Research On Insulation Properties Of SF₆ Substitutes In HVDC-GIL

Gas Insulated Transmission Lines, which is short for GIL, has got rapid development and application, because of its advantage in large transmission capacity, low energy loss, less area, high reliability and small influence on the environment. And with more HVDC projects putting into operation, researchers have paid more attention to DC GIL. The applied AC GIL almost use pure S or SF6/N2 gas mixtures as its insulation dielectrics. However, the Global Warming Potential (GWP) of SF6 is very high, many countries have accepted the protocol to limit its use before 2020, so studying on SF6 substitutes in GIL. especially in DC GIL is becoming more and more vital.In this paper, feasibility of SF6 alternatives are studied from the respect of gas insulation properties. First researchers have determined two schemes, one is partly substituting project by applying SF6/N2 gas mixtures, the other is totally substituting project by using CF3I/N2 gas mixtures. From the reality of DC GIL, by applying gas insulated properties experiment platform, researchers have confirmed to study the gas gap breakdown characteristics and surface flashover properties of two kinds of SF& alternatives, by applying negative DC or negative LI voltage, and making an insulation properties comparison between SF6 substitutes and pure SF6.Insulation characteristics of pure SF6 and SF6/N2 gas mixtures show that, from the view of gas insulation properties,30%SF6/70%N2 gas mixtures under 0.6MPa and 20%SF6/80%N2 gas mixtures under 0.7MPa indicate almost equivalent characteristics as pure S in 0.5MPa, and they have large potential to substitute SF6 to use in HVDC-GIL.And insulation properties of pure CF3I and CF3I/N2 gas mixtures show that, insulation strength of pure CF3I is not always higher than that of pure SF6,when pd value reaches or exceeds 0.3mm·MPa in uniform field, pure SF6 shows better insulation behavior. And there are by-products through gas discharge of CF3I, and these substance could affect the following discharge. Insulation properties of all studied CF3I/N2 gas mixtures are roundly lower than pure SFe and SF6/N2 gas mixtures under the same gas pressure. If considering using CF3I/N2 gas mixtures to replace SF6 in HVDC-GIL, researches on enlarge the insulation dimension should be taken into account, in order to fulfill the requirements of insulation properties.