Study On Gas Discharge Parameters And Insulated Synergistic Effect Of SF₆ Alternative Gases

SF6 gas has excellent insulation and arc extinguishing performance.However,SF6 is also a greenhouse gas,the Kyoto Protocol signed in 1997 designated SF6 gas as one of the six emission-limited gases.Therefore,it is urgent to find SF6 alternative gas which could be used as a kind of new dielectric for various high voltage electrical equipment,and the gas should be conducive to green environmental protection.At present,the research has not found that any pure gas can replace SF6 completely.Most studies are focused on gas mixtures.If we want to obtain insulation characteristics of gas mixtures under different types,different mixing ratios and different pressure conditions,it means that a lot of experimental work and enormous cost.Thus,electron swarm parameters such as the ionization and attachment coefficients in different gas mixtures could be obtained by experimental and computational methods in this paper.The relation of effective ionization coefficient with field strength and the critical values of electric strength(E/N)lim will be obtained.The synergistic effect between different gas mixtures would be analyzed,which could help to optimize the composition of more types of gas mixtures.In this paper,test platform was built based on the SST experimental principle.The ionization and attachment coefficients in C3F7CN/CO2 and C3F7CN/N2 gas mixtures with different proportions were obtained,and the ionization and attachment coefficients in SF6 gas mixtures and CF3I gas mixtures with different proportions were calculated by the Boltzmann equation on the basis of various collision cross sections of SF6,CF3I,CO2 and N2 gases.The relationship between electrical strength {E/N)lim and mixing ratio was also obtained.The results showed that the electric strength of the C3F7CN gas mixtures was significantly higher than that of other gas mixture.When the content of C3F7CN in the gas mixtures was less than 30%,the electric strength of C3F7CN/N2 gas mixtures was higher than that of C3F7CN/CO2 gas mixtures.And when the content of C3F7CN gas was higher than 30%,the electric strength of C3F7CN/N2 gas mixtures was lower than that of C3F7CN/CO2 gas mixtures.The electric strength of SF6/N2,CF3I/N2 gas mixtures was lower than that of SF6/CO2,CF3I/CO2 gas mixtures and with the increase of the content of SF6 or CF3I in the gas mixture,the gap decreased.the synergistic effect coefficient was introduced to characterize the synergistic effect of electric strength in this paper.The results showed that the synergistic effect of C3F7CN/CO2 and C3F7CN/N2 gas mixtures were approximately equal at low ratios.When the content of C3F7CN gas was more than 20%,the synergistic effect of C3F7CN/CO2 gas mixtures was better than that of C3F7CN/N2 gas mixtures,and the difference between them increased with the increase of C3F7CN gas content.The synergistic effect of SF6/N2 gas mixtures was better than that of SF6/CO2 gas mixtures.The synergistic effect of CF3I/CO2 gas mixtures was the same as CF3I/N2 gas mixtures.Finally,the relationship between the electric strength of gas mixtures and the breakdown characteristics was analyzed,and it was found that they may be inconsistent.Therefore,it is inaccurate to select a substitute only based on the electrical strength of gas mixtures.It is necessary to combine the electric strength and the synergistic effect coefficient of gas mixtures to select SF6 alternative gases.According to the analysis results in this paper,the SF6/N2 gas mixtures and the low ratio C3F7CN/CO2 and C3F7CN/N2 gas mixtures were selected as potential SF6 alternative gases.