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 most common 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 in mixtures with other gases. The insulation properties of gas insulation depends on several parameters, e.g. surface roughness. An electrode with a built-in precision motor was used in a quasi-uniform gap. This thesis aims to investigate the impact of needle protrusions with lengths in the range 0.05 – 2 mm in SF6 at 4 bar absolute pressure and CO2 at 6 bar absolute pressure. Another aim is to investigate the applicability of the enlargement law. The results with a single needle were used to create predictions, according to the enlargement law, for 20 and 100 identical needle protrusions. The electrode used allowed needle arrays of 20 and 100 needles to be tested with needle lengths in the range 0.05 – 2 mm. 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 from the single needle results and agreed well with the predictions for both polarities. With positive polarity and 100 needles, there was no significant change in the results between the 20 and 100 needles and there was a discrepancy with the predictions. With 100 negative needles there was a good agreement between predictions and results, showing a decrease from the results with 20 needles. The sandblasted surfaces showed a decrease from the small area (2.383 cm2) to the large area (64.465 cm2). The reduction in E50 showed good agreement with predictions with negative and disagreed significantly with positive polarity. The obtained results in CO2 showed an insensitivity to single needle protrusion with needle lengths shorter than 1 mm with positive and 0.75 mm with negative polarity. With 20 needles, there was an increased sensitivity and reduction for both polarities. E50 with positive polarity decreased with increasing needle lengths. Negative polarity had a significant decrease of E50 from 0.1 to 0.5 mm where it seemed to saturate between 0.5 – 2 mm needle lengths. 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. The results with positive polarity decreased further. With negative polarity similar behavior as with 20 needles was seen, where the saturation occurred at 0.2 mm needle lengths and ended at the same field strengths as with 20 needles. The sandblasted surfaces showed much 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 empirical cumulative distribution function, which was fitted with the three parameter Weibull distribution. From the distribution functions the 2 % average background breakdown field was found. Possible physical explanations and breakdown mechanisms of the results were discussed. The results showed that SF6 was more sensitive to the needle protrusion than CO2. The main conclusion is that the enlargement law must be used with care. It is possible that the mechanisms involved in breakdowns scale differently than just with increased number of needles or area and require further investigation to understand.