INMR Volume 21 - Number 2 （Issue 100) | Page 62

Photos: INMR ©
With a sprawling network comprising some 300,000 km of transmission and distribution lines, Eskom has had a long history of problems due to insulator failures. When the country’ s 400 kV‘ backbone’ transmission grid was established between 1966 and 1969, the first such lines employed glass cap & pin strings with the relatively short creepage of 15 mm per kV. In coastal areas, however, industrial and marine pollution combined with local weather patterns caused a high incidence of flashovers, especially during late winter and early spring. In addition, vandalism at various locations resulted in numerous broken glass discs. Locally produced porcelain cap & pin insulators were tried as an alternative but soon abandoned because of frequent punctures in areas with lightning.
Given these types of problems, by the late 1970s some of the glass cap & pin strings on Eskom’ s transmission system began to be replaced by porcelain long rods having a specific creepage distance of 25 mm / kV. Although superior performance was expected due to their single unit construction and aerodynamic profile, the number of flashovers actually increased. Various mitigation techniques, such as hand washing, were then considered but such maintenance proved costly and mostly ineffective. By 1987, a program of washing from helicopters was introduced in its place, partly because of the speed with which the work could be done and also because it was often difficult for trucks to reach affected lines.
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corona rings on polymeric tension insulators, surge arresters and composite transformer bushings attest to localized pollution affecting 400 kV Stikland Substation near Cape Town.
According to Eskom maintenance personnel, porcelain long rods required frequent washing in some areas and it was difficult to predict when was the correct time. To help, a device was built based on evaluating the radio interference generated when pollution reached a critical level. Nevertheless, the cost of helicopter washing also proved high and, during extreme pollution events, even periodic washing did little to prevent flashovers. Nor was increasing specific creepage distance of strings an option because of limitations associated with minimum clearances.