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

To address the issues of greenhouse gases and resultant global warming, there is an urgency to find an electronegative gas to replace SF6 in global power system networks. This article 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 (BDVs) under both polarities rise linearly with gap distance in quasi-uniform fields with the positive being higher. Increasingly nonuniform fields lead to saturation in the case of positive BDV, while in the negative polarity case, the BDV increases linearly. As a result, the two polarities’ BDV magnitudes crossover as field nonuniformity 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 2–5 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 indoor applications in cold-climate countries.