Swissgrid is planning to replace the existing 220 kV line between Niederwil and Obfelden, which is around 17 kilometres long, with a 380 kV line. The new line will make a significant contribution to the security of supply in the cantons of Aargau, Zurich and Lucerne.
The Swiss Federal Office of Energy (SFOE) has proposed a corridor, including transmission technology (overhead line or underground cable) for the specific planning of the new line. It is based on the recommendation made by a monitoring group. The proposed planning corridor runs as an overhead line from the Niederwil substation south-east of the settlement area of Besenbüren and then passes under the Reuss as an underground cable south of Jonen. From there, it is once again planned as an overhead line to the Obfelden substation.
The SFOE placed the proposed planning corridor in a public consultation procedure from 2 December 2019 to 29 February 2020, during which communities, associations and private individuals could make statements. The cantons of Aargau and Zurich were also consulted. Particularly strong opposition was expressed to the proposed transmission technology.
The Federal Council determines the planning corridor and technology
The Federal Council supported the variant proposed by the monitoring group and defined the planning corridor and transmission technology on 31 August 2022. Swissgrid is preparing the construction project on this basis. Among other things, it will determine the specific line routing and the locations of the transitional structures for the underground cable section, as well as defining deadlines and costs and holding negotiations on transit rights.
This project provides for the upgrade or replacement of the 17-kilometre 220 kV line between the substations at Niederwil (AG) and Obfelden (ZH) with a 380 kV line. The line is part of the upgrade programme from Beznau to Mettlen. The Beznau – Mettlen line currently causes congestion in the Swiss and European transmission grid. The upgrade of the line will improve energy distribution in Central Switzerland and the supply for greater Zurich. The north-south connections have to be upgraded, which includes upgrades beyond the borders. The new production patterns, such as the expansion of wind power in Northern Europe and shutdown of nuclear power plants, require that Switzerland increases its exchange of electricity with Europe.
For each grid project, Swissgrid checks both cable and overhead line options. The Federal Council reaches a decision on whether to install an underground cable or an overhead line during the sectoral plan process. A monitoring group deployed by the Swiss Federal Office of Energy provides the Federal Council with recommendations in defining the planning corridor and in deciding on the technology (overhead line or underground cable). For this purpose, it discusses and evaluates the versions prepared by Swissgrid.
The experts assessed the corridor variants according to the criteria set out in the transmission line evaluation scheme (regional planning, technology, environment and economic efficiency) before submitting a recommendation to the Federal Council. The solution envisaged consisted of an overhead line with almost four kilometres of underground cabling on the section crossing the protected Reuss landscape area between Besenbüren and Jonen (AG).
The best possible compromise is sought
When drawing up the detailed plan, Swissgrid attaches great importance to routing the lines in a way that is as compatible with residential areas and as gentle on the landscape as possible. The corridor allows for a line routing that is far enough from residential areas to significantly reduce the visual impact thanks to the dismantling of the existing 220 kV line. In addition, Swissgrid is examining options for bundling its 380 kV line with Axpo’s 110 kV line along two sections of the planning corridor. The construction project should meet the requirements of regional planning as well as landscape and environmental protection, and take into account the concerns of the local population as far as possible, whilst being both technically feasible and economically justifiable.
Swissgrid is responsible for drawing up the construction project on the basis of the planning corridor defined by the Federal Council. During the construction project phase, Swissgrid will regularly exchange information with the affected communities in the Reuss Valley, local and regional interest groups, environmental associations and cantons about the current status of the plans and the next steps. The aim is to find the best possible compromise to take into account the numerous demands on the project.
Swissgrid started working on the construction project as soon as the planning corridor had been defined by the Federal Council on 31 August 2022. Once the construction project has been drawn up, probably in 2024, Swissgrid will submit the planning approval application with all the necessary documents to the competent approval authority (Federal Inspectorate for Heavy Current Installations, ESTI). This application will then be made public. On obtaining the legally binding planning approval decision, the line can be built, which will take about four years. After the commissioning of the new 380 kV line, the dismantling of the existing 220 kV line will begin. In the event of appeals and legal proceedings, commissioning may be delayed by several years.
Overhead lines are visible objects in the landscape and can have a negative impact on the landscape depending on their location. The interventions must be assessed by the federal government and the cantons in accordance with the protection objectives and the consideration of compliance with the federal tasks (ensuring the supply of energy).
Underground cables are generally assessed more favourably in relation to the landscape than overhead lines. While the cable route is not visible on open terrain, it remains clearly visible in forested areas as a necessary zone without deep-rooted and high trees must be maintained.
Interventions in groves by route clearing and/or the suppression of growth. Higher pylons make it possible to cross above the forest. However, the effects on the landscape are greater. The land under overhead lines can be used for agricultural purposes.
Interventions in groves are greater than the interventions required for an overhead line due to the need for route clearance. The cable route is clearly visible in the forest because, for underground cables, the area above the line has to be kept free of high trees and deep-rooted plants. The soil above the underground cable can be used for agricultural purposes.
Soil sealing takes place on a small scale around the pylon foundations. Temporary interventions, especially around the pylon locations, during the construction phase.
Extensive impact during the construction phase. Around 20,000 m2 of soil is impacted to construct an underground line 1 kilometre long (construction, installation locations, construction tracks). Depending on the electricity load, the electrical losses heat the underground cable and the surrounding soil. Measuring installations are used to measure and monitor the soil temperature. Soil sealing takes place on a small scale around the coupling shafts.
The noise emissions are caused by electrical discharge on the conductors (corona noises), which occur on high-voltage overhead lines under certain weather conditions, such as fog, rain or snow.
In the operating phase, the noise emissions originate from compensation systems. Compensation systems emit a humming noise similar to transformers.
The magnetic field depends on the current and the phase arrangement of the conductor. In an overhead line, the limit of 1 microtesla is complied with from a distance of about 60 – 80 metres from the line axis.
In underground cables, the magnetic field depends on the current, the installation depth and the cable arrangement. The spatial expansion of the magnetic field is lower than for overhead lines, but the strength directly over the underground cable is much higher than under an overhead line. The limit of 1 microtesla is complied with from a distance of about 6 – 8 metres from the line axis of the underground cable. The magnetic field can amount to more than 100 µT directly above the line. A value of 100 μT must not be exceeded from a soil distance of 20 cm.
Electric power transmission leads to electrical losses. A small portion of the power is transformed into heat. The voltage causes losses as well. With overhead power lines, these are known as «corona losses,» because the air serves as an insulator. In cables, it is the insulating material that conducts a small portion of the power, likewise resulting in losses. Since cables have a significantly larger cross section than overhead lines, they experience much lower conductor losses than the latter. However, the interplay of cable and grid generates reactive power, which has to be compensated for. These losses are much higher with cables than with overhead lines.
