INMR Volume 21 - Number 2 (Issue 100) | Page 69

At the moment, Eskom’ s transmission network is about to undergo a fundamental shift, with a planned new backbone of 765 kV. Says Nishal Mahatho, an insulator specialist at the utility’ s Research & Innovation Department,“ much of our system is still based on 400 kV and 275 kV, although we have two 765 kV lines insulated with glass that operate in the middle of the country. But now we have changed philosophy and are moving in a phased approach toward a 765 kV grid and by 2020 to a ± 600 kV DC system as well. This means that there is even more work than normal going on when it comes to insulator research.”
One example is a project already underway at KEPCO’ s Gochang Test Station in Korea where five different designs of composite insulators, already approved by Eskom, are being compared to one another and also to the standard glass strings used on existing 765 kV lines in South Africa. The environment is coastal and every effort has been made so that the comparison is as realistic as possible, even using the same planned conductor bundle configuration.
Mahatho explains that leakage current is being measured on each insulator design and correlated against weather conditions. The insulators are installed in double I-strings and are apparently among the first to be monitored using sophisticated new leakage current sensors from EPRI in the U. S. Periodic UV and IR testing with special cameras is also being performed on each.
Says Mahatho,“ the idea was to compare how the different materials and designs from Eskom approved manufacturers perform in a full scale energized environment. For example, because of the high voltage, electric field will be one of the issues that might contribute most to premature ageing.” He goes on to state that the test is expected to last 2 years with at least 90 % of this time under voltage and that parameters
“ With the new 765 kV composite insulators planned for our network, corona ring design and placement will be even more vital to performance than the silicone rubber housing material.”

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400 kV lines re-insulated with silicone rubber.
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