Effect of pollution on high-voltage outdoor insulators

High-voltage insulators may flashover at working voltage when polluted and wet. Pollution is deposited by various forces, of which aerodynamic ones are shown to be usually the most important. Theories to explain the pollution-flashover process are described. The performance of insulators in a polluted environment may be assessed by tests in natural conditions, or by the use of artificial methods, which are critically examined. The influence of a number of parameters, such as surface conductivity, overall length, leakage-path length and profile on insulator flashover is discussed. Studies of the effect of voltage waveshape show that direct voltage gives flashover at the lowest peak stress for a given pollution severity. Pollution flashover can be prevented by washing or greasing or by employing oil-bath or resistive-glazed insulators or special insulator assemblies. The advantages and disadvantages of these remedies are outlined. An analysis of the behaviour of polluted surge diverters and insulators with special functions introduces extra problems which are discussed. Organic insulating materials are degraded by tracking or erosion, but insulators using these materials have advantages, especially for ultra-high-voltage systems. An important aspect of the pollution problem which needs further study is the measurement of the site-pollution severity, so as to classify insulation requirements, and several methods of measurement are described.