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Sulphur hexafluoride (SF6), 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 C3F7CN&#x002F;CO2 gas mixtures was assessed as a potential retro-fill solution for existing SF6-filled gas insulated lines (GIL) and busbars (GIB). A reduced-scale coaxial prototype was developed to establish the breakdown strength of 20&#x0025; C3F7CN &#x002F; 80&#x0025; CO2 and 16&#x0025; C3F7CN &#x002F; 84&#x0025; CO2 gas mixtures in direct comparison with pure SF6 under the standard lightning impulse (1.2&#x002F;50&#x00A0;&#x03BC;s). Breakdown results demonstrate that a mixture of 20&#x0025; C3F7CN &#x002F; 80&#x0025; CO2 exhibits comparable insulation capability to pure SF6 in coaxial geometries with similar field uniformity to GIL&#x002F;GIB. This initial finding has led to the construction of a full-scale GIB demonstrator rated for 420&#x002F;550&#x00A0;kV. Type tests according to IEC 62271&#x2013;204 showed that the 20&#x0025; C3F7CN &#x002F; 80&#x0025; CO2 gas mixture has passed all the required voltage levels as SF6. The research findings in this paper are an encouraging step towards a technically viable SF6-free retro-fill solution for existing GIL&#x002F;GIB installed for the 400&#x00A0;kV transmission network in the UK.

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Technical Viability of Retro-Filling C3F7CN&#x002F;CO2 Gas Mixtures in SF6-Designed Gas Insulated Lines and Busbars at Transmission Voltages