Retrofill replacement of SF6 to accelerate the decarbonisation of electricity networks

Narrative

As the UK transitions towards net zero, demand for electricity is set to quadruple. Keeping the existing network safe, reliable and able to function for as long as possible is key. Gas-insulated switchgear (GIS) devices, that transform and distribute electricity throughout electrical networks, are essential in maintaining network reliability. Unfortunately Sulphur hexafluoride (SF₆), the most commonly used gas insulator within GIS devices, is a potent greenhouse gas with a global warming potential 25,200 times greater than CO₂. Approximately 10,000 tonnes are installed annually across the energy sector. As we transition toward Net Zero, SF₆ must be replaced with environmentally sound alternatives but without jeopardising the safety and reliability of the electrical network. The scale of the challenge grows when the replacement of current infrastructure is considered. Replacing national SF₆ infrastructure with new build alternatives would cost billions of pounds, a cost that would be levied directly on UK electricity consumers. Retrofill of existing GIS devices with SF₆ alternatives offers a faster, easier and cheaper solution to replacement. Unfortunately, it requires more rigorous validation of SF₆ alternatives to ensure compatibility with existing infrastructure, and requires development of safe, affordable and reliable approaches for retrofill.

To achieve these goals, Manchester’s team led by Dr Tony Chen partnered with National Grid Electricity Transmission (NGET) to develop a retrofill solution that would allow NGET to replace the SF₆ in existing assets. Within Manchester’s High Voltage Laboratory – the biggest electrical infrastructure test and research facility in UK academia – the team assembled a 420/550 kV rated GIS demonstrator using equipment designed for SF₆. This demonstrator allowed them to undertake a unique balancing act, between identifying the suitable gas mixture and finding the right operating conditions at which its properties most closely mirror those of SF₆. Working with C₃F₇CN, a gas identified by as being a good SF₆ alternative for new equipment that’s been optimised by manufacturers, the Manchester demonstrator allowed the team to identify, optimise and retrofill a suitable mixture within existing busbar and bushing equipment. As part of this process, the team addressed additional technical challenges such as IEC type test using full-scale equipment and material compatibility. Through this meticulous approach, Manchester researchers were able to both prove the technical viability of retrofilling existing assets using C₃F₇CN based mixture, plus optimise and validate a C₃F₇CN gas mixture and show - through rigorous type testing in accordance with International Electrotechnical Commission (IEC) standards - provide the equivalent electrical performance and more than a 99% reduction in carbon footprint when compared to SF₆. Note that carbon footprint reduction calculation is taking into account the different densities of C₃F₇CN mixture and SF₆ at the same pressure.

This breakthrough unlocks the potential of replacing SF₆, without having to replace the equipment completely. As a result of Manchester’s demonstration Retrofill is now considered a viable intervention option and part of the NGET roadmap to be SF₆-free by 2050. The retro-fill approach adopted is estimated to be one order of magnitude cheaper than a new-build approach and minimises by years the timescale for new-build replacement of existing SF₆-GIS substations. When potential savings are applied to NGET’s retrofill intervention strategy this solutions represents a saving of billions of pounds for UK electricity consumers by 2050. In Dec-2021, NGET energised the world’s first retrofilled SF₆-free substation equipment in Richborough, Kent. The operation removed 755 kg of SF₆ from service equivalent to 19,026 tonnes of CO₂. This saving is equivalent to removing over 11,000 cars from UK roads for one year.

Impact date	Jul 2021
Category of impact	Economic, Environmental, Policy, Technological
Impact level	Adoption