Technical Viability of Retro-Filling C3F7CN/CO2 Gas Mixtures in SF6-Designed Gas Insulated Lines and Busbars at Transmission Voltages

• Published in: IEEE transactions on power delivery 2020-10, Vol.35 (5), p.2394-2402
• Main Authors: Loizou, Loizos, Chen, Lujia, Liu, Qiang, Cotton, Ian, Waldron, Mark, Owens, John G.
• Format: Article
• Language: English
• Subjects: Bars; Breakdown; Busbars; Carbon dioxide; Compressed gas; Dielectric breakdown; Dielectrics; gas insulated lines and busbars; gas insulation; Gas mixtures; Gases; Greenhouse effect; Greenhouse gases; Heptafluoro-iso-butyronitrile (<named-content xmlns:xlink="http://www.w3.org/1999/xlink" xmlns:ali="http://www.niso.org/schemas/ali/1.0/" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" content-type="math" xlink:type="simple"> <inline-formula> <tex-math notation="LaTeX"> text{C}_3 \text{F}_7 \text{CN}</tex-math> </inline-formula> </named-content>); Insulation; lightning impulse breakdown; Prototypes; Sulfur hexafluoride; sulphur hexafluoride (<named-content xmlns:xlink="http://www.w3.org/1999/xlink" xmlns:ali="http://www.niso.org/schemas/ali/1.0/" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" content-type="math" xlink:type="simple"> <inline-formula> <tex-math notation="LaTeX"> text{SF}_6</tex-math> </inline-formula> </named-content>)
• ISSN: 0885-8977; 1937-4208
• DOI: 10.1109/TPWRD.2020.2967582

Sulphur hexafluoride (SF 6 ), the most popular dielectric medium adopted in compressed gas insulated equipment, has been identified as a highly potent greenhouse gas. This has led to increased interest in finding a more environmentally friendly replacement candidate. In this paper, the technical viability of C 3 F 7 CN/CO 2 gas mixtures was assessed as a potential retro-fill solution for existing SF 6 -filled gas insulated lines (GIL) and busbars (GIB). A reduced-scale coaxial prototype was developed to establish the breakdown strength of 20% C 3 F 7 CN / 80% CO 2 and 16% C 3 F 7 CN / 84% CO 2 gas mixtures in direct comparison with pure SF 6 under the standard lightning impulse (1.2/50 μs). Breakdown results demonstrate that a mixture of 20% C 3 F 7 CN / 80% CO 2 exhibits comparable insulation capability to pure SF 6 in coaxial geometries with similar field uniformity to GIL/GIB. This initial finding has led to the construction of a full-scale GIB demonstrator rated for 420/550 kV. Type tests according to IEC 62271-204 showed that the 20% C 3 F 7 CN / 80% CO 2 gas mixture has passed all the required voltage levels as SF 6 . The research findings in this paper are an encouraging step towards a technically viable SF 6 -free retro-fill solution for existing GIL/GIB installed for the 400 kV transmission network in the UK.