Environmentally friendly gases have been widely investigated to replace sulphur hexafluoride (SF6) in compressed gas insulated equipment for the power industry. This work presents the experimental investigations on the gas stability of HFO1234ze(E) (hereinafter referred to as C3H2F4) where a unique first breakdown voltage behavior is found for AC, DC and lightning impulse (LI) voltages. A splatter of dark colored soot is observed on the plane electrode after the first breakdown that is followed by consecutive LI breakdowns chopped on the front. The breakdown characteristic stabilizes after the first voltage withstand and the 50% breakdown voltage (U50) is calculated using the subsequent stabilized data series. This first breakdown behavior of C3H2F4 is consistently observed for different tested electrode materials and configurations. The voltage difference between the first breakdown and U50 varies significantly due to the work function of the electrode material and electric field uniformity. For AC breakdowns with comparatively higher discharge energy, the data series is more consistent after the first breakdown but with significant level of soot formation indicating that this is the main influencing factor for the first breakdown effect. Soot is identified as fluorinated carbon through X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) analyses.

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.

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.

Sulphur hexafluoride (SF6) is an insulating medium widely used in the power industry due to its high dielectric strength and arc quenching capability. There is a growing interest in identifying an environmentally friendly alternative to SF6 and a mixture of C3F7CN and CO2 is one potential candidate. This project investigates the effect of different electrode materials on the breakdown performance of the aforementioned gas mixture over 300 breakdowns under negative DC. Three sets of rod-plane electrodes manufactured in stainless steel, aluminum and brass are tested for a fixed gap distance of 3 mm and pressure of 4.8 bar absolute. The results show that stainless steel has the highest breakdown voltage and the lowest change in surface roughness compared to aluminum and brass. The post-testing gas analyses have shown negligible reduction in the C3F7CN content after 300 breakdowns.

SF6 alternatives with significantly lower environmental impact have been extensively researched. This paper investigates two electrode configurations and test pressures to mimic the conditions as found in MV/HV equipment to systematically compare the breakdown performance of SF6, C3H2F4 and a mixture of 30% CF3I / 70% CO2 under AC and LI stresses. SF6 possesses comparatively better breakdown performance than the other alternatives investigated in this study. C3H2F4 shows "first breakdown" behavior and only the stable data series is considered for evaluation of 50% breakdown voltage, U50. The lower boiling points of C3H2F4 and CF3I limit its potential application to medium-voltage equipment with a low operating pressure. Due to pyrolysis, there is formation of soot and iodine post-breakdown in C3H2F4 and 30% CF3I / 70% CO2 respectively. This will reduce their dielectric performance and could pose health hazard to maintenance personnel.