Study On Power Frequency Discharge Law Of C₄F₇N/CO₂ Mixed Gas

SF₆ gas is widely used in electrical equipment such as Gas-insulated switchgear?GIS?and Gas-insulated transmission line?GIL?in high voltage engineering.However,SF₆ has strong greenhouse effect.It is a major demand of the electric power industry to study SF₆ alternative gas.The mixture of perfluoroisobutyronitrile?abbreviated as C₄F₇N?and CO₂ developed jointly by GE and 3M Company has a promising prospect as an insulating medium to replace SF₆.Knowing the power frequency discharge law of C₄F₇N/CO₂ mixed gas is the premise of equipment insulation design.The discharge voltage of the mixed gas is mainly affected by the mixing ratio,pressure,electrode distance,electrode surface roughness,environmental temperature and so on.Most of the existing studies use the discharge test method to obtain the discharge law under the influence of some factors such as the mixing ratio and pressure.But limited by the test conditions,there are few data points under high pressure and large spacing,and there is no universally applicable law.Moreover,few studies consider the influence of electrode surface roughness and temperature,so the discharge law obtained is not comprehensive.Therefore,in this paper,the power frequency discharge law of C₄F₇N/CO₂ gas mixture is studied by the method of combining calculation with experiment,considering three main factors of mixing ratio,electrode surface roughness and ambient temperature.The main work and conclusions are as follows.?1?The influence of mixing ratio on the discharge of C₄F₇N/CO₂ mixture was studied.In order to solve the problem that the data of ionization and attachment coefficient of mixed gas are only in a specific ratio,and it is impossible to study the discharge in arbitary ratio,this paper proposes the Wieland approximation method to calculate the net ionization coefficient of C₄F₇N/CO₂ mixed gas in arbitary ratio,from which the critical reduced electric field is obtained.By comparing with the data in the literature,the Wieland approximation method is verified to be effective.Based on the streamer criterion,the calculation model of discharge voltage of mixed gas is established.The calculated discharge voltage shows synergistic effect with the increase of ratio,which is consistent with the breakdown test results reported in the literature.In order to explain the synergistic effect,the physical meaning of the synergistic effect coefficient is analyzed.The theoretical derivation shows that the synergistic effect coefficient is equal to the ratio of the slope of the net ionization curve of two pure gas components.At a lower ratio,the ratio is smaller,which makes the mixture have a strong synergistic effect.When the synergistic effect is fully utilized,the relationship between the optimized mixing ratio and the coefficient of synergistic effect is put forward through theoretical deduction,and the optimized ratio of C₄F₇N/CO₂ is in the range of 9%?15%.?2?The discharge law of C₄F₇N/CO₂ gas mixture considering the electrode surface roughness was studied.In order to solve the problem of huge error caused by the saturation of discharge voltage under high pressure,this paper establishes the calculation model of discharge voltage of mixed gas with the consideration of electrode surface roughness by referring to Pedersen model.The calculation formula of partial distortion electric field in Pedersen model is optimized,and the law of exponential attenuation is put forward to fit the electric field distribution near the rough electrode surface.It is found that the effect of roughness is more significant when the pressure is higher.When the product of pressure and roughness is smaller than the figure of merit of the mixture,the effect of roughness can be ignored.The figure of merit of C₄F₇N/CO₂ gas mixture is calculated,and the discharge test under rough electrode is carried out to verify the effectiveness of the calculation method.At the same time,it is found that the roughness can be obtained by experiment first,and then the discharge voltage of different gases can be calculated by using the model.The electrode model of 220 k V equipment is developed,and the discharge test of C₄F₇N/CO₂ gas mixture is carried out.The discharge voltage of such equipment can be calculated with the roughness of 150?m after calibration.Through calculation,it is found that the roughness control value in C₄F₇N/CO₂ mixture decreases with the increase of pressure,and decreases with the increase of ratio.If the increase of pressure makes the discharge voltage of the mixture gas and SF₆ gas the same,then the roughness control value in the mixed gas in the ratio range of 4%?30%can use the 6.3?m standard of the existing SF₆ equipment.?3?The discharge law of C₄F₇N/CO₂ gas mixture considering ambient temperature was studied.In order to solve the problem that the influence of temperature on the discharge voltage is still unknown when the ambient temperature is lower than the liquefying temperature of the mixed gas under extreme climate conditions,based on the calculation model the saturated vapor pressure curve of C₄F₇N,the variation law of the discharge electric field strength of the mixed gas with temperature is calculated.The results show that the discharge voltage of the mixed gas is not affected by the temperature when there is no liquefaction,but decreases with the decrease of the temperature when there is liquefaction.The turning point temperature is the liquefaction temperature.In order to verify the validity of the calculation model,a temperature controlled gas discharge test device was established to carry out the discharge test of mixed gas in the temperature range of-35?20?,and the test results are close to the calculation results.The ratio and pressure of the mixture at different temperatures and the lowest available temperature at different ratios and pressures are obtained by using the model.In conclusion,by introducing the calculation method,improving the existing model and combining the discharge test,this paper studies the power frequency discharge law of C₄F₇N/CO₂ mixture under the influence of mixing ratio,electrode surface roughness,and ambient temperature.The discharge voltage calculation of C₄F₇N/CO₂ mixed gas at arbitary ratio is realized,the cause of synergistic effect is explained,and the optimal ratio range based on synergistic effect is proposed.The discharge voltage under high pressure and large size electrodes is calculated,and the surface roughness control value of C₄F₇N/CO₂ mixed gas electrode is proposed.Based on the calculation of discharge voltage in extremely cold condition,the application scheme of C₄F₇N/CO₂ mixed gas in different environment is put forward.The research results of this paper have important theoretical and practical value for the popularization and application of C₄F₇N/CO₂ gas mixture.The method proposed in this paper has reference significance for the research of discharge law of new type gas mixture in the future.