Effects of DC Polarity and Field Uniformity on Breakdown of SF6 and C3F7CN/CO2 Mixture

To address issues of greenhouse gases and resultant global warming, there is an urgency to finding an electronegative gas to replace SF6 in global power system networks. This paper provides an experimental study of various factors on DC breakdown of electronegative gases. Rod-plane geometries with a range of rod diameters (3.5-12.5 mm) and electrode separations (5-55 mm) are used to provide different degrees of field uniformity. SF6 and a 20% C3F7CN / 80% CO2 mixture are tested under pressures ranging between 1 and 5 bar. Breakdown voltages under both polarities rise linearly with gap distance in quasi-uniform fields with the positive being higher. Increasingly non-uniform fields lead to saturation in the case of positive breakdown voltage, while in the negative polarity case the breakdown voltage increases linearly. As a result, the two polarities’ breakdown voltage magnitudes crossover as field non-uniformity increases. The value at which the negative value exceeds the positive is dependent on the field uniformity, pressure and gas medium. A simulation based on the streamer criterion model provides a good agreement with experimental results for positive DC breakdown in the range of 2 to 5 bar. In terms of the insulation characteristics, 20% C3F7CN / 80% CO2 could provide a valuable alternative to SF6 in high voltage plant for outdoor applications in hot-climate countries and indoors for cold-climate countries.