The 3S‑SupraStromSchiene project set out to demonstrate the feasibility of a high‑current direct‑current (DC) transmission line based on high‑temperature superconductors (HTS). The core technical goal was the construction of a 25‑metre long demonstrator that could carry a nominal 20 kA of DC while being fully integrated with cryogenic and electrical infrastructure. The line combined superconducting sections made from REBCO (Rare Earth Barium Copper Oxide) tapes with conventional copper or aluminium conductors, thereby allowing a direct comparison of losses and operational costs. Two manufacturers—D‑Nano and Theva—provided REBCO tapes produced by different fabrication routes, enabling the project to evaluate the influence of tape quality and cost on system performance. By arranging the superconducting tapes in a parallel configuration that reduced the required number of tapes by 50 %, the design achieved a significant reduction in material cost without compromising the 20 kA current capability.
The cryogenic system was a key component of the demonstrator. A liquid‑nitrogen‑based cooling loop, powered by a cryogenic pump and a pulse‑tube refrigerator built on the pulse‑tube principle, maintained the superconducting tapes below their critical temperature. The project quantified the heat load on the cryogenic system and found it to be larger than initially anticipated, which had a decisive impact on the overall system design. The integration of the cryogenic loop with the electrical layout required careful thermal management, and the experience gained highlighted the importance of rigorous safety protocols when handling liquid nitrogen in an industrial setting.
Electrical connections between the superconducting and normal‑conducting sections were addressed through the development of a novel contact technology. The design ensured low‑resistance, mechanically robust joints that could withstand the high currents and thermal cycling inherent in a DC power system. The demonstrator’s modular construction allowed for the replacement of individual sections, facilitating future scaling to longer lengths and higher currents. The project’s results indicate that, for currents above 20 kA and distances of 20–30 m, the operational losses of the superconducting line are lower than those of comparable copper or aluminium conductors, despite the higher upfront investment in material and cryogenic equipment.
Beyond the technical achievements, the project served as a platform for knowledge transfer among its partners. The Karlsruhe Institute of Technology (KIT) led the feasibility study and coordinated the overall project, while Vision Electric GmbH provided industrial expertise and project management. D‑Nano and Theva supplied the superconducting tapes and contributed to the design of the cryogenic system. The project was funded by the German Federal Ministry of Economics and Energy, with the German Research Center for Energy (Jülich) acting as the project sponsor. Over a period of approximately three years, the partners collaborated to move from conceptual design through manufacturing, assembly, and testing, culminating in the successful operation of the 20 kA demonstrator.
The 3S‑SupraStromSchiene project has produced a set of practical insights that will inform future industrial deployments of HTS DC transmission lines. The demonstrator proved that high‑current superconducting conductors can be integrated into existing power infrastructure, offering reduced electrical losses and potential space savings. The experience gained in cryogenic system design, tape arrangement, and joint technology will accelerate the transition of HTS technology from laboratory prototypes to large‑scale, commercially viable power transmission solutions.