Environment-friendly insulating gases for HVDC gas-insulated transmission lines

Environment-friendly gas insulating mediums adapted to a DC gas-insulated transmission line (GIL) electric field condition is the key to the next generation of Environment-friendly HVDC GILs. In this paper, we review the literature on sulfur hexafluoride (SFö) alternatives including the scientific understanding, control, and implementation of gas-solid systems in this type of power transmission. First, the structure-activity relationship between the molecular structure and physico-chemical properties of Environment-friendly insulating gases is presented. Then, the search and prediction of important physicochemical properties of gases are summarized. Subsequently, in view of the potential of environmental friendly insulating gases, the swarm parameters of gas discharge and breakdown properties in a quasi-uniform field, inhomogeneous field, and at the gas-solid interface, that need to be taken into account with industrialized DC GILs are discussed. The latest research progress on insulation characteristics, especially the polarity effect in DC gas-solid insulation systems, the sensitivity to the electrode surface state, and the non-uniformity of the electric field, and the influence of metal particles and their variation with air pressure, is highlighted. In addition, the heat transfer characteristics of insulating gases, related to DC GIL transmission with a large current-carrying capacity and the influence of alternative gases on the heat transfer characteristics are described. Finally, aiming at solving the contradiction of low environmental impact, high dielectric strength and low liquefaction temperatures in the selection of alternative gases, an coordinated regulation model for Environment-friendly gases in DC GILs is established. Considerations for future work on this topic are also presented.