Abstract
In this paper, the dielectric breakdown properties in SF6/CO2 mixed gas, the development of the streamer in SF6/CO2 mixed gas, and the distribution of each component with time were studied. First, the electron transport parameters (mean energy, longitudinal diffusion coefficients, Townsend coefficient, critical reduced electric field coefficients, and electron energy distribution function) in SF6/CO2 mixtures with different ratios in the E/N range of over 50–1000 Td were obtained by two-term Boltzmann equation analysis. Then, coupled with the Boltzmann drift–diffusion equation and Poisson equation, the hydrodynamic model of discharge of SF6/CO2 mixtures in a strongly non-uniform electric field was established. Many different influence factors are considered, such as the gas mixture ratio, applied voltage, space temperature, space pressure, and electrode structures. The results indicate that the increase in SF6 content in the mixed gas will reduce the ionization rate of the total mixed gas, and therefore, it takes a longer time for breakdown. The higher the pressure, the more concentrated the form of the streamer. As the temperature increases, the shape of the streamer head becomes more scattered, and it loses its contoured shape at about 3000 K; in addition, the existence of the maximum electron number density value appears at the tip of the rod electrode rather than at the streamer head. The simulation also revealed that the dielectric strength of SF6/CO2 mixtures is stronger than that of SF6/N2 mixtures and reached a turning point at an SF6 ratio of 60% under extremely non-uniform electric fields, which agreed well with experiments.
Highlights
With its excellent electrical insulation properties,1 sulfur hexafluoride (SF6) is widely used as a gas insulating medium for high-voltage circuit breakers, switch cabinets, and other electrical equipment in power production and often replaces oil-filled circuit breakers (OCBs) containing harmful polychlorinated biphenyls (PCBs).2 according to the assessment of the Intergovernmental Panel on Climate Change, SF6 is a gas with a strong greenhouse effect
The rod-plate electrode model and the hydrodynamic model of SF6/CO2 mixed gas discharge in a non-uniform electric field were established by solving the Boltzmann drift–diffusion equation coupled with the Poisson equation
When the value of E/N increases to 100 Td, the electron mean energy of the SF6/CO2 mixtures is dominated by CO2, which leads to higher electron mean energy
Summary
With its excellent electrical insulation properties, sulfur hexafluoride (SF6) is widely used as a gas insulating medium for high-voltage circuit breakers, switch cabinets, and other electrical equipment in power production and often replaces oil-filled circuit breakers (OCBs) containing harmful polychlorinated biphenyls (PCBs). according to the assessment of the Intergovernmental Panel on Climate Change, SF6 is a gas with a strong greenhouse effect. In the study of SF6 mixed with non-fluorine-containing gas, the breakdown voltages of SF6/N2, SF6/CO2, SF6/air mixtures under the condition of uniform field were calculated by Malik and Qureshi.. The breakdown process of mixed gas cannot be visually observed under the calculation of electron swarm parameters; it can be perceived intuitively in the process of simulation of streamer development. The spatiotemporal evolution of electrons, ions of each component (SF6+, SF6−, CO2+, etc.), and electric field strength during the development of the streamer were calculated These simulation results can provide a reference for the insulation performance and breakdown process of SF6/CO2 mixtures as an insulating medium as well as an alternative to pure SF6
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