Analysis of Breakdown and Decomposition Characteristics of C4F7N/N2/O2 Ternary Mixed Gas Based on Density Functional
Year: 2024
Type: article
Abstract: Objective In the field of electrical equipment insulation, the application of gas insulation is becoming increasingly prevalent. SF6 gas has been extensively utilised due to its stable chemical properties and its excellent insulation and arc-quenching capabilities. Nevertheless, SF6 is a powerful greenhouse gas with the potential to persist in the atmosphere, underscoring the urgency of identifying a suitable replacement. C4F7N, with its superior insulation properties and environmental benefits, has the potential to be developed as an eco-friendly insulating gas that could replace SF6. Owing to the elevated liquefaction temperature of C4F7N, it is frequently employed in engineering applications in conjunction with other gases including CO2 and N2. At present, CO2 is predominantly employed as a buffer gas for C4F7N, typically constituting in excess of 90% of the mixture. Nevertheless, as CO2 remains a greenhouse gas with a substantial greenhouse effect, the utilisation of N2 as a buffer gas is egarded as a means of markedly enhancing the environmental benignity of the mixture. Nevertheless, the C4F7N/N2 mixture may undergo decomposition during discharge, which could result in the formation of solid carbon deposits on metal electrodes. This phenomenon has the potential to significantly impair the normal operation of gas-insulated equipment. Some researchers have put forth the suggestion of incorporating oxygen as a second buffer gas into C4F7N mixtures with the aim of enhancing the arc-quenching performance of binary mixtures. The incorporation of O2 can facilitate the breakdown characteristics of C4F7N/N2 binary mixtures, while simultaneously inhibiting the formation of decomposition products. Nevertheless, the microscopic precise mechanism by which O2 exerts its influence on the breakdown and decomposition characteristics of C4F7N/N2 mixtures remains unclear. The objective of this study is to investigate the microscopic mechanism by which oxygen influences the breakdown and decomposition characteristics of C4F7N/N2 binary mixtures.Methods This study employs density functional theory to determine the electrostatic potentials of the gas molecules present in C4F7N/N2/O2 mixtures. The electron affinity and electronegativity of O2 are calculated, as are those of the C4F7N/N2 mixture. Molecular dynamics methods based on the principle of energy optimisation are employed to derive the structural formulas of N2/O2 and C4F7N with varying molecular quantities (1, 2, 3, and 4 molecules). The interaction energy and charge transfer between varying quantities of N2 and O2 with C4F7N gas are calculated. The Fukui function of the C4F7N molecule is calculated based on Hirshfeld atomic charges, and the bond dissociation energies of C4F7N/N2/O2 gas molecules are determined. The quantum chemistry calculations employ the B3LYP functional and the 6–G(d,p) basis set, while the molecular dynamics simulations utilise the COMPASS force field. The results of the calculations are used to analyze the radical reaction processes involved in the discharge decomposition of C4F7N with O2. This analysis investigates the impact of O2 on the breakdown and decomposition characteristics of C4F7N/N2 mixtures from a microscopic perspective.Results and Discussions The findings demonstrate that O2 exhibits a greater electron affinity and electronegativity in comparison to N2. The incorporation of O2 results in a reduction of free electrons within the gaseous environment, thereby diminishing the likelihood of collision ionisation during the discharge process. This in turn, gives rise to an enhancement in the breakdown voltage of C4F7N/N2 mixtures. In the case of N2/O2 gases, it can be observed that as the number of molecules present increases, the interaction energy also increases approximately in proportion. The interaction energy between O2 and C4F7N is approximately five times that between N2 and C4F7N. In the C4F7N/O2 molecular system, the C4F7N molecule is observed to a negative charge, indicating that electrons are gained by C4F7N within the system. As the number of molecules increases, C4F7N gains more electrons, resulting in an overall increase in charge transfer. In the C4F7N/N2 system, the C4F7N molecule is observed to carry a positive charge, indicating that electrons are lost by C4F7N within the system. As the number of molecules increases, C4F7N loses electrons in greater quantities. In the absence of discharge decomposition, the interactions between the buffer gas and C4F7N molecules are predominantly physical in nature. During the discharge process, O2 provides a substantial number of oxygen radicals. Once the chemical bonds of C4F7N have been broken, forming small molecular radicals, these react with ${\rm O}^{\,\cdot}$ which has been produced by the homolytic cleavage of O2. The addition of O2 results in the consumption of the ${\rm CF}_2^{\,\cdot} $ and ${\rm C}^{\,\cdot} $ radicals produced by the decomposition of C4F7N, thereby reducing the adherence of elemental carbon on the surface of metal electrodes.Conclusions In conclusion, the addition of oxygen can enhance the decomposition characteristics of the C4F7N/N2/O2 ternary gas mixture. Nevertheless, further research is required to ascertain the precise molecular ratio of the C4F7N/N2/O2 mixture.
Authors: Ti LIU, Cong CHEN, Zhen WU, Huan CHEN
Language: English
Topic: Vacuum and Plasma Arcs
Subfield: Atomic and Molecular Physics, and Optics
Field: Physics and Astronomy
Domain: Physical Sciences
Open Access status: green
OpenAlex ID: W6979897108
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