A Comparative Study on the Breakdown Characteristics of SF6 and 20% C3F7CN / 80% CO2 Gas Mixture in a Coaxial Configuration

Gas Insulated Lines (GIL) are considered an attractive alternative technology to overhead lines for transmission of high power over long distances. However, a key drawback is the use of sulphur hexafluoride (SF6) as the main dielectric medium. SF6 is listed as a greenhouse gas with a Global Warming Potential (GWP) of 23,500 times higher than CO2. As SF6 is widely used in the power industry, there are increasing research activities to find a cost-effective and technically viable alternative candidate with significantly lower environmental impact. This paper investigates the feasibility of replacing SF6 in high voltage equipment with a more environmentally friendly gas named C3F7CN. A reduced-scale coaxial prototype was developed which replicates the quasi-uniform electric fields as found in a full-scale GIL. The test samples were subjected to standard lightning impulse (1.2/50 μs) of both polarities. The results have shown that the mixture of 20% C3F7CN / 80% CO2 can reach a dielectric strength equivalent to 100% SF6 gas. In coaxial configurations, the positive lightning impulse breakdown voltage is in general higher than the negative polarity, especially at higher pressures. In conclusion, C3F7CN mixtures can be a potential candidate of replacing SF6 for insulation applications such as gas insulated lines and busbars which is an encouraging step towards a more environmentally friendly future transmission network.