DC Breakdown Characteristics of C3F7CN Gas Mixtures for High Voltage Insulation Applications

Abstract

The drive towards decarbonising the electricity grid and the high global warming potential (GWP) of sulphur hexafluoride (SF6), 24,300 times greater than CO2 and with an atmospheric lifetime of 3,200 years, has led to an increasing level of research activity to find a suitable alternative candidate with a significantly lower GWP. This thesis investigates the technical viability of adopting C3F7CN/CO2 gas mixtures as an alternative insulation medium to SF6 gas for DC gas insulated lines (GIL) and gas insulated busbars (GIB). A single protrusion streamer criterion model has been developed to accurately predict the DC breakdown voltage for screening new SF6 alternatives without recourse to experiment. Two types of electrode configurations are used to examine the DC breakdown characteristics of SF6 and CO2 gases and mixtures of 4% C5F10O, 7% C3F7CN and 20% C3F7CN mixed with CO2 at 1-9 bars of pressure and rod-plane and reduced-coaxial electrodes with a range of field uniformities between 0.07-0.65 and 0.48-0.69 respectively, including the effects of electrode material and conductor surface roughness. This provides a range of field uniformities and conductor practical surface finishes as found in the designs of GILs, GIBs and switches inside ring main units. Results demonstrate that 20% C3F7CN / 80% CO2 and SF6 have comparable DC breakdown characteristics in the investigated geometries and surface roughness. Thus, 20% C3F7CN / 80% CO2 could provide a valuable alternative to SF6 in a high-voltage plant. It is also noted that CO2, 4% C5F10O / 96% CO2 and 7% C3F7CN / 93% CO2 show a weaker dielectric performance than SF6. The calculated results are in strong agreement with experimental results for positive DC breakdown in a rod-plane configuration with a pressure range of 2-9 bars. In coaxial geometry close to the optimal ratio, 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 relevant to practical applications. Therefore, this modelling approach has the ability to provide an accurate estimation of DC breakdown voltage over a range of representative test conditions, optimising the selection process of new SF6 alternatives.