In a few years, use of the extremely potent greenhouse gas SF₆ will no longer be permitted in new circuit breakers in the power system. The circuit breakers in the power system connect and disconnect electricity and can interrupt the current if a fault occurs.
“SF₆ gas is very effective for this purpose, but it is also a greenhouse gas that is 24 300 times more potent than CO₂. The power system therefore needs new and equally reliable alternatives to SF₆ breakers,” says Nina Sasaki Støa-Aanensen, senior research scientist at SINTEF.
Through the EU project MISSION, led by SINTEF Energy Research, Siemens Energy is developing and testing an SF₆-free alternative for 420 kV outdoor circuit breaker.
Siemens Energy has successfully tested the prototype of the SF₆-free 420 kV air-insulated circuit breaker based on vacuum technology (3AV2FI 420 kV).
“The tests carried out on the prototype proved the technical feasibility of a circuit breaker based on vacuum technology for 420 kV. A fully industrialised solution will be installed by the transmission system operators RTE in France and Statnett in Norway in the second half of 2026,” says Maurice Lesser, Head of Product Management Air Insulated Switchgear at Siemens Energy.
Different temperature zones
The pilot projects will take place at the Marsillon substation in southeastern France and at the Dagali substation, located near a mountain plateau in southern Norway.
The French site is characterised by a maritime climate, with summer temperatures consistently hovering above 30°C, while the temperatures at the Norwegian substation can drop as low as -50°C in the winter. Testing the new circuit breaker at these two locations will therefore provide valuable insights into its performance under extreme temperature conditions.
“Statnett has used vacuum technology as alternative to SF₆ for 145 kV voltage rating (AIS and GIS), but never for 420 kV rating. Statnett is very proud to host world’s first pilot installation of a 420kV AIS SF₆-free circuit-breaker, at the Dagali 420 kV substation. This project showcases Statnett’s commitment to the environment, developed in partnership with Siemens Energy, to deliver the first 420 kV AIS SF₆-free circuit breaker designed for operation at -50°C”, says Guilhem Blanchet, VP for substation technology at Statnett.
An old substation in need of upgrades
Dagali substation is located near the Hardangervidda mountain plateau – the largest of its kind in Europe. The area is known for its year-round cold alpine climate and is home to one of Norway’s largest glaciers.
Dagali is an air-insulated substation, located at 900 metres above sea level. Built in 1978, its high voltage equipment is now aging, and reinvestment is necessary to ensure a reliable power supply.
The substation operates at 420 kV and consists of one 120 MVA transformer bay and four overhead line bays that transmit power across the Hardangervidda plateau, spanning high mountains and deep fjords.
The new SF₆-free 3AV2FI 420 kV breaker will be installed in the Dagali-Sima 420 kV bay. It will replace an existing 20-year-old SF₆ AIS Circuit-breaker containing 40 kg of SF₆ gas corresponding to 1 000 ton of CO₂ equivalents. This 50 km overhead line crosses the Hardangervidda plateau, with transmission towers reaching altitudes of 1 450 metres.
Temperatures at the substation range from -50°C in winter to +20°C in summer. This 70-degree temperature span can cause liquefaction and other issues with SF₆ gas.
This particular bay has one of the highest recurrence rates of phase-to-ground and phase-to-phase short circuit faults in Norway, making it a highly relevant location for such a pilot installation.
Additionally, access to the substation can be difficult for Statnett personnel during winter due to snow and ice, reinforcing the need to have a robust solution that minimises outages and downtime for end users.
Replacing old technology
The Marsillon substation is located in the southwestern part of France, between Pau and Bayonne. The region enjoys an oceanic climate, influenced both by the Atlantic Ocean and the Pyrenees.
During the summer, temperature in the area can exceed 30°C for several hours a day.
The 400 kV substation is connected to RTE’s transmission network via two 400 kV overhead transmission lines (Cazaril 1 and 2). Two 240 MVA transformers and one 600 MVA autotransformer supply the region’s 90 kV and 225 kV networks.
As part of the MISSION project, RTE aims to replace one of the two 400 kV circuit breakers on the Marsillon-Cazaril overhead transmission line with a new 400 kV vacuum circuit breaker developed by Siemens Energy. The current circuit breaker has been in service since 1986 and contains 50 kg of SF₆, equivalent to 1 215 tonnes of CO₂. It is operated by a hydraulic operating mechanism – an outdated technology.
The feeder circuit breaker must protect the overhead line connecting the Marsillon and Cazaril substations, a distance of over 100 km. This line has experienced at least one short-circuit event per year over the last ten years. A rapid auto-reclosing function is also required for this circuit breaker.
“By participating in the project, RTE aims to demonstrate the feasibility of replacing aging SF₆ circuit breakers with SF₆-free technologies – without compromising engineering and maintenance operations”, says Minh Nguyen, circuit breaker specialist at RTE.
About MISSION
MISSION (Emission-free HV and MV transmission switchgear for AC and DC) is a Horizon Europe project coordinated by SINTEF Energy Research, involving 12 partners from 9 countries. Co-funded by the EU, it brings together research scientists, the switchgear industry and grid operators across Europe, to tackle the challenge of developing suitable SF₆-free technologies.
The research and innovation project aims to develop and test three SF₆-free switchgear technologies for use in different parts of the power grid. In addition to a 420 kV AC air-insulated live-tank vacuum circuit breaker, which will be tested in these two pilot projects, MISSION will also develop:
- A 550 kV HVDC gas-insulated switchgear with pressurised technical air for the safe operation of HVDC links and grids, and
- A 12 kV MVDC air-insulated circuit breaker for future MVDC grids, including superconducting grids.
Due to decarbonisation efforts to meet climate targets, there will be a significant need in the coming years to develop and expand the power grid in both Norway and Europe.
From 2028, all new switchgear in Europe up to 145 kV must be SF₆-free. For voltage levels above 420 kV, the corresponding deadline is 2032, in accordance with EU’s revised F-gas regulations.
By developing SF₆-free switchgear for different parts of the power system, the MISSION project contributes to reducing greenhouse gas emissions while enabling the safe and sustainable transmission of new renewable energy.
