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Surface charging of spacers is one of the main factors restricting the industrialization of DC gas insulated lines (GILs). The effects of ion mobility, gas pressure and temperature on surface charge behavior are important issues to be clarified. In this paper, a selection method of ion mobility is reviewed and reconsidered. A multiphysics-coupled model considering ion mobility, species of ions, temperature, and gas pressure is investigated. A mathematical model of ion mobility under low and high electric fields is presented and the critical electric field strengths are calculated, based on the current density vs electric field strength. The results show that the two critical electric field strength have different characteristics with gas pressure and temperature. The ion mobility in SF 6 gas has a completely different calculation model in high electric field and low electric field, and these two ion mobilities are equal only at a specific temperature and gas pressure. This paper can provide a reference for the selection of ion mobility in DC GILs under conditions of temperature and pressure. [ABSTRACT FROM AUTHOR]

C 4 F 7 N/CO 2 is one of the most promising alternatives to SF 6 in high-voltage electrical equipment such as gas insulated switchgear (GIS) because of its excellent electrical performance and low environmental impact. However, the insulation characteristics of C 4 F 7 N/CO 2 is still unclear regarding the effect of conductor surface roughness. To obtain a figure of merit of C 4 F 7 N/CO 2 gas mixtures and to evaluate its sensitivity to local inhomogeneous electric field, the breakdown voltage of electronegative gas considering surface roughness is developed using the streamer discharge theory. A gas discharge experimental platform for C 4 F 7 N/CO 2 mixed gas is set up and the discharge test is carried out under gas pressures from 0.15∼0.5 MPa having a maximum protrusion height of 15 μm. The results demonstrate that when the volume fraction of C 4 F 7 N is 10%, the figure of merit of C 4 F 7 N/CO 2 mixture is slightly higher than that of SF 6 gas. Therefore, at the same pressure, the standard of conductor surface roughness for C 4 F 7 N/CO 2 equipment can refer to the original SF 6 gas equipment. [ABSTRACT FROM AUTHOR]