AC2DC

The energy transition poses growing challenges for electricity grids: The increasing feed-in of renewable energies, the expansion of electromobility and the coal phase-out by 2038 require new solutions for grid stabilization. The AC2DC project is a highly innovative research project with practical relevance that addresses these issues. It promises exciting developments in power electronics and grid integration and will have a direct impact on the future of electricity grids and the energy transition. It starts there by converting alternating current (AC) into direct current (DC) lines. 

Although alternating current (AC) dominates the electricity grid today, direct current (DC) offers decisive advantages: higher efficiency, lower losses and better controllability. In view of the growing challenges posed by decentralized feed-in, electromobility and the coal phase-out, AC distribution grids are increasingly reaching their limits. DC technology enables direct connection of generators and consumers without conversion losses, uses existing cable infrastructures more efficiently and reduces the need for costly grid expansion. Currently, DC transmission technology is only used in the construction of new HVDC lines. Converting existing AC cables to DC transmission has not yet been considered and, due to the high costs, only preliminary work on medium-voltage DC connections has been carried out to date. 

The project therefore represents a completely new approach, which is why there have only been limited investigations into the recommissioning of cable routes, power quality limits in DC grids and suitable selective protection algorithms. However, this innovative technology allows the existing grid infrastructure to be used more efficiently, costly grid reinforcements can be avoided and grid quality can be improved. New DC/DC converters, protection and control systems as well as multilevel converters ensure a flexible, stable and future-proof power supply.

As part of the project, a successful consortium meeting has already been held to coordinate the research partners, initial system concepts for DC grids have been developed and field tests are planned to validate the technologies developed. 

The AC2DC project can only be realized through close cooperation with strong partners from science and industry. TU Dresden is working on grid protection, voltage quality and the load on cable systems and is developing new methods and test procedures for recommissioning. Maschinenfabrik Reinhausen (MR) is developing a new type of galvanically isolating DC/DC converter system for field testing, while HIGHVOLT Prüftechnik Dresden is designing a monitoring system for converted AC cables and characterizing their properties before and after conversion to DC. We at Fraunhofer ISIT are working on modular multilevel converters (MMC) for efficient AC/DC conversion and implementing innovative control strategies for grid support using full-bridge cells with local control boards and an individual protection system. Südkabel GmbH contributes its expertise in the field of high and extra-high voltage cables and provides support with its know-how on field control using insulated materials. In addition, the grid operators NetzeBW, SachsenNetze and Thüringer Energienetze are providing the necessary test infrastructure and supporting the implementation of the technology in real grid environments.