Swissgrid is upgrading the 150-kilovolt line between Samstagern, Thalwil, Waldegg (Zurich) and Obfelden to 220 kilovolts (kV) in stages. By doing so, Swissgrid will strengthen the security of supply for the city of Zurich and the western shore of Lake Zurich in the long term.
Currently, the greater Zurich area is much better connected to the extra-high-voltage grid from the north than from the south. In the north, the 220 kV grid reaches as far as the city of Zurich via the Auwiesen and Fällanden substations. In the south, on the other hand, the Obfelden and Samstagern connection points are only connected to the city boundary via one 150 kV line each. Swissgrid is therefore planning to upgrade the line between Samstagern (Richterswil), Thalwil, Waldegg (Zurich) and Obfelden to 220 kilovolts (kV) in six sub-projects. The upgrade is part of Swissgrid’s Strategic Grid.
With the connection of the Thalwil and Waldegg substations to the extra-high-voltage grid and the reinforcement of the lines between Obfelden and Samstagern, the extra-high-voltage grid will also reach the city of Zurich from the south. This will increase the transport capacity as well as the security of supply for the city and the entire Zurich region. The project is also very important for Swiss Federal Railways (SBB), as two 132 kV railway power lines will be carried on one of the sections. These two lines will connect the Sihlbrugg substation with the Zurich substation, significantly improving the security of supply for SBB in the greater Zurich area.
The expansion of the line between Samstagern (Richterswil), Thalwil, Waldegg (Zurich) and Obfelden consists of six sub-projects. The construction costs of the entire project amount to around CHF 170 million. The work on the various sections and the dismantling of the existing line sections is expected to take until 2030.
At the Samstagern substation, modifications to the substation and line must be implemented in order to connect the line, which is currently operated at 150 kV, to the 220 kV grid.
Between Samstagern and Schweikrüti (municipality of Thalwil), the line is already designed for extra-high voltage, but is only operated at 150 kV. Swissgrid is planning to increase the voltage to 220 kV. In September 2024, Swissgrid submitted the planning approval application for the voltage increase between Samstagern and Schweikrüti. The line project complies with all applicable laws and ordinances. This section of the line was already designed for 220 kV when it was built in the 1980s, which is why only the replacement of the underground cable and the occasional installation of phase spacers is required. The Samstagern substation will be structurally adapted and two new pylons will be built in its vicinity. According to the current schedule, the voltage will not be increased until 2027 at the earliest.
A legally binding construction permit for a new overhead line along a modified route has been issued for this section. Following the commissioning of the newly constructed line section between Schweikrüti and Sihlhalden in 2023, Swissgrid will begin the second construction phase between Sihlhalden and Kilchberg in spring 2025. The construction of an underground cable is not possible on this section because two 132 kV SBB lines will be carried on the same pylons as the Swissgrid line. For technical reasons, the SBB lines must necessarily run above ground on this section. The Thalwil substation, which is important for supplying the western shore of Lake Zurich, will be connected to the Schweikrüti – Kilchberg section. As soon as the Thalwil substation is connected to the extra-high-voltage grid, Axpo’s 110 kV Obfelden – Thalwil line (known as the Albis line), which served as an alternative measure, will be dismantled. This will substantially reduce the burden on the residential areas of Langnau and Gattikon. At Kilchberg, the overhead line will be lowered into the ground via a new transition structure and then continue underground.
The new line from Kilchberg to Frohalp near Zurich Manegg station will be built underground along the motorway. It will then be routed through the conduit blocks built during the construction of the two tunnel tubes of the Uetliberg tunnel before continuing to the Waldegg substation via a two-kilometre underground tunnel (with a diameter of approx. four metres).
Swissgrid is planning a new underground substation with ewz. From here, electricity will flow into the city of Zurich. The substation will be built underground to minimise the impact on the landscape and the visibility for the neighbouring residential area.
The line between Obfelden and Waldegg (Zurich) is already designed for extra-high voltage, but is only operated at 150 kV. Swissgrid plans to increase the voltage to 220 kV by 2027, so that the Waldegg substation can then be connected to the transmission grid. This will probably not require any major structural measures on the line.
Construction work on the new line between Schweikrüti and Kilchberg has been divided into two stages. Swissgrid completed the first stage between Schweikrüti and Sihlhalden within a year.
The foundations, which consist of 2,200 tonnes of concrete, were built between October 2022 and March 2023. The pylon assemblers then erected six new pylons between April and July 2023 – they carried out this work at a dizzying height, as the pylons are 58 to 85 metres tall. The pylons contain a total of 330 tonnes of steel and 16,800 screws. Finally, from July to September 2023, around 32 km of conductors were installed on the first section of the line, which measures 1.7 km. This was no easy undertaking, as the line crosses the A3 motorway twice.
Exactly one year after work began, Swissgrid put the new line into operation at an initial voltage of 150 kV so that the old line can be dismantled along this section by the end of 2024. This will visibly improve the landscape around Gattikerweiher and the residential area.
Work on the second stage between Sihlhalden and Kilchberg will take place between October 2024 and December 2025. Here, the new line will largely run along the existing route. If everything goes to plan, the overall project will be completed by 2030. The remaining parts of the old line will then be dismantled.
