Flumenthal – Froloo grid project

An extra-high-voltage line is planned between Flumenthal (canton of Solothurn) and Froloo (municipality of Therwil, canton of Basel-Country) to replace the existing power line. In the long term, this new transmission line will strengthen the security of supply in the greater Basel area and across Switzerland.


Your contact for this grid project

Sandra Bläuer
Sandra Bläuer

Phone +41 58 580 21 11
info@swissgrid.ch

Planning area
Planning area
Existing line
Existing line
Substation
Substation

The most important facts at a glance

  • The existing 145-kilovolt line between Flumenthal and Therwil, which is operated by IWB (Industrielle Werke Basel), will be replaced by a 220-kilovolt line.
  • The increase in voltage between Flumenthal and Froloo is important for the security of supply in the greater Basel area.
  • At the beginning of April 2022, Swissgrid submitted an application to the Swiss Federal Office of Energy (SFOE) to initiate the sectoral planning process. The sectoral planning process for transmission lines (SÜL) consists of two steps. The first step involves defining a planning area. Swissgrid then draws up possible corridor variants within this area, including the transmission technology (underground cable, overhead line or a combination of the two).
  • The defined planning area runs from Flumenthal in a northerly direction towards Therwil, the municipality where the Froloo substation is located.
  • Swissgrid recommends three possible corridor variants. In all cases, the aim is to find a solution that has the minimum impact on residential and protected areas. The focus is on protecting people and nature.
  • Next, a monitoring group appointed by the SFOE will discuss and assess the corridor variants.
  • The Federal Council is expected to define the corridor between Flumenthal and Froloo, and therefore the technology of the line, by the end of 2023. Swissgrid will then establish the specific 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 the public disclosure of the project.

Project

Initial situation

The 145-kilovolt line between Flumenthal and Froloo (municipality of Therwil), which is operated by IWB (Industrielle Werke Basel) and covers a distance of 33 kilometres, is to be replaced by a 220-kilovolt line. The new line is part of the Strategic Grid being implemented by Swissgrid, which is therefore responsible for the planning. The extra-high-voltage line between Flumenthal and Froloo ensures electricity supply redundancy in the greater Basel area and thus strengthens the security of supply across Switzerland in the long term.

Why the project is important

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 Basel’s supply. Distributing the electricity flows along several axes instead of just a few or a single one, as is the case today, increases grid stability. The distribution of energy flows along several north-south axes also increases grid stability and therefore contributes to the long-term security of supply in the greater Basel area and across Switzerland. That is why the new 220-kilovolt line from Flumenthal to Froloo is needed.

The current power line between Flumenthal and Therwil runs over the Rüttelhorn mountain.
The current power line between Flumenthal and Therwil runs over the Rüttelhorn mountain.

Three possible corridor variants

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 SÜL process consists of two steps:

Step 1: Planning areas are first drawn up, i.e. geographic areas in which the new line should be built. The planning area is defined by the Federal Council. In the Flumenthal – Froloo project, there was insufficient scope for several planning areas, which is why Swissgrid and the cantons concerned, i.e. Basel-Country, Solothurn, Berne and Jura, agreed on a single planning area. This area stretches from Flumenthal in the canton of Solothurn in a northerly direction towards Therwil, the municipality in the canton of Basel-Country where the Froloo substation is located.

Step 2:In the application made to initiate the sectoral planning process, Swissgrid made a recommendation for three possible corridor variants within the defined planning area. The transmission technology for each corridor is always defined in advance, i.e. whether the line will be planned as an overhead line, an underground cable or a combination of the two. For each project, Swissgrid develops at least one corridor variant per technology. The focus is on protecting people and nature. Swissgrid therefore strives to create solutions that minimise the impact on residential and protected areas.

For this project, Swissgrid identified three possible corridors within the planning area, which largely overlap. First, a spatial overview was drawn up. The location was then assessed, and the third step involved determining which areas were – from an overall perspective – suitable for a line and which were not. The advantages and disadvantages of the three corridors were evaluated with various specialists, taking into account feedback from stakeholder groups.

Overview of the three corridor variants

1/4: Planning area
2/4: «Overhead line» corridor
3/4: «Underground cable» corridor
4/4: «Combination» corridor

«Overhead line» corridor

This corridor offers operational advantages and comparatively low costs. The challenges are to minimise the impact on protected areas, to ensure that the overhead line, which runs across the valleys in places, is carefully integrated into the landscape, and to maintain a suitable distance to residential areas, villages and individual farms.
Cost: approximately CHF 60 – 80 million.

«Underground cable» corridor

This corridor variant offers the greatest possible protection of residential areas, landscapes and protected areas between Flumenthal and Therwil, with the exception of wooded areas, which it would be necessary to cut through. Compared to the other two options, an underground cable is more expensive. Further challenges are posed by the geological conditions, which make tunnel construction and the protection of groundwater more difficult, as well as placing increased demands on grid operations.
Cost: approximately CHF 230 – 290 million.

«Combination» corridor

This corridor largely corresponds to the overhead line corridor. As a result, it has comparable advantages and disadvantages. The five-kilometre-long cable route between Froloo in Therwil and Ettingen would provide relief for the residential areas of Therwil, Reinach and Ettingen, as well as the landscape conservation area to the east. However, it presents an additional challenge, because it would be necessary to provide as much protection as possible for the amphibian spawning area, which is of national importance.
Cost: approximately CHF 80 – 100 million.

Dismantling the old line

The project will reduce the impact on the residential areas between Flumenthal and Therwil. The plan is to install the new line as far away as possible from residential areas. In addition, the existing 145-kilovolt line, which runs partly through the middle of heavily populated areas, will be completely dismantled after the commissioning of the new 220-kilovolt line.

1/2
2/2

The dismantling of the existing 145-kilovolt line will noticeably relieve the burden on localities such as Herbetswil.


Next steps

Next, a monitoring group appointed by the SFOE will discuss the corridors evaluated by Swissgrid and assess the effects on regional planning, the environment, technology and costs. The monitoring group’s recommendation will then proceed to the public collaboration stage. The Federal Council is expected to define the corridor between Flumenthal and Froloo, and therefore the technology of the new line, by the end of 2023. Swissgrid will then establish the specific 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 the public disclosure of the project.

Schedule in the best case scenario. If it comes to legal proceedings, the timetable will be delayed by two to four years.

Information events

In Therwil and Flumenthal

Dialogue with the citizens, municipalities, authorities and associations is important to Swissgrid. Swissgrid will be holding information events on the project at 6.30 p.m. on Tuesday, 28 June 2022 in Therwil (BL) and at 6.30 p.m. on Thursday, 30 June 2022 in Flumenthal (SO).


Downloads

Contact

Title

Please select a title.

Please enter your first name.

Please enter your last name.

Please enter a valid e-mail address.

Please enter your message.

Please click the checkbox.

Contact

Title

Please select a title.

Please enter your first name.

Please enter your last name.

Please enter a valid e-mail address.

Please enter your message.

Please click the checkbox.