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 (Rz) 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.