Swissgrid is planning an extra-high-voltage line between the substations in Flumenthal (canton of Solothurn) and Froloo (municipality of Therwil, canton of Basel-Landschaft) in order to strengthen the supply of electricity in the greater Basel area and throughout the two Basel cantons. The new 220-kilovolt line will replace the existing 145-kilovolt line operated by IWB (Industrielle Werke Basel).
The current line between the Flumenthal and Froloo substations is in operation as a two-strand 145-kV line. It is part of the national distribution system. The IWB line, which is around 33 kilometres long, has to be replaced due to its age. At the same time, the operating voltage on one of the strands will be increased to 220 kilovolts. This means that the line will be part of the Swiss transmission grid in the future and, as such, will be operated by Swissgrid. The second strand on the line, which will have an operating voltage of 145 kilovolts, will continue to be operated by the relevant distribution system operators as before.
The extra-high-voltage line between Flumenthal and Froloo will ensure electricity supply redundancy in the greater Basel area and strengthen security of supply across Switzerland in the long term.
A key aspect of this project is grid redundancy; the Froloo substation plays a key role in supplying the greater Basel area. Today, it is connected to the transmission grid in the stub, i.e. via a single line. If this line fails, electricity will no longer flow to the substation, which would substantially weaken supply in the region. Distributing the electricity flows along several north-south axes instead of just a few or a single one, as is the case today, will increase the security of supply in the region and, in the long term, the grid stability of Switzerland as a whole.
At the beginning of April 2022, Swissgrid submitted an application to the Swiss Federal Office of Energy to initiate the sectoral planning process. The sectoral plan for transmission lines (SÜL) is the Swiss government’s superordinate planning and coordination tool for the construction and expansion of transmission lines.
The planning area stretches from Flumenthal (canton of Solothurn) north over the Weissenstein and Schelten mountains to the Laufen basin as far as the substation in Brislach (Laufen district, canton of Basel-Landschaft), which will be connected to the new line. The line will continue from the substation in Brislach via the Blauen mountain range to Therwil (canton of Basel-Landschaft), the municipality where the Froloo substation is located.
Swissgrid developed various working corridors within this planning area.
In accordance with the sectoral plan for transmission lines (SÜL), a monitoring group consisting of various technical experts appointed by the SFOE assessed the variants for the planning corridor put forward by Swissgrid. Three corridors were examined in depth within the planning area: an overhead line corridor, a partial underground cable corridor and a total underground cable corridor. The monitoring group recommends the overhead line corridor.
The SFOE monitoring group submitted a recommendation to the Federal Council in favour of the overhead line corridor shown on the map. The working corridor put forward by Swissgrid in the SÜL application was optimised following an on-site inspection and various technical discussions.
Recent studies have expressed fundamental reservations about the use of long sections of underground cable in the extra-high-voltage grid for technical and operational reasons. On the one hand, long cable lines lead to undesirable reactive power compensation. On the other hand, cable systems can generate resonances that can disrupt or even seriously jeopardise stable grid operation. What is more, cable lines in the extra-high-voltage grid are also a hindrance when it comes to black start capability – i.e. the ability to restart the grid autonomously from a switched-off state. The operational risks increase for Swissgrid when there is a greater density of underground cables in the extra-high-voltage grid. In the future, it will only be possible to plan underground cables in the transmission grid where they are absolutely necessary. There is clearly no such need in the Flumenthal – Froloo project.
Before the Federal Council makes its decision, private individuals are free to give their views on the monitoring group’s recommendation during the consultation and participation procedure.
The project documents will be available from the relevant municipal administrations for 30 days from 2 May 2025. During this period, private individuals can take advantage of the public participation procedure and contact the canton they live in:
The project documents can also be consulted on the SFOE website during the above-mentioned period.
The Federal Council is expected to choose the corridor for the new 220-kV line by the end of 2025. Swissgrid will then prepare the construction project and submit the planning approval application to the competent authority, the Swiss Federal Inspectorate for Heavy Current Installations (ESTI). This will be followed by another public consultation. In the best-case scenario, provided there are no legal proceedings, Swissgrid will begin the construction of the line in 2030, and the work will take around two years. Once the new line has been commissioned, the existing 145-kV IWB line will be dismantled.
The following schedule is based on the current planning status and represents the best possible scenario, without taking into account potential legal proceedings.
| Start of sectoral plan for transmission lines (SÜL) | April 2022 |
| Participation procedure | May 2025 |
| Federal Council decision on corridor and technology | End of 2025 |
| Development of construction project | 2026-2027 |
| Planning approval procedure | 2028-2029 |
| Construction of the new 220-kV line | 2030-2032 |
| Dismantling of the existing 145-kV line | 2033 |
The existing 145-kV line runs close to the villages in some places. When planning the new 220-kV line, Swissgrid is taking care to ensure that the line is routed in a way that protects the landscape and residential areas as much as possible. Once the new line has been commissioned, the existing 145-kV line will be dismantled. This will considerably reduce the impact on residential areas, especially at the southern foot of the Jura and near the Laufen basin.
Media release: «Strengthening the security of supply in the greater Basel area»
