| Stable chemical properties,good insulation strength and excellent arc extinguishing effect make SF6 gas widely used in insulation protection of high voltage equipment.However,because of its high greenhouse effect coefficient,high liquefaction temperature and sensitive to uneven electric field,the application of SF6 gas is greatly restricted.In view of this,SF6 gas and its alternative gas have been widely studied at home and abroad.It has not been found that SF6 can be replaced in the comprehensive properties of insulation and arc extinguishing characteristics.In the short term,the more effective method is to mix with other gases as an alternative solution.In this paper,the discharge characteristics of two mixtures of SF6/N2 and SF6/CF4 under various electrodes and different conditions are studied.The theoretical calculation of the discharge process of mixed gas is carried out.At the same time,the accuracy of the calculation is tested by the measurement experiment of the discharge characteristic of mixed gas.Through the basic research,a reliable theoretical evidence is provided for the application of SF6/N2 and SF6/CF4 mixed gas in the field of high voltage insulation.Theoretical calculation will start from the dynamic foundation of electron collapse to streamer development during gas discharge,and establish a streamer discharge model developed in the course of gas discharge.The spatial number density distribution of electrons,positive ions and negative ions in the discharge process is calculated,and the distortion effect of space charge on electric field is analyzed.A method of calculating the discharge voltage of gas discharge with electrode structure is proposed.The discharge process of flow injection under different electrode shape,voltage polarity and gas pressure is analyzed.The synergistic effect between different molecules is studied and the polarity effect of mixed gas is analyzed.The discharge process of mixed gases is studied from two aspects of macroscopic and microscopic aspects.The results are as follows:at 100Td,after the gas mixture interaction,the gas dissociation ionization coefficient?/N and the adsorption coefficient?/N are obtained.The?/N of the N2molecule is 4.86×10-24,the SF6 molecule is 3.05×10-23,and the?/N of SF6 molecule is 1.12×10-20.When N2 and SF6 are mixed,the effective ionization coefficient of gas is lower than the sum of the weights of the two.The essence of synergy in discharge of mixed gases is elucidated in microcosmic theory.When the discharge time is 4ns,the density of 80%CF4electrons is 2.9×1016m-3,5.68 times that of pure SF6,and the growth rate of gas electron density is slower under rod electrode.When the electronic number density reaches 1017m-3,the electric field is distorted,the pressure is 0.1MPa and the distance is 5mm,the peak value of the electric field distortion of the pure SF6 gas discharge is 30.7kV/mm.At the same time,the corona starting voltage is calculated and tested,and the experimental results verify the effectiveness of the calculation method.By analyzing the synergistic effect of mixed gas,when the pressure is 0.1 MP and the content of SF6 is 20%,C value of mixed gas is 0.33 and0.54,when the gas pressure is about 0.3MPa,the C value of the mixed gas reaches the minimum value and synergistic reaction decreases with the increase of?SF6.In polar effect,the corona onset voltage is more positive than negative polarity,and breakdown voltage reverses with the increase of pressure at 5mm spacing.When electrode spacing is 5mm,the gas pressure is less than the critical pressure,polarity is higher than negative polarity,and polarity reverses thereafter.When the electrode spacing increases,the breakdown voltage of the mixed gas is high negative polarity.When the non-uniformity of electric field is greater,the corona onset voltage and breakdown voltage of mixed gas are more sensitive,and the gas voltage values all increase slowly.With the increase of gas pressure,the polarity of the breakdown voltage is very different,when electrode spacing is 9mm,air pressure is 0.6MPa the difference value is up to 21.9kV,which is not conducive to the application of gas insulation.The difference value decreases when?SF6 decreases.It can be seen that adding CF4?and N2 gas is beneficial to improving the application value of gas. |
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