Study on the Effect of Ag Surface on Oxidation of SF6 Primary Decomposition Products
Li Haotian1, Zeng Fuping1, Yan Yiming1, Zhu Kexin1, Yao Qiang2, Tang Ju1
1. School of Electrical Engineering and Automation Wuhan University Wuhan 430072 China; 2. State Grid Chongqing Electric Power Company Electric Power Research Institute Chongqing 401123 China
Abstract：Operational experience shows that the thermal stability of SF6 in the gas-insulated equipment is lower than expected, and the metal surface inside the equipment may have a significant effect on SF6 primary decomposition and secondary reactions, but the mechanism is not yet clear. First, it is necessary to study how the O2 molecule dissociates and provides the reaction system with O atoms, which requires high energy; second, it needs to reveal the reasons for the significantly lower stability of SF6 than expected in the actual fault state; third, it is necessary to investigate in depth the mechanism of the metal surface inside the GIS on the generation process of important characteristic products. Among the metal materials used in GIS, silver is usually plated on the internal contacts, electrical connections and other locations to lower the contact resistivity where the local over-thermal faults occasionally occur. In this case, the Ag surface is in direct contact with SF6 gas and other impurity gases, while the mechanism of the interfacial interaction is not known and needs to be studied. First, the adsorption configuration of O2 on the Ag surface is obtained based on density functional theory, and the reaction model of low-fluorine sulfides with the O atom on Ag surface is established, which initially reveals the mechanism of oxidation reaction of SF6 primary decomposition products on the Ag surface under GIS over-thermal fault state; Second, based on transition state theory and reaction kinetic theory, the energy barrier and reaction rate constant of interfacial reaction are calculated and compared with the corresponding gas-phase reaction, respectively. Last, the main reaction sites of the oxidation reaction paths of the three low-fluorine sulfides are identified separately, and the promotion/inhibitory mechanism of the reaction by the Ag interface is analyzed. (1) The presence of the Ag interface enables the SOF4 generation reaction, which is otherwise difficult to occur in the gas-phase space at both room temperature and superheated state, to proceed at room temperature, and its reaction rate is further enhanced with increasing temperature, which can be completed instantaneously with a low half-life of 5.05×10-9 h at 400℃. Compared with the gas-phase reaction, the energy barrier of this reaction is reduced by 71.45% under the catalysis of the Ag surface. (2) The Ag surface also significantly reduces the activation energy required to generate SOF3, allowing it to be generated rapidly at room temperature; the gas-phase formation reaction of SOF3 is sensitive to temperature, which makes it have a faster reaction rate under over-thermal conditions. Both the Ag surface and the gas-phase space are important reaction sites for the formation of SOF3. (3) Unlike SOF4 and SOF3, the Ag surface does not have a promotion effect on the generation of SOF2. It raises the energy barrier of the reaction and significantly reduces the rate constant of the reaction. Accordingly, it is judged that SOF2 is mainly generated through the direct reaction between SF2 and O2 in the gas phase space. The main reason may be the small molecular mass of SF2 and the strong adsorption effect of Ag surface on it, which leads to the difficulty for the product to leave from the surface after SF2 interacting with surface O atoms. Thus, the generation of SOF2 gas molecules is inhibited. The results of this paper reveal the mechanism of gas-solid reactions involving low-fluorine sulfides, oxygen-silver surfaces. The catalytic effect of Ag on several of these reactions is found, thus explaining previous doubts about the cause of SF6 stability failure. The research methodology established in this paper is expected to be an inspiration in gas-solid reaction studies.
李昊天, 曾福平, 颜伊鸣, 朱可馨, 姚强, 唐炬. Ag表面对SF6初级分解产物氧化反应的影响作用研究[J]. 电工技术学报, 2024, 39(9): 2841-2850.
Li Haotian, Zeng Fuping, Yan Yiming, Zhu Kexin, Yao Qiang, Tang Ju. Study on the Effect of Ag Surface on Oxidation of SF6 Primary Decomposition Products. Transactions of China Electrotechnical Society, 2024, 39(9): 2841-2850.
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