Electrical Thermal Decomposition And Metal Compatibility Mechanism Of Eco-friendly Insulating Gas C₄F₇N-CO₂

Sulfur hexafluoride（SF₆）,which is currently used in various types of gas-insulated transmission and distribution equipment,is an extremely strong greenhouse gas with the greenhouse effect potential（GWP）23500 times that of CO₂ and the atmospheric life span of3200 years.With the increasingly severe climate and environmental issues,it is urgent to seek eco-friendly gas insulating medium and apply it in power transmission and distribution equipment.The research and application of eco-friendly gases are not only in line with the green and low-carbon development concept,but also a fundamental strategy to solve the reliance on the strong greenhouse gas SF₆ in the power transmission and distribution manufacturing industry.In recent years,the Perfluoroisobutyronitrile（C₄F₇N）gas mixture has been regarded as a potential SF₆ substitute gas due to its great eco-friendly and insulation properties.In this paper,theoretical and experimental researches were carried out on the electrical and thermal decomposition characteristics and gas-metal compatibility of C₄F₇N-CO₂ gas mixture.The application feasibility of C₄F₇N-CO₂ was also evaluated in combination with physicochemical properties of the decomposition products as well as the insulation reliability of gas mixture.Considering the structural characteristics of C₄F₇N molecule,this thesis firstly constructed the decomposition and product generation pathways of C₄F₇N-CO₂ gas mixture systematically.Then the geometric structure,energy information,vibration frequency and other microscopic parameters of the reaction particles were calculated based on density functional theory and transition state theory.The enthalpy,activation energy of various reaction pathways and the influence of temperature on them were analyzed.The main decomposition and characteristic product generation pathways of C₄F₇N-CO₂ gas mixture were extracted.The characteristic decomposition product detection method for C₄F₇N-CO₂gas mixture and the spark discharge,partial discharge（PD）decomposition experiments were conducted to reveal the composition,content and change regularities of the discharge decomposition products.The theoretical analysis was also compared with the test results to confirm the discharge decomposition mechanism of C₄F₇N-CO₂ gas mixture.The thermal decomposition simulation of C₄F₇N and C₄F₇N-CO₂ gas mixture were conducted based on the Reax FF reaction molecular dynamics method to obtain the product particle types,content and generation rate at different temperature conditions.The thermal stability and decomposition properties of C₄F₇N-CO₂ gas mixture was also explored experimentally based on the local thermal decomposition simulation platform.The influence of temperature and gas pressure on the composition and production of thermal decomposition products was obtained.This thesis also analyzed the differences between the discharge and thermal decomposition characteristics of C₄F₇N-CO₂ gas mixture,and the decomposition products which can characterize discharge or overheating faults were proposed.The interaction model between C₄F₇N and the commonly used metal（copper,aluminum,silver）interface in the device was systematically established to analyze the interaction energy,charge transfer,density of states and charge density difference of the gas-solid interface interaction process based on density functional theory.The enthalpy and activation energy of the adsorption-dissociation reactions were also calculated.The interaction mechanism between C₄F₇N and metal interface was further revealed combined with the stability tests.The effects of C₄F₇N-metal interface reaction on the gas composition,metal morphology as well as surface element composition were analyzed and the optimization scheme of the gas-metal interface stability was proposed.Finally,the physiochemical properties,ionization parameters（ionization energy,electron affinity energy,band gap）and biological safety of C₄F₇N-CO₂ characteristic by-products were discussed.The insulation recovery performance of C₄F₇N-CO₂ gas mixture as well as the morphology and element composition of solid precipitates generated by PD decomposition were revealed.Combining the related electrical,thermal and metal compatibility test results,the significant component parameters that should be monitored in the operation and maintenance of C₄F₇N-CO₂ gas mixture equipment and related operation,maintenance,safety protection suggestions for person who might contact or operate C₄F₇N gas mixture were given.On the whole,this thesis has systematically revealed the decomposition mechanism of C₄F₇N-CO₂ gas mixture under discharge,overheating conditions and gas-metal interface compabitility through a large number of theoretical and experimental studies.The application feasibility and operation attention points of C₄F₇N-CO₂ in transmission and distribution equipment was also evaluated.Relevant results provide an important theoretical and technical references for the engineering application of C₄F₇N-CO₂ gas mixture.