July 27, 2021
6 min read

Siemens Energy’s transformer factory in Nuremberg is supporting highly efficient HVDC power transmission to drive Germany’s energy transition

SuedOstLink and ULTRANET

Siemens Energy is contributing to the Ultranet and SuedOstLink projects, two of the planned electricity highways aimed at advancing the energy transition in Germany. The transmission capacity of these two corridors will be 4 GW in total, and SuedOstLink will be the most powerful high-voltage direct current (HVDC) system in voltage-sourced converter (VSC) technology in the world. Siemens Energy will supply more than 50 HVDC converter transformers from its transformer factory in Nuremberg – key components to enable transmission of renewable energy from northern to southern Germany.

“DC highways” from north to south – when AC cables just won’t get the job done

The decision to shut down Germany’s nuclear and fossil-fuel power plants in pursuit of the country’s energy transition increases the share of power demand being met by renewable power generation in northern Germany. HVDC is the transmission solution delivering the highest energy efficiency and return on investment to meet the challenge of tackling imbalances between power generation from northern onshore and offshore installations, and energy consumption in western and southern Germany. With its HVDC Classic (line-commutated technology) and HVDC PLUS (voltage-sourced technology), Siemens Energy is the market leader in efficient transmission of renewable energy to load centers.

The converter transformer for the Ultranet project: the prototype was manufactured at Siemens Energy’s power transformer factory in Nuremberg

Grid operators Amprion and TransnetBW announced in 2015 that Siemens would be their reliable partner for the Ultranet power highway. For this project existing AC overhead transmission lines will be converted to DC over a distance of 340 km. The project was meanwhile extended by another transmission line via DC cables with approx. 300 km length and now includes three HVDC PLUS converter terminals with full-bridge technology. The framework agreement calls for Siemens Energy’s power transformer lead factory in Nuremberg to deliver 39 HVDC converter transformers (13 units per terminal). 

Leading manufacturer of HVDC equipment and record-breaking projects

Siemens Energy leads the market not only because of its two outstanding HVDC converter station technologies, but also by manufacturing innovative HVDC converter transformers.

Taking into account the advantages of HVDC, such as reduced energy transmission losses and smaller footprint as well as reduced right-of-way costs, investments in HVDC equipment pay off easily. HVDC converter transformers can also be filled with bio-degradable ester fluids. The advantage, besides the improved green footprint, is the reduced fire hazard due to higher flash and fire points.

Deviations in winding geometry would significantly impact the performance and efficiency of the HVDC converter transformer, and therefore the whole HVDC system. This is why Siemens Energy develops, designs and manufactures its largest transformer class to the highest precision standards.

We always put great emphasis on the quality of our HVDC converter transformers, and are proud of our outstanding abilities in designing and manufacturing these transformers. In 2017, we enabled the first 1,100 kV HVDC transmission link, delivering the world’s first converter transformer in that voltage class to China.

Eduardo Terzi

Siemens Energy’s Senior Vice-President for Non-Switching Products

Siemens Energy is furthermore involved in many projects for transmitting high-voltage direct current between countries or to load centers, such as the ALEGrO project linking Germany and Belgium, the Viking Link between Denmark and the UK, the Attica-Crete line in Greece, the Belo Monte project transporting hydro power to Rio de Janeiro, and numerous offshore grid connections, the latest of which include Borwin 3, Dolwin 6, and Borwin 5.


Ultranet, a joint power transmission project by grid operators Amprion and TransnetBW, counts among the three main electricity highways in Germany’s energy transition plan. It calls for the southern section of the planned corridors to utilize HVDC technology from north to south. The 340 km link between the installation in Osterath and Philippsburg (formerly a nuclear power plant) was planned as the “A-Süd” corridor. Ultranet marks the first time that an existing 380 kV AC line in the southern section will be combined with a newly laid DC line. The northern section of the line will be implemented entirely as underground cables.


The two transmission system operators 50Hertz Transmission GmbH and TenneT TSO GmbH are the main contractors building the SuedOstLink – one of the electricity highways in the German energy transition plan. It will transport renewable energy from Wolmirstedt substation in the state of Saxony-Anhalt in north-central Germany to the connection point of a (then) former nuclear power plant near the city of Landshut in Bavaria. When excess wind power is generated, the northern Wolmirstedt converter station will convert the alternating current into direct current. The energy will mainly be transported south via underground cables at a voltage of 525 kV. The southern terminal will convert the incoming direct current back into alternating current and feed it into the grid. The transmission capacity will be sufficient to supply more than four million households with electricity.