Insulation And PD Decomposition Characteristics And Mechanism Of C₄F₇N And Its Mixture Gas

SF6 insulated electrical equipment has the advantages of small floor area,high reliability and small maintenance,which is widely used in various fields of power industry.More than 90%of SF6 gas produced in China is used in power industry per year.However,as a kind of greenhouse gas with great harm to the atmospheric environment,the environmental problems caused by SF6 have also become an urgent problem to be solved in the power industry.In recent years,the requirement of environmental protection is urgent all over the world,and environmental protection gas insulated electrical equipment has gradually become one of the research hotspots in the power industry.The insulation and environmental protection characteristics of SF6 substitutes are the core issues in the development of environmental protection gas insulated electrical equipment.Fluoronitriles(C4F7N)and its mixture are considered to be environmental friendly insulating gas with great potential to replace SF6.In this paper,the molecular structure of C4F7N gas is studied,quantum chemical calculation,molecular reaction dynamics and experimental research are used to study the insulation performance and decomposition mechanism of C4F7N and its mixture gas.The following work is focused on:Firstly,22 molecular descriptors of 47 kinds of gases are calculated based on the intrinsic characteristics of gas molecular structure.The quantitative structure-property relationship(QSPR)model of gas insulation performance and the QSPR improved model of gas molecular insulation performance with double or triple bonds are established by multiple linear regression method.The key structural parameters affecting gas insulation performance are obtained:molecular surface electrostatic potential positive value area and negative area.On the basis of this,using the theory of quantum chemical calculation,the change rule of the positive and negative area of the electrostatic potential on the molecular surface under the action of dipole moment electric field is calculated,and a new method for calculating the relative dielectric strength of insulating gas is proposed.At the same time,based on the intermolecular interaction of the mixed gas,the interaction factor,molecular configuration and intermolecular binding energy of C4F7N gas mixed with CO2 or N2 are studied,and a method of C4F7N gas mixture ratio considering the intermolecular synergistic relationship is proposed.Secondly,in order to obtain the synergistic effect on the electrical strength of C4F7N gas mixture,the I-d relationship between C4F7N and N2,CO2 gas mixture was measured by the steady-state Townsend method(SST),and the variation rules of ionization coefficient,attachment coefficient and effective ionization coefficient of C4F7N gas mixture with reduced field strength were obtained.The results show that the critical electrical strength of C4F7N gas mixture increases linearly with the increase of C4F7N content.According to the test results,the empirical formula and correlation coefficient of the electrical strength of C4F7N gas mixture are proposed.The synergistic effect index C of C4F7N/CO2 and C4F7N/N2 are calculated.It is found that the two gases have a strong positive insulation synergistic ability when the content of C4F7N is in a certain range.Then,a calculation model of the electrical resistance of C4F7N gas mixture based on the electrostatic potential area,the electrostatic potential imbalance and the synergistic effect index C of molecular surface is proposed and verified effectiveness.Thirdly,the breakdown characteristics of C4F7N gas mixture were studied under different voltage forms,different air pressure and different electric field structures.The factors that affect the breakdown voltage of C4F7N gas mixture were analyzed.A set of empirical formula for the breakdown field strength of C4F7N/CO2 gas mixture with electrode surface roughness and air pressure as parameters was proposed.The results show that the ability of C4F7N/CO2 mixture to restrain negative PD of HVDC is better than that of SF6/N2 mixture,and the ability to restrain positive PD of HVDC is equivalent to that of SF6/N2 mixture.Fourth,the energy change and kinetic process of C4F7N/CO2 mixture decomposition were studied by means of quantum chemistry and molecul ar reaction dynamics.The main decomposition path of C4F7N/CO2 mixture was analyzed.The change rules of free radicals and product contents in the decomposition process of C4F7N/CO2 mixture under different reaction temperature,concentration and time were obtained.The decomposition mechanism of C4F7N/CO2 under different reaction conditions was shown.Using the method of simulated partial discharge,the relative content of decomposition products of C4F7N/CO2 mixture under the condition of corona discharge and suspended discharge was studied.The "self inhibition" model for calculating the relative content of saturated fluorocarbon products and the"competition" model for calculating the relative content of unsaturated fluorocarbon products were proposed.The effects of different discharge forms and micro water on the formation of gas decomposition products were analyzed.The ratio of CF3CN/CF4 and CF3CN/C3F8 was used to judge the suspended discharge and corona discharge.QSPR calculation model and test method were used to analyze the effect of the product on gas insulation,and the physical-chemical properties and toxicity of the products were given.Finally,based on the properties of gas insulation,GWP and liquefying temperature,the application scheme of C4F7N/CO2 was proposed.In this paper,the insulation and decomposition properties of C4F7N and its mixture are studied.The results of this paper can provide theoretical reference and technical support for the research methods and engineering applications of C4F7N and its mixture.