The enhancement of the Bassecourt – Mühleberg extra-high-voltage line is part of the «Strategic Grid 2025». Swissgrid needs the 380-kV connection between Bassecourt and Mühleberg to ensure the security of supply in the medium-term. This is particularly important in the winter months, when Switzerland relies on additional energy imports.
The decommissioning of the Mühleberg nuclear power plant will result in the removal of part of Swiss energy production in Central Switzerland from the end of 2019. In the medium-term, this missing feed-in needs to be offset by higher production by Swiss power plants or energy imports from abroad. To enable the additional imports, the existing capacities of the extra-high-voltage lines and transformers between Bassecourt and Mühleberg need to be expanded. This is particularly important in the winter months, when Switzerland relies on additional energy imports.
In a normal supply situation, Greater Bern will receive an adequate supply of electricity even after the decommissioning of the Mühleberg nuclear power plant. 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 (redispatch) or measures on the power market (NTC adjustments), to respond to the situation and ensure grid stability. Swissgrid needs the 380-kV connection between Bassecourt and Mühleberg to ensure the security of supply in the medium term.
The Federal Council has enacted various measures to strengthen security of supply in the short term. These measures are ready to be implemented in the coming winter in order to deal with extraordinary and critical situations. They include plans for temporary voltage increases on the Bickigen – Chippis (Gemmi line) and Bassecourt – Mühleberg transmission lines.
To prevent congestion in the transmission system in the coming winter, the Federal Council is allowing a temporary increase in the operating voltage of the two transmission lines between Bickigen and Chippis (Gemmi line) and Bassecourt and Mühleberg from 220 kilovolts (kV) to 380 kV in the period from January to April 2023. This will relieve congestion in the Swiss transmission grid so that the energy from the pumped storage power plants in the Alps can be transported in full to Central Switzerland. In an emergency, Swissgrid can increase the transfer capacity of the extra-high-voltage grid on the basis of criteria set by the federal government.
Swissgrid is making the necessary technical preparations to operate the Bickigen – Chippis and Bassecourt – Mühleberg lines at a voltage of 380 kV on a temporary basis in the event of a tense supply situation. The measure is only planned for the period between January and April 2023. Test operations will take place on both lines between mid-December 2022 and the end of February 2023. This will involve temporarily changing the voltage of the existing lines from 220 kV to 380 kV. The test operation with 380 kV will only be carried out if the grid situation allows it. The combined test operations will verify the lines from an operational point of view. The two voltage increases influence each other and can shift congestion depending on grid utilisation. The scope and duration of the test operations were approved by the Federal Inspectorate for Heavy Current Installations (ESTI).
Swissgrid had already successfully tested the operation of the Bassecourt – Mühleberg line and the transformer in Mühleberg at a voltage of 380 kV in autumn 2021. This means that Swissgrid will be technically ready to operate this line at 380 kV on a temporary basis in the event of a critical supply situation. The planned construction measures necessary for the permanent voltage change to 380 kV are being carried out in two stages from mid-August 2022 until the end of 2023.
The Bassecourt – Mühleberg extra-high-voltage line is 45 kilometres long and was built in 1978. The Federal Inspectorate for Heavy Current Installations (ESTI) approved the line for an operating voltage of 380 kV back in 1978. However, it has been operated with a voltage of only 220 kV since that time. The voltage increase will take place in compliance with the limits specified in the Ordinance on Protection against Non-Ionising Radiation (NISV).
The line will be modernised along the existing pylons and will not change the landscape. In total, construction work is planned on 56 of 142 pylons as well as on the guyed gantries of the two substations in Pieterlen and Bassecourt: individual pylon foundations will be reinforced, double anchor chains will be installed, or the cable winch tension will be increased. The future operation at 380 kilovolts will result in an increase in noise emissions that will be hardly noticeable.
Swissgrid is investing approximately 17 million Swiss francs in the modernisation of the Bassecourt – Mühleberg line. The voltage increase project is part of the «Strategic Grid 2025», Swissgrid’s long-term grid planning.
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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. Everything is progressing according to plan, and the line should be ready for operation at 380 kilovolts by the end of 2023. In recent months, individual pylon foundations have been reinforced and double anchor chains have been installed. These are used to increase security, in particular at crossings with roads and railway lines or near residential areas. Work on the pylon foundations will continue until around mid-2023. The line will be temporarily taken out of service in autumn 2023 to allow installation work to be carried out on the pylons. The conductors will be tightened to gain more distance from the ground. This is one of several measures being taken to comply with all currently valid regulations and limits on electromagnetic fields and noise.
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.
