Energy transition: New electricity motorways for Germany

New electricity motorways are to transport wind power from the north to the south of Germany in future

According to calculations by the German government, the production of renewable electricity will increase by around 90 GW by 2037 - and with it the need for transport. Based on the Netzentwicklungsplans 2037/2045 (2023) of the federal government, 69 line projects with a length of 12,300 km are to be added in the future to ensure overland transport over long distances from the producer to the consumer. The so-called electricity motorways play a special role here. But what does this mean in practice for the responsible long-distance grid operators or project organisers?

Published by: Federal Network Agency
© GeoBasis-DE / BKG 2023
© Transmission system operator Status: 30.06.2023

1. German electricity motorways: planning status of two pilot projects

Since 2015, the Bundesbedarfsplangesetz has declared underground cabling to be the rule for the construction of electricity motorways using extra-high-voltage direct current (HVDC) transmission lines. Overhead line sections are only still possible in strictly defined exceptions. With a length of 2,300 km, HVDC lines comprise around a third of the entire transmission grid currently being planned or constructed. The responsible transmission system operators describe specific HVDC expansion measures in the grid development plan - as is the case with the two central infrastructure projects SuedLink and SuedOstLink.

Both projects are also categorised by the European Union as a "Project of Common Interest" (PCI), which means that they are also of great importance for security of supply and the expansion of renewable energies at a European level.

a) SuedLink: Wind energy for the German south

With a total length of around 700 km and a total transmission capacity of 4 GW, SuedLink consists of two 525 kV direct current lines that are being planned, built and operated in parallel.

They run from Brunsbüttel in Schleswig-Holstein in the north to Großgartach (Leingarten) in Baden-Württemberg, north of Stuttgart and between Wilster in Schleswig-Holstein and Bergrheinfeld West in Lower Franconia.

The project developer and transmission system operator TenneT is responsible for the northern section of the route and the converters in Schleswig-Holstein and Bavaria. TransnetBW is the project developer responsible for the southern section of the route and the converter in Baden-Württemberg.

TenneT and TransnetBW are planning the joint installation of the Wilster-Bergrheinfeld West line along the majority of the route. In the grid development plan, this joint section is referred to as "Corridor C". The designation "SuedLink" in turn summarises both underground cable projects.

A converter system built in 2021 by grid operator TenneT in Wilster, Schleswig-Holstein

Construction of the first converter at the Leingarten substation near Heilbronn by TransnetBW began in July 2023. The official start of construction of the first northern section of the "ElbX" line in Wewelsfleth (Schleswig-Holstein) was 11 September 2023: In the first construction section, which is considered technically challenging and is around 5.2 km long, an accessible tunnel with an internal diameter of 4 m is to be built under the Elbe.

Commissioning of the power line, which is intended to secure the supply of wind power (onshore and offshore) to Bavaria and Baden-Württemberg, is planned for 2028. As the line is designed for direct current, it can only go into operation as a whole and not in sections.

b) SuedOstLink: Backup power for Isar 2 nuclear power plant

SuedOstLink, which will realise two underground direct current connections (2 GW each) at the
525 kV voltage level, runs for around 540 km between Wolmirstedt near Magdeburg (Saxony-Anhalt) to the Isar site near Landshut (Bavaria).

The project developer is 50Hertz, which is responsible for the northern section of the electricity motorway. The southern part of the route, located in Bavaria, is the responsibility of the transmission system operator and project developer TenneT. Construction of the converter at the Wolmirstedt site (Magdeburg) began in March 2023.

The southern part of the route, which is located in Bavaria, is in turn the responsibility of the transmission system operator and project developer TenneT.
On 11 December 2023, even before the planning approval decision was made, the start of construction work for the converter at the Isar site near Landshut was celebrated in the presence of Bavaria's Minister President Dr. Markus Söder and Minister of State Hubert Aiwanger.

Opening of the SuedOstLink construction work in Isar: Minister President Markus Söder and Minister of State Hubert Aiwanger present pieces of an HVDC cable as part of the local team

From 2027, green electricity is to be reliably transported to Bavaria from the offshore wind farms in northern Germany and from the wind farms in Saxony-Anhalt, Brandenburg and Mecklenburg-Western Pomerania in order to compensate for electricity shortages caused by the decommissioning of the Isar 2 nuclear power plant in the district of Landshut, among other things. As with SuedLink, the power line does not allow partial operation due to the direct current and can only start work once the project is completed.

Underground installation of green power lines: pipeline construction in practice

a) Laying techniques

Open and closed construction method
Different techniques are used for underground installation of HVDC cables, depending on the geographical conditions or the development on site.

The open construction method is used wherever no obstacles - such as roads or bodies of water - need to be crossed underneath. In this variant, an excavator digs a cable trench in the respective construction section. A protective pipe is placed at the bottom of the trench and the trench is backfilled with soil. With the help of a cable winch, the power cable is then pulled into the empty conduit piece by piece. Once the construction work has been completed, nothing stands in the way of immediate renaturalisation or further agricultural use of the area, the so-called protection strip.

If obstacles, important infrastructure or sensitive areas need to be tunnelled under, the closed construction method is the method of choice. This includes various underground drilling methods such as pipe jacking, microtunnelling or flush drilling. In flush drilling (HDD: Horizontal Directional Drilling), for example, sections of the route are first bored underneath. Once the empty conduit has been inserted into the channel, the underground cable can be pulled in. This installation technique can currently only be used for sections of a maximum length of 1 km.

SuedLink underground cable from the Prysmian Group

b) HVDC cables: characteristics and advantages of underground installation
Extra-high-voltage direct current transmission lines have so far been used as overland lines or submarine cables. If they are laid underground, they require a converter station at their start and end points, which converts the direct current back into three-phase current.

Materials and structure
The core of HVDC cables consists of a copper conductor - which transmits the direct current - a metal shield and a cable sheath. The solid outer sheath protects the cable from possible damage. So-called fibre optic cables (glass fibres) are also integrated into the cable, which enable the current flow to be monitored.

The underground cables from the Prysmian Group, for example, which are also used in the construction of SuedLink and SuedOstLink, weigh 41 kg per metre and have a diameter of 15 cm.

Technological characteristics and significance for the transport of renewable electricity
Due to their nature, HVDC cables enable an efficient, very high transmission capacity of up to several 1000 MW with voltages of up to 1 MV. When bridging long distances, transmission losses are significantly lower compared to AC cables. The cable insulation also results in almost complete shielding of the static magnetic field that is generated, the field strength of which (comparable to the earth's magnetic field) is barely measurable on the earth's surface.

Renewable energies such as wind and solar power are subject to natural regional or seasonal fluctuations. HVDC technology can better control the resulting load flows and integrate them into the electricity grid, which also contributes to its stability.

Underground cable work for NordLink: Underground cable conduits are made of particularly hard-wearing materials

c) Cable ducts for HVDC lines

The underground installation of HVDC power lines in the form of so-called unpressurised lines places special demands on the conduit systems used: During operation, they are permanently exposed to high thermal loads of up to 70 °C and voltages of 525 kV.

Cable conduits for both open and closed installation must therefore be particularly resistant in terms of their material properties and designed for a long service life.

Special materials made of polyethylene (PE) or polypropylene (PP) have the corresponding properties. These are permanently heat-resistant and offer the greatest possible safety for people and the environment when used with HVDC cables.

Conclusion:

With the increasing realisation of numerous new line projects, the energy transition is also progressing in the German electricity sector. The gradual phase-out of fossil energy generation - towards greater sustainability and the goal of decarbonising Germany by 2045 - is therefore within reach.