An intervention under high voltage: Modernizing the Daxlanden substation

Germany’s Daxlanden substation ranks among the largest in Europe. Now, Siemens Energy is modernizing it step by step to prepare for the energy transition and the surge in electricity demand.

Towering steel rods rise into the sky like antennas – sometimes standing alone, sometimes in T- or H-shapes – thick black cables weaving between them. Then there are the gray giants: container-like housings that seem to silently monitor everything. This is likely how a casual observer would describe the Daxlanden substation near Karlsruhe in Southwest Germany, which spans approximately 145,000 square meters and ensures that electricity reliably reaches consumers.

On this site, power lines of various voltage levels converge in a star-like pattern. They are connected and disconnected via the steel rods, which are actually circuit breakers. The gray giants, meanwhile, are transformers – responsible for adjusting the voltage of electricity for further transmission.

The substation is operated by the transmission system operator TransnetBW, which has commissioned Siemens Energy to carry out the modernization. The upgrade of the Daxlanden substation is expected to be completed by 2035. It is part of the major project “Netzverstärkung Badische Rheinschiene” (NBR), a program to expand the Baden-Rhine corridor from 220 to 380 kilovolts and modernize several substations between Karlsruhe-Daxlanden and Eichstetten near Freiburg.

Michael Jesberger, Chief Operating Officer of TransnetBW, underscores the project’s national and regional relevance: “Rebuilding Daxlanden is emblematic of the grid transformation Germany needs to make its energy transition a success. For Karlsruhe and all of Baden-Württemberg, it delivers extra resilience, capacity, and climate-neutral power. Working shoulder to shoulder with Siemens Energy, we’re proving that even the most complex upgrades can be completed safely while the lights stay on.”

The power grid can be compared to a road network. The transmission grid is like the highway system – it connects regions across Europe and individual countries, transporting large volumes of electrical energy. It operates at voltages of 220 or 380 kilovolts. The distribution grid, running at 110 kilovolts, is like a system of country and district roads. It receives energy from the transmission grid and distributes it to municipal utilities and larger industrial customers. The voltage is then reduced to medium-voltage levels between 1 and 60 kilovolts to carry electricity into cities, and finally down to 400 volts in the low-voltage grid.

And the Daxlanden substation? It’s essentially a highway interchange where electrons can switch from the 380-kilovolt level to the 220-kilovolt level, or vice versa. It plays a central role in ensuring security of supply for the Karlsruhe region. That makes its modernization both essential and challenging. A substation of this size can’t simply be taken offline – it must be rebuilt while remaining in operation.

“Fields” in this case refer to air-insulated switchgear. One field is about 20 meters wide. In Daxlanden, these switchgear units are now being dismantled and replaced with two gas-insulated switchgear units as part of the modernization project. These are significantly more space-efficient, requiring only a third of the area of the old systems. This means more transmission capacity on the same footprint in the future.

With the switch from air- to gas-insulated switchgear, new busducts are also needed to carry electricity to other components such as transformers. Siemens Energy is installing busducts from its Blue Portfolio: instead of using the insulating gas sulfur hexafluoride, they use processed air – known as Clean Air – for insulation, enabling more climate-friendly operation.

At the same time, the substation must also become smarter. “New measurement technology is important, too. Substations today must operate more flexibly and manage power flows in both directions,” says Walczak. “In the past, there was one power plant feeding in energy. Electricity was transported from the plant to the substation, transformed to different voltage levels, and distributed to consumers. Today we have a gas plant here, a solar installation there, and a wind farm elsewhere. So the grid today has to be much smarter – with far more data collection – than it was back then.” Back then, that was in the early 1960s, when the Daxlanden substation was built.

The Daxlanden substation may not be a large construction site on its own, but it is part of the larger construction site that is the European power grid. According to an analysis by the International Energy Agency, 40 percent of Europe’s power grids are more than 20 years old –meaning they’ve reached about half of their average service life. In 2024 alone, 1,650 gigawatts of solar and wind projects worldwide were in advanced stages of development, waiting to be connected to the grid.

Strong coordination and intensive dialogue between governments, transmission system operators, regulatory authorities, developers, and manufacturers are essential to expand the supply chain sufficiently and sustainably. Only then can transmission projects be implemented on time and the energy transition continue to move forward.

Because the need for expansion and modernization is enormous. Ultimately, it can only be tackled together – step by step, just like in Daxlanden.

About the author: Christina Hümmer works in communications at Siemens Energy, where she has specialized in grid-related topics for many years.