed by a section which explains some of the ways in which suspension insulators deteriorate due to pollution, including cement growth, corroa ion of metal parts and deterioration of the shell surface.This paper concludes on the importance of insulator behaviour in the design and construction of transmission lines in polluted areas, stating that both shape and leakage distance play a major role in determining that behaviour.