This selective re-insulation reportedly resulted in customers experiencing from 100 to 200 fewer voltage depressions each year. In addition, there were cost savings as well from no longer having to wash lines. Excluding customer benefits, the US $ 7 million reportedly invested in the project had a payback of less than 10 years while also solving persistent past problems such as vandalism and pin corrosion on glass strings located near the seacoast.
Over the following years, successive generations of composite insulators found their way onto Eskom’ s network and the utility began developing expertise in assessing which designs were good and which were not. The basic strategy was to become‘ an informed buyer’ rather than simply depending on external sources. Part of this philosophy also involved fully understanding the actual service environment for insulators by monitoring pollution levels on a regular basis( see article on p. 76).
Eskom insulator expert, Wallace Vosloo, has helped implement this policy for years and published an influential text covering the field. He remarks,“ Many utility people at distribution voltage levels of 132 kV and below thought that IEC 61109 Annex C was all that was needed to guide their specification of composite insulators, as at transmission voltages. But, in our case, we soon found out that it was simply not applicable to our environment.”
As proof of this statement, Vosloo notes that in 2003, when Eskom instituted its program of qualifying potential insulator suppliers at the Koeberg Insulator Pollution Test Station( KIPTS), only 60 % of the products tested over the subsequent 3-year period passed. Moreover, many of those that failed the tests also failed in service, even though these often fulfilled all the requirements set down in IEC 61109 Annex C. This, he concludes, demonstrated that meeting this standard alone – at least as far as South Africa was concerned – was not sufficient proof of expected good performance in service.
These days, even with a fastgrowing population of composite insulators selectively installed throughout the country, widespread flashovers still occur in parts of South Africa due to different underlying causes. Sanjay Narain, one of Eskom’ s Chief Engineers in transmission line engineering and insulation design, explains that in KwaZulu Natal, for example, these problems are usually linked to high local humidity combined with the frequent burning of sugar cane. The Western Cape, by contrast, tends to be dry and widespread bush fires occur in cycles of about every 7 years, due to the time needed for re-growth of vegetation. These fires create high loading of airborne particulate matter that drifts out to sea and mixes with salt, before being blown back to land and settling onto lines. Clean insulators can become critically polluted in a very short time. The last major flashover problem in the area occurred in 2007 and the one before that, in 2000, apparently blacked out almost the entire Cape Town region.
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( left) Greased porcelain breaker housing at Ascot 66 / 11 kV Substation in Cape Town shows high pollution loading. Pollution deposition on polymeric arrester at 132 / 66 kV Belhar Substation.
Photos: INMR ©
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