Breakdown at Multiple Protrusions and Surface Roughness in SF6 and CO2 - A Study of the Enlargement Law
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- Institutt for elkraftteknikk 
Gas insulated components are important in the modern transmission system. The mostcommon gas used in high pressure insulation is Sulphur Hexafluoride (SF6). SF6 has beenclassified as a greenhouse gas and a potential replacement is CO2, either by itself or inmixtures with other gases. The insulation properties of gas insulation depends on severalparameters, e.g. surface roughness. An electrode with a built-in precision motor was usedin a quasi-uniform gap. This thesis aims to investigate the impact of needle protrusionswith lengths in the range 0.05 2 mm in SF6 at 4 bar absolute pressure and CO2 at 6bar absolute pressure. Another aim is to investigate the applicability of the enlargementlaw. The results with a single needle were used to create predictions, according to theenlargement law, for 20 and 100 identical needle protrusions. The electrode used allowedneedle arrays of 20 and 100 needles to be tested with needle lengths in the range 0.05 2mm. The enlargement law was also investigated for two sandblasted surface areas.The results in SF6 showed a decrease in the 50 % average background breakdown field,E50, with increasing needle lengths for both polarities. With 20 needles E50 decreased fromthe single needle results and agreed well with the predictions for both polarities. Withpositive polarity and 100 needles, there was no significant change in the results betweenthe 20 and 100 needles and there was a discrepancy with the predictions. With 100 negativeneedles there was a good agreement between predictions and results, showing a decreasefrom the results with 20 needles. The sandblasted surfaces showed a decrease from thesmall area (2.383 cm2) to the large area (64.465 cm2). The reduction in E50 showed goodagreement with predictions with negative and disagreed significantly with positive polarity.The obtained results in CO2 showed an insensitivity to single needle protrusion with needlelengths shorter than 1 mm with positive and 0.75 mm with negative polarity. With 20needles, there was an increased sensitivity and reduction for both polarities. E50 with positivepolarity decreased with increasing needle lengths. Negative polarity had a significantdecrease of E50 from 0.1 to 0.5 mm where it seemed to saturate between 0.5 2 mm needlelengths. For 20 needles both polarities agreed well with predictions. With 100 needles,there was no longer an agreement between predictions and results for either polarities. theresults with positive polarity decreased further. With negative polarity similar behavioras with 20 needles was seen, where the saturation occurred at 0.2 mm needle lengths andended at the same field strengths as with 20 needles. The sandblasted surfaces showedmuch more of a decrease than what was predicted based on the small area.The experimental results were analyzed with Turnbull s algorithm to determine the empiricalcumulative distribution function, which was fitted with the three parameter Weibulldistribution. From the distribution functions the 2 % average background breakdown fieldwas found. Possible physical explanations and breakdown mechanisms of the results werediscussed. The results showed that SF6 was more sensitive to the needle protrusion thanCO2. The main conclusion is that the enlargement law must be used with care. It is possiblethat the mechanisms involved in breakdowns scale differently than just with increasednumber of needles or area and require further investigation to understand.