Photos courtesy Eskom
“ If you go from guessing ESDD to actual measurement, your band of uncertainty reduces and you can fine tune selection of suitable insulator dimensions and geometry.”
I Example of testing at NETFA for corona on different insulator and grading ring assemblies. examined for each insulator will include hydrophobicity and electric field distribution, in addition to leakage current. It is also planned to remove the insulators after a year to allow close-up visual examination before putting them back up.
Narain refers to the upcoming generation of insulators planned for Eskom’ s future 765 kV network and notes,“ we are dealing in this case with composite insulators where corona ring design and placement may prove even more vital to performance than the silicone rubber housing. Narrain also points out that Eskom philosophy has always been that insulator corona rings must take care only of the insulator and not necessarily aim to also protect associated hardware.“ For this reason,” he says,“ you will find separate grading rings for hardware on a lot of our 400 kV and 765 kV lines.”
In many ways, South Africa’ s topography can be compared to an
upside down saucer in that there is a sharp rise from most coastal areas to plateaus that typically reach 1500 m or more. This means that a high proportion of Eskom lines have their insulation affected by altitude and therefore local test laboratories, such as the National Electric Test Facility have been intentionally sited to operate under reduced air density( in the case of NETFA at 1625 m).
The entire process of insulator selection at Eskom has been systematically refined in recent years so as to remove decisions based mostly on past experience. Instead, a large body of reliable information has been compiled on pollution across the country to record and track environmental stresses at every line and substation.
Says Narain,“ in the past, selecting insulation was driven mainly by experience and perceptions. Now, we are moving to eliminate such subjectivity from decision-making and rather base our line insulation specifications not on guessing ESDD levels but rather on actual measurements that provide us with a statistical representation of the real pollution situation. This reduces the band of uncertainty and means we can fine-tune our selection of ideal insulator dimensions and geometry. For example, instead of specifying from 30-40 mm / kV, we can be precise and state we want e. g. 35-36 mm / kV.”
Narain goes on to explain that pinpointing pollution levels across the country and correlating these against the flashover behavior of actual insulator profiles from a test laboratory makes it possible to plot stress versus strength.“ We put all this type of information together to arrive at the optimal insulator choice,” he says.“ In other words, once we know the flashover characteristics of any particular insulator’ s profile( i. e. the flashover voltages under different pollution severities) we can be certain of
68