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