Negative DC Breakdown Characteristics of C₃F₇CN / CO₂ Gas Mixture for Application in High Voltage Accelerators

Abstract

This study investigates the technical feasibility of retro-filling a C 3 F 7 CN/CO 2 gas mixture for existing SF 6 -insulated accelerators. Negative DC breakdown characteristics of 20% C 3 F 7 CN / 80% CO 2 gas mixture and SF 6 for varying pressure, gap distance and field uniformity are analyzed experimentally using four distinctly different electrode configurations. The C 3 F 7 CN/CO 2 gas mixture shows a higher breakdown strength than SF 6 for configurations with high field uniformity, whereas SF 6 outperforms C 3 F 7 CN/CO 2 gas mixture under the non-uniform field. Importantly, 20% C 3 F 7 CN / 80% CO 2 gas mixture exhibits comparable insulation capability to SF 6 in a 10/30 mm coaxial geometry with similar field uniformity as found in the practical gas-insulated equipment. The results of this work contribute to the development of a potential retrofill solution using 20% C 3 F 7 CN / 80% CO 2 gas mixture for accelerators.

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

“…A voltage ramp rate of 5 kV/s and a time interval of 2 minutes between each breakdown and next voltage ramp were selected. Ramp rate and time intervals were previously shown to have a negligible effect on the BDV [6]. In the result figures, each data point represents the average BDV, U50 of 30 breakdowns and the error bar is represented by the standard deviation, σ.…”

“…For negative polarity, as the test gap widens (i.e., non-uniform field), SF6 has a comparatively higher BDV than 20% C3F7CN / 80% CO2 mixture. This can be attributed to CO2 being a weakly attaching gas, which might result in a higher probability for detachment of electrons or negative ions [6], [18].…”

“…elevated pressures. A mixture of 20% C3F7CN content could potentially replace SF6 gas as it matches its breakdown strength in uniform and quasi-uniform field configurations under lightning impulse (LI) [4], AC [5] and DC [6] voltage stresses. Furthermore, a 20% C3F7CN / 80% CO2 mixture has a liquefaction temperature of -8 °C at 5 bar pressure [5], where it meets the minimum temperature requirement for outdoor applications in hot-climate countries and indoor applications for cold-climate countries in accordance with IEC 62271-203: 2011 [7].…”

Negative DC Breakdown Characteristics of C3F7CN and CO2 Mixture with Different Electrode Materials

“…Previous investigations show that a 20% C3F7CN / 80% CO2 gas mixture have comparable breakdown performance to SF6 when tested for uniform and quasi-uniform field configurations in lightning impulse (LI), AC and DC voltage stresses [1][2][3][4]. As a result, 20% C3F7CN / 80% CO2 mixture ratio has been chosen for this study.…”