# Green Gas for Grid as an Eco-Friendly Alternative Insulation Gas to SF\({}_{6}\): A Review
## 3 Green Gas for Grid  
Although NovecTM 4710 has excellent dielectric strength and an arc-quenching ability, it cannot be used alone due to its high-liquefaction temperature. Therefore, it is necessary to mix it with other gases such as CO2 and N2. Addictive NovecTM 4710 in gas mixtures is in the range of 5-20% [26; 27]. Table 1 shows the fundamental physical and chemical properties of SF6, NovecTM 4710, CO2, and N2. The dielectric strength of g3 gas has been investigated by breakdown voltage and PD. Figure 1 shows the AC breakdown voltage depending on the NovecTM 4710 and CO2 mixture ratio, as well as the gas pressure in the uniform field that was configured using parallel disk electrodes with a gap distance of 2.54 mm [66]. The AC breakdown voltage was also measured on a 145 kV GIS with NovecTM 4710 ratio from 0% to 20% and CO2. It was indicated that the dielectric strength of pure CO2 was equivalent to about 40% that of SF6 at atmosphere pressure. Adding 6% to 7% of NovecTM doubled the dielectric strength, and g3 gas with 18% to 20% of NovecTM 4710 had the equivalent dielectric strength to SF6 at atmosphere pressure [30; 67; 68]. Breakdown tests under AC at different pressures and electrode distances were conducted in plane-plane, sphere-plane, rod-plane, and needle-plane electrode configurations. In the uniform field, equivalent dielectric strength to SF6 at 100 kPa can be reached by a mixture containing 15% NovecTM 4710. In the nonuniform and highly nonuniform fields, SF6 exhibited higher dielectric strength than a 20% mixture. The 20% NovecTM 4710/80% CO2 mixture at 100-120 kPa was verified as an optimal potential replacement of SF6 as the dielectric medium in MV switchgears if attention is paid to the equipment design to avoid nonuniform fields [44]. The comparison of AC breakdown voltages in the sphere-to-sphere electrode arrangement showed that the 3.7% NovecTM 4710/96.3% CO2 mixture constituted a good compromise between high-voltage apparatus insulation and a low ambient temperature application [28]. \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Chemical Formula** & **SF6** & **C4F7N** & **CO2** & **N2** \\ \hline Molecular weight (g/mol) & 146.06 & 195 & 44.01 & 28.014 \\ Number of electrons & 77 & 48 & 16 & 7 \\ Relative dielectric strength to SF6 & 1 & \(>\)2 & \(<\)0.4 & \(<\)0.4 \\ Liquefaction pressure at \(-30\) \({}^{\circ}\)C (MPa) & 0.52 & 0.0311 & 1.43 & - \\ Freezing point (\({}^{\circ}\)C) & \(-\)51 & \(-\)118 & \(-\)78.5 & \(-\)210 \\ Boiling point at 0.1 Mpa (\({}^{\circ}\)C) & \(-\)63 & \(-\)4.7 & \(-\)79 & \(-\)196 \\ Ozone depletion potential & 0 & 0 & 0 & 0 \\ Global warming potential (GWP) & 23,500 & 2100 & 1 & 0 \\ Atmospheric lifetime (year) & 3200 & 30 & 5–200 & - \\ Toxicity (LC 50 (rat)) & \(>\)500,000 & 10,000–15,000 & \(>\)300,000 & - \\ Flammability & No & No & No & No \\ Corrosion & No & No & No & No \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Fundamental physical and chemical properties [20; 25; 65]. The positive and negative DC breakdown tests were carried out in a plane-plane electrode arrangement. As shown in Figure 2, the negative DC breakdown strengths of 4% NovecTM 4710/96% CO\({}_{2}\) and 8% NovecTM 4710/92% CO\({}_{2}\) mixtures at 0.7 MPa were 81.21% and 96.48% of the value for SF\({}_{6}\) at 0.5 MPa [57, 69]. The lightning breakdown voltages of 3.7% NovecTM 4710/96.3% CO\({}_{2}\) at pressures of 0.88 MPa and 1.04 MPa were almost equivalent to that of SF\({}_{6}\) at 0.55 MPa and 0.65 MPa, respectively [28]. PDIVs of SF\({}_{6}\) and NovecTM 4710/CO\({}_{2}\) were measured in protrusions on the conductor and enclosure of GIS. As shown in Figure 3, PDIVs in g3 gas with 4% NovecTM 4710/96% CO\({}_{2}\) in protrusion defects were measured as 76-84% of that of SF\({}_{6}\). In addition, the average apparent charge and pulse count of PD in g3 were higher compared with those in SF\({}_{6}\)[70]. The PDIV in NovecTM 4710/N\({}_{2}\) showed that the dielectric strength of the gas mixture was weaker than that of pure SF\({}_{6}\)[27].