Research On The Insulation Characteristics Of Multi-component Gas Mixtures Based On The Gas Molecule Microstructures

Gas insulation has been widely used in high-voltage electrical equipment with small size,low cost,high reliability and easy maintenance advantages.But sulfur hexafluoride（SF₆）,the most commonly used insulating gas,has great greenhouse effect which may cause seriously environment problems.It is necessary to find the alternative gases or solutions to SF₆.In the past decade,various gas solutions including SF₆-N₂,SF₆-CO₂ gas mixtures,octafluorocyclobutane（c-C₄F₈）,trifluoroiodomethane（CF₃I）and Alstom g3gas mixtures have been proposed successively.Considering the insulation strength,environment effect,gas costs,stability,liquefaction temperature and many other conditions,so far it has not been able to find a comprehensive alternative solution to SF₆.Most researches on gas insulation were based on the experimental phenomenon,while there was few researches focus on the gas micro-molecular structures in fundamental level.For new substitute gases,the method in micro molecule structure level which can quickly determine the gas insulation performance may help to improve the efficiency of alternative gas selection,and even synthetic new gases.Multi-component gas mixtures can combine the advantages of each component in different aspects and reduce or even avoid the disadvantages,such as liquid temperature and gas cost.The molecule structure research on insulating gases also can lead methods to optimize the gas compositions and achieve the positive synergistic effects among different gases and higher insulation strength of the gas mixtures,which plays as the crucial point for gas mixtures.The research started in theoretical simulation calculation and experimental measurement.Firstly,I found the molecule structure parameters which influence the gases and their mixtures insulation characteristics by the quantum chemistry method.Then the relationship between molecule structure and synergistic effect of various mixture components including N₂,CO₂,SF₆,c-C₄F₈ and CF₃I was explored by the gas mixtures breakdown tests with power frequency voltage and lightning impulse,which may provide components screening basis and help us to optimize the gas mixtures.In the first part of this thesis,the research background and current situation of gas insulation was summarized.Several common alternative gas solutions such as SF₆ gas mixtures,c-C₄F₈ and CF₃I were compared and analyzed from the aspects including insulation performance,greenhouse effect,liquid temperature,gas cost and toxicity.At the same time,the research results on the stability and decomposition products of gases during the breakdown tests were introduced.Due to the suitable multi-component gases mixtures can reduce or avoid the disadvantages of each gas which cannot meet the practical application conditions,this thesis used the method of quantum chemistry and molecular orbital theory to optimize the components and propose the theoretical basis of the synergistic effect among gases.By using density functional theory（DFT）,the most basic organic hydrocarbon gases were used as the example to calculate the microcosmic structure parameters of gas molecules including the vertical electron affinity,the lowest unoccupied molecule orbital energy and its distribution,the probability volume of molecular electron cloud,also to analyze the relationship among them and gas insulation characteristics.The results showed that the vertical electron affinity and the lowest unoccupied molecule orbital energy of gas molecules have important effects on the free electron attachment process and the insulation performance in macroscopic level.For the gases mixtures,it has an important effect on the synergistic effect between gas components whether the excitation and ionization energies of gas components fit appropriate.On the basis of this theory,the microscopic molecule parameters of gas mixture components N₂,CO₂,SF₆,c-C₄F₈ and CF₃I were also calculated and analyzed.The insulation characteristics of different components gas mixtures with 90%N₂ and CO₂ as buffer gas and 10%SF6,c-C₄F₈ or CF₃I as the main insulating gas had been tested with 5mm-20mm electrode gaps in slightly non-uniform electric field under the power frequency voltage and positive and negative lightning impulse breakdown.The results of calculations and experiments all showed that the synergistic effect between SF₆ and N₂ is better than that of CO₂,while c-C₄F₈ has better synergistic effect with CO₂.The gases mixtures of CF₃I-N₂ and CF₃I-CO₂ had similar breakdown voltages.By comparing the unoccupied orbits energy distributions of each component,the mixtures with c-C₄F₈,CF₃I and CO₂ had the better combination of excitation energy and ionization energy.The relative higher insulation performance during the breakdown tests was consistent with the calculated results.These were the best gas mixture components with stability and cost advantages over each single gas in the screening range in this thesis.According to the analysis on the gas microscopic molecule structure,this thesis reveals the fundamental effects of the vertical electron affinity and the lowest unoccupied orbit energy distribution on the gas insulation characteristics and proposed a method to evaluate the gas insulation performance quickly.The relationship between the microscopic structure and the chemical bonds that make up the gas molecules can also help the substitute screening and synthesis of insulating gases.Based on the above theory,this thesis attempted to select the components of gases mixtures by the analysis of the gas molecule structure parameters and the breakdown tests.The mixtures of c-C₄F₈,CF₃I and CO₂ which had great positive synergistic effect,lower greenhouse effect,low gases cost and other aspects of improvement were selected.The theory can also help to select gas mixture components in a larger range.