SF6-free gas-insulated switchgear: where to go?
At present, technology is converging towards three types of insulation gases that can replace SF6.
by René Smeets
1. A mixture of N2 and O2 natural origin gases (NOG). For switching purposes, this gas mixture is not fit, and vacuum interrupters are mostly used, having a N2/O2 mixture for outside insulation. Whereas these gases have GWP < 1, the life-cycle ecological footprints of switchgear in which they are used are not necessarily the lowest. The relatively low dielectric strength of the NOG mixture may require bigger insulation distances or high pressures at bigger sizes and more material. In Japan, there is a lengthy experience with high-voltage vacuum circuit breakers, though with SF6 as outside insulation.
2. Live tank gas circuit breakers. (for outdoor air-insulated stations) using a NOG mixture of CO2 and O2 for insulation and interruption are now reaching a voltage of 420 kV. This development started with application for very low ambient temperatures. Also, these gases have GWP < 1.
A variety of mixtures of the synthetic fluorinated gas C4F7N (C4-FN) with carrier gas CO2, CO2-O2 or N2 [i]. These (manufacturer dependent and mostly patented) mixtures are used both for insulation and switching and go without vacuum interrupters, which makes the technology better scalable to higher voltages. Because of the C4-FN additive, the high dielectric strength of the mixtures enables equipment size that is comparable to that of SF6 equipment, though at higher filling pressure. Pure C4-FN has a high boiling temperature and therefore only a limited fraction of this gas can be accepted in mixtures for switchgear, especially those operating at low ambient temperature. Therefore, a trade-off between applicability at low ambient temperature and adequate dielectric strength is the key factor in the choice of the proper C4-FN content in the mixture.
The GWP of the mixture is in the hundreds, and therefore the gas is not compatible with the new EU fluorinated-gas regulations [ii]. Moreover, C4-FN is a member of the per- and polyfluoroalkyl (PFAS) family of substances and is under scrutiny of the European Chemical Agency (ECHA) for future restrictions[iii].
[i] Perret, M., Gatzsche, M., et al., “Handbook C4FN mixtures for high-voltage equipment” (2024) https://www.gevernova.com/grid-solutions/sites/default/files/resources/products/reference/c4-fn_handbook.pdf
[ii] EU F-gas 2024 regulation: https://climate.ec.europa.eu/eu-action/fluorinated-greenhouse-gases/eu-rules_en
[iii] ECHA 2025 PFAS restrictions proposal update: https://echa.europa.eu/nl/hot-topics/perfluoroalkyl-chemicals-pfas
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