Overview
The electricity system in Europe and Switzerland is undergoing radical transformation. Due to decarbonisation, power plants with guaranteed capacity are being taken off the grid, and new, renewable energies are being expanded. These developments affect grid operators in various ways and present them with major challenges. North-south connections such as the Bassecourt – Mühleberg line are not only important for transporting energy away from Swiss production sites, but also increase Switzerland’s import capacity, as the country is unable to cover its electricity requirements from its own production in winter.
In a normal supply situation, Greater Bern receives an adequate supply of electricity, even after the decommissioning of the Mühleberg nuclear power station. In the event of short-term grid congestion, the Swissgrid grid control room can take various measures, such as switching operations or interventions in electricity generation (redispatching) or measures on the power market (NTC adjustments), to respond to the situation and ensure grid stability. Swissgrid needed the 380-kV connection between Bassecourt and Mühleberg to ensure security of supply in the medium-term. Swissgrid has reached another milestone following the completion of this project from the «Strategic Grid 2025». However, the further modernisation of the transmission system must be implemented rapidly in order to increase grid security and security of supply in Switzerland in the long term.
Project
The 45-kilometre Bassecourt – Mühleberg line was built in 1978 and had already been approved for a voltage of 380 kV at the time. Swissgrid operated it at 220 kV until the end of July 2023 before disconnecting it from the grid for three months. During this three-month outage period, installation work was carried out to modernise the line. Swissgrid had carried out the civil engineering work in advance, starting in summer 2022, while the line was still in operation. The structural measures were necessary because the regulations and limits regarding electromagnetic fields and noise have become stricter since the line was built in 1978. After around 15 months of construction, Swissgrid successfully put the line into operation at a voltage of 380 kV on 21 November 2023.
The voltage was increased in compliance with all applicable regulations and limits regarding electromagnetic fields and noise.
The line was modernised along the existing pylons, and the landscape has not changed. In total, Swissgrid carried out work on 56 of the 142 pylons and on the guyed gantries of the two substations in Pieterlen and Bassecourt: individual pylon foundations were reinforced, twin tension strings were installed, or the cable winch tension was increased. The operation at 380 kilovolts has resulted in an increase in noise emissions that is hardly noticeable.
Swissgrid invested approximately 17 million Swiss francs in the modernisation of the Bassecourt – Mühleberg line. The project is part of the «Strategic Grid 2025», Swissgrid’s long-term grid planning.
Schedule
On 22 August 2019 the Swiss Federal Office of Energy approved the voltage increase of the extra-high-voltage line between Bassecourt and Mühleberg from 220 to 380 kilovolts and dismissed the objections. These generally related to the load from magnetic fields and the noise or demanded that the line route be relocated or cabled. Various appellants have appealed to the Federal Supreme Court against the decision made by the Swiss Federal Office of Energy. The Federal Supreme Court confirmed the planning approval for increasing the voltage of the Bassecourt – Mühleberg line, rejecting all complaints, on 23 March 2021.
Swissgrid started the necessary renovation work in August 2022. After around 15 months of construction, Swissgrid successfully put the line into operation at a voltage of 380 kV on 21 November 2023.
The renovation work on the existing line at a glance
Existing electricity pylons and other components were modified during the modernisation work. The line routing remained unchanged, as did the landscape. In total, renovation work was carried out on 56 of the 142 pylons and on the guyed gantries of the two substations in Pieterlen and Bassecourt. The concrete foundations, pylons and portals were reinforced, and twin suspension strings were installed. These serve to increase safety, especially near residential areas or at intersections with roads or railway lines. The cables were tightened in order to increase the distance between the ground and the live conductors. By modernising the line, all current ordinances and limits regarding electromagnetic fields and noise are now complied with during operation at 380 kV.
Substations
New transformers in the Mühleberg and Bassecourt substations
Swissgrid has commissioned a new transformer in the Mühleberg substation. Together with the increase in voltage of the extra-high-voltage line, the transformer in Mühleberg will ensure that current can be transported with a voltage of 380 kV as well as 220 kV.
The last of four transformer poles was delivered to Mühleberg in mid-September 2020. The new transformer was transported from the port Auhafen in Basel to Mühleberg in four heavy-load transporter trips. Every transformer pole has a transport weight of 170 tonnes and is around 11 metres long, almost 3.6 metres wide and 4.5 metres high. The transformer has been operational since the end of 2020.
The renovation work for the new 380/220 kV phase-shifting transformer in the Bassecourt substation has been completed. The new transformer has been in operation since May 2020 and enables better control of the energy flows from France.
