Investigation On The Insulation Characteristics And Synergistic Effect Of SF₆ Alternative Ternary Gas Mixtures

Sulfur hexafluoride（SF₆）is an insulating gas widely used in power equipment and has strong greenhouse effect.Therefore,the development of environment-friendly alternative insulating gas is of great significance to China’s strategic goals of“peaking carbon dioxide emissions”and“achieving carbon neutrality”.Due to the liquefaction temperature,the existing environmentally friendly insulating gas must be mixed with the buffer gas before it can be used in power equipment.The study on the insulation performance and synergistic effect of the SF₆ alternative ternary gas mixtures can provide a theoretical framework for the selection of insulating gases in power equipment,and formulate an alternative plan that not only meets the requirements of environmental protection and safety,but also that of the insulation and operating temperature of power equipment.In this paper,electron transport parameters of CF₃I,c-C₄F₈,C₃F₈ and buffer gases CO₂,N₂,CF₄ are calculated by using Boltzmann equation.The critical electric field intensity（E/N）_cr and synergistic effect coefficient of ternary gas mixtures with different mixing ratios are obtained.The results show that CF₃I gas molecules with CO₂/N₂ or CO₂/CF₄ ternary gas mixtures have better insulation performance for single-strength ternary gas mixtures,whereas the ternary gas mixture composed of c-C₄F₈ and N₂/CF₄ has better insulation performance.The synergistic effect coefficient also conforms to the above-mentioned trend.Meanwhile,the20%c-C₄F₈/20%N₂/60%CF₄ gas mixtures have the largest critical breakdown field strength,（E/N）_cr=241.1Td,which is about 67.7%of the SF₆critical breakdown field strength.For double-strength ternary gas mixtures,c-C₄F₈/CF₃I combination has better insulation performance.Positive synergy even occurs when the proportition of strength insulating gases are high.Among the three buffer gases,the combination of CO₂ and strong insulating gas has a good synergistic effect,while the combination of CF₄ and these gases has a negative synergistic effect.However,because of CF₄gas molecules’good insulation properties,the ternary gas mixture formed with CF₄has better insulation performance.The critical breakdown field strength of5%c-C₄F₈/15%CF₃I/80%CF₄ is 243.8Td,which is about 68.5%of the critical breakdown field strength of SF₆.Through macroscopic experiments,the insulation performance changes and synergistic effect of single-strength and double-strength ternary gas mixtures with slightly uneven electric fields under different voltage types,gap distances,and air pressures are studied.The results show that for single-strength ternary gas mixtures,the insulation performance of c-C₄F₈ gas mixtures is better than that of the CF₃I gas mixtures with the same component mixing ratio;the synergistic effect of CF₃I/CO₂/N₂ and c-C₄F₈/CO₂/CF₄ is better under the power frequency;and the synergistic effect of CF₃I/N₂/CF₄ and c-C₄F₈/N₂/CF₄ is better under the lightning impulse voltage.For double-strength ternary gas mixture,CF₃I/c-C₄F₈/N₂ has a higher breakdown voltage at low pressure,whereas CF₃I/c-C₄F₈/CF₄exhibits a better insulation performance as the air pressure increases.In short,the ternary gas mixture shows a good synergistic effect under both power frequency and lightning impulse voltage.Through mathematical regression analysis and similar comparative analysis,a comprehensive analysis of the microstructure parameters of the gas molecules,the microscopic and macroscopic parameters of the ternary gas mixture is carried out.It is found that there is a strong correlation between the lowest unoccupied molecular orbital energy,the highest occupied molecular orbital energy,and the gas insulation performance.At the same time,there is also a strong correlation between the vertical ionization energy,vertical affinity energy,and the synergistic effect coefficient of the ternary gas mixture.The order of the correlation between microscopic parameters and the relative insulation performance of the ternary gas mixture and is:effective ionization coefficient>electron diffusion coefficient>drift velocity of electron>average energy per electron;whereas the order of the correlation between microscopic parameters and the synergistic effect coefficient is:average energy per electron>drift velocity of electron>electron diffusion coefficient>effective ionization coefficient.In the macroscopic parameters,the relationship between the voltage type and the breakdown voltage of the ternary gas mixtures is:50%positive electrode breakdown voltage>50%negative electrode breakdown voltage>power-frequency breakdown voltage.As the air pressure and gap distance increase,the breakdown voltage increases.However,macroscopic parameters do not have a strong influence over the synergistic effect.On the other hand,the influence of the voltage type on the synergistic effect is:power frequency voltage synergistic effect coefficient>positive lightning voltage synergistic effect coefficient>negative lightning voltage synergistic effect coefficient.The influence of air pressure and gap distance on the synergistic effect coefficient is basically consistent with the law that the synergistic effect coefficient increases as the air pressure and gap distance increase.Furthermore,the liquefaction temperature and GWP value of the ternary gas mixtures that meet the environmental requirements are also calculated,and it is found that the application of the buffer gas combination effectively reduces the liquefaction temperature of the ternary gas mixtures.In addition,the GWP value of the ternary gas mixtures is much lower than that of SF₆.This indicates that the studied ternary SF₆ alternative gas mixtures have important environmental protection value.