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

Abstract

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/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% C3F7CN / 80% CO2 and 16% C3F7CN / 84% CO2 gas mixtures in direct comparison with pure SF6 under the standard lightning impulse (1.2/50 μs). Breakdown results demonstrate that a mixture of 20% C3F7CN / 80% CO2 exhibits comparable insulation capability to pure SF6 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% C3F7CN / 80 % 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 / GIB installed for the 400 kV transmission network in the UK.

Negative DC Breakdown Characteristics of C3F7CN / CO2 Gas Mixture for Application in High Voltage Accelerators

“…Note that for the 15/30 mm coaxial configuration, the breakdown voltage of SF 6 is comparatively higher than 20% C 3 F 7 CN / 80% CO 2 gas mixture for all tested pressures. This is consistent with LI breakdown results reported for small-scale coaxial geometries, where the coaxial geometrical ratio closer to the ln(r 2 /r 1 ) = 1 attains the higher breakdown strength [16].…”

“…Both SF 6 and C 3 F 7 CN have a high electron affinity and are expected to behave similarly. Based on existing literature, for uniform and quasiuniform field, 20% C 3 F 7 CN / 80% CO 2 gas mixture is comparable or slightly better than SF 6 [14,16], as observed in Figures 7 and 8.…”

“…In the limited research on SF 6 alternatives research under DC, uniform field breakdown tests were conducted using low concentrations of C 3 F 7 CN (4% and 8%) in the pressure range of 3-7 bar absolute [15]. Results for negative DC indicate that a higher concentration of C 3 F 7 CN is required to match the performance of SF 6 in order to develop a retro-fill solution as demonstrated in [16] for a 420/550 kV rated AC GIL equipment.…”

“…The ends of the enclosure were flared to ensure that the E max is located at the center of the coaxial conductor. Detailed design description can be found in [16].…”

Negative DC Breakdown Characteristics of C3F7CN and CO2 Mixture with Different Electrode Materials

“…However, the gas possesses a high global warming potential (GWP) that is 23,500 times greater than CO2 and a long atmospheric lifetime of 3,200 years. The cumulative environmental impact could pose a significant problem for the power industry [1].…”