Experimental Research On Power Frequency Breakdown Characteristics Of CF₃I/N₂ Gas Mixtures Under Different Electric Field

Researchers have been striving to find an environmental friendly gas to replace SF6 gas. Recent years, CF3 I and its mixed gases are considered to be the most promising alternative gases to replece SF6 as an insulating gas. Foreign scholars have studied some researchs about breakdown performance of CF3 I and its mixed gases, but mainly concentrated on pure CF3 I and CF3 I gas mixtures under narmal pressure. The power frequency breakdown performance considering muti-factors is still lack of systematic research.Based on the disadvantage of the existing gas-insulated test device in the laboratory, a new experimental device has been designed. The device uses inner tank nested within the outer tank to improve the experimental feasibility and accuracy. A needle-plate, a hemesphere ball rod-plate and a ball-ball electrode models were designed to simulate extremely non-uniform, slightly non-uniform and quasi-uniform electric field respectively. The relationship between power frequency breakdown voltage of the CF3I/N2 gas mixtures and gas pressures, mixing ratios, electric field utilization factor was studied through power frequency breakdown test, and the power frequency breakdown voltage of the CF3I/N2 gas mixtures was comparatively analyzed with the SF6/N2. Pure CF3 I gas has an excellent insulating recovery property. The electric fields sensitivity of pure CF3 I is higher than SF6. Frequency breakdown strength of pure CF3 I under extremely non-uniform, slightly non-uniform and quasi-uniform electric fields is 0.76, 0.93, 1.04 times of SF6 respectively, and CF3I/N2 gas mixtures are 0.88, 0.93, 0.97 times of SF6/N2 under the three electric fields respectively. The more uniform of electric fields, the more excellent power frequency breakdown characteristics of CF3I/N2 compared with SF6/N2 gas mixtures. Overall, power frequency breakdown strength P values of CF3I/N2 compared with SF6 gas increase as the increase of gas pressure and CF3 I volume fraction. Power frequency breakdown performance of CF3I/N2(50%/50%) gas mixtures is almost the same with pure SF6 when the gas pressure is 0.3MPa. From the perspective of frequency breakdown performance, CF3I/N2 of high CF3 I volume fraction and high pressure has more application potential.The paper proposed a quantitative analysis method using the C value to determine the type and strength of gas mixtures synergism. CF3I/N2 gas mixtures present a positive synergistic effect totally under extremely non-uniform electric field, its synergistic effect is much better than SF6/N2 of C value 0.27, which makes up for the poor insulation performance of pure CF3 I under extremely non-uniform electric field. So CF3I/N2 gas mixtures have more value than pure CF3 I. C value of CF3I/N2 gas mixtures is 0.22 and 0.34 under slightly non-uniform and quasi-uniform electric field. CF3I/N2 gas mixtures show a synergistic effect, but weaker than SF6/N2 of C value 0.20 and 0.22. With the increase of electric field utilization factor, synergistic effect of CF3I/N2 gas mixtures becomes weaker. The paper make a quantitative analysis for the CF3 I volume fraction from the perspective of liquefaction temperature, CF3I/N2 gas mixtures have application value when CF3 I volume fraction less than 50%.The related properties of CF3 I, SF6 and its gas mixtures with N2 were compared, considering the power frequency breakdown performance, liquefaction temperature and environmental impact of the three factors, CF3I/N2(CF3I contains 20%~50%) gas mixtures may replace SF6 gas for gas-insulated equipment in certain situations.