Impact Mechanism of Liquefaction Phenomenon on Insulation Properties of $\text{SF}_{6} / \mathrm{N}_{2}$ Gas Mixtures Under Cryogenic Conditions

The $\text{SF}_{6} / \mathrm{N}_{2}$ gas mixture has become a crucial alternative to pure $\text{SF}_{6}$ gas in GIS/GIL equipment within the State Grid power system due to its excellent insulation performance and relatively low Global Warming Potential (GWP). However, in high-altitude cold regions, the liquefaction of $\text{SF}_{6}$ gas within the mixture can degrade its insulation performance. This paper carried out liquefaction and breakdown experiments on gas mixtures under low-temperature and high-pressure environments. The study found that under the conditions of $-60^{\circ} \mathrm{C}$ and 0.8 MPa, the liquefaction ratio limit of the gas mixture is a combination of 19 % sulfur hexafluoride ($\text{SF}_{6}$) and 81 % nitrogen ($\mathrm{N}_{2}$). With the increase of the proportion of sulfur hexafluoride and the decrease of temperature, the breakdown voltage of the gas mixture shows a trend of first increasing and then decreasing. Specifically, when the proportion of sulfur hexafluoride is 19 % and the temperature is $-50^{\circ} \mathrm{C}$, it reaches extreme values of 57.6 kV and 56.9 kV respectively. The breakdown voltage increases monotonically with the increase of pressure, but when the pressure is relatively high, the liquefaction phenomenon of sulfur hexafluoride will slow down the growth rate of the breakdown voltage. This study clarifies the mechanism by which the liquefaction of sulfur hexafluoride affects its insulation performance, and provides key theoretical support for the design, operation and maintenance of GIS/GIL equipment using sulfur hexafluoride/nitrogen gas mixtures in high-latitude regions cold areas.