Modeling of DC Breakdown Characteristic in Coaxial Geometries for SF₆ and its Alternatives

There is an increasing scrutiny on the use of sulfur hexafluoride (SF₆) due to its high global warming potential (GWP) with numerous alternative gases and mixtures being researched. This work proposes a single protrusion streamer criterion model to predict the DC breakdown voltage for SF₆ alternatives in coaxial geometries close to the optimal ratio as found in practical equipment. DC breakdown characteristics of SF₆ and CO₂ gases and mixtures of 7% C₃F₇CN, 20% C₃F₇CN and 4% C₅F₁₀O with CO₂ are experimentally analyzed for pressures of 1-9 bar absolute, conductor/enclosure diameters of 8/30, 10/30, 15/30, 15/45 and 20/60 mm, and maximum surface roughness (R_z) between 8-121 µm. Results show that, 20% C₃F₇CN / 80% CO₂ mixture and SF₆ have comparable DC breakdown characteristics in the investigated coaxial field geometries and surface roughness. The majority of the experimental and calculated results are within ±5% for different coaxial sizes, pressures (>1 bar), gas mediums, DC polarities and conductor surface roughness. The key advantage of this modeling approach is the ability to provide a reasonable estimation of DC breakdown voltage over a range of representative test conditions, which optimize the selection process of new SF₆ alternatives without having to perform significant level of experimental activity.