Blue Technology in Switchgear for Sustainable Power Transmission Systems: Dielectric, Switching, Life-Cycle and Techno-Economic Modelling of SF₆-Free Clean-Air and Vacuum Gas-Insulated Substations

Sulphur hexafluoride (SF₆) has been the reference insulating and arc-quenching medium of high-voltage switchgear for six decades, but with a global warming potential of about 24 300 and an atmospheric residence time of the order of 3 200 years it is now the single largest direct greenhouse-gas liability of the transmission sector. "Blue" switchgear technology — vacuum interruption combined with clean-air (80% N₂ / 20% O₂) insulation — removes fluorinated gases from the substation entirely. This thesis develops an integrated engineering model of blue switchgear for sustainable power transmission and evaluates it against SF₆ and fluoronitrile (C₄-FN) alternatives for a 145 kV, 40 kA reference bay. A streamer-criterion dielectric model with Schwaiger field-utilisation and an empirical design-derating factor yields a clean-air design point of a 150 mm coaxial gap at 6.2 bar absolute for a 650 kV lightning impulse withstand level, against 95 mm at 4.4 bar for SF₆; the enclosure diameter grows from about 300 mm to 475 mm and the bay footprint by roughly 22%. A vacuum-arc recovery model shows a dielectric-recovery rate of approximately 18 kV/µs against a transient recovery voltage of 2.0 kV/µs, giving an interruption margin of about nine. A Rayleigh-number-based thermal model shows that the lower volumetric heat capacity of clean air requires a 28% larger conductor cross-section to hold the temperature rise below 65 K at 2 500 A. A cradle-to-grave life-cycle inventory gives 377 t CO₂-eq for the SF₆ bay against 92 t CO₂-eq for the blue bay, a 75.7% reduction, of which 77% of the SF₆ burden is gas leakage and end-of-life release. A discounted life-cycle cost model with a 9.5% capital premium gives a break-even at service year 19 and a net present advantage of 3.9% of capital cost at 40 years; sensitivity analysis shows that the business case is governed by capital premium, gas-handling and compliance cost, and discount rate, and only weakly by carbon price. A four-layer system architecture linking the primary plant, an IEC 61850 digital layer, an asset and carbon-accounting layer and the regulatory interface is proposed, together with a weighted multi-criteria sustainability index in which blue technology scores 0.89 against 0.73 for the fluoronitrile route and 0.54 for SF₆.