HV megawatt charging system targets heavy goods traffic

Technology News |
By Christoph Hammerschmidt

In a joint effort, Fraunhofer and several partners develop central charging components for heavy electric vehicles.

The charging infrastructure for battery-powered heavy goods and passenger transport plays a decisive role in a successful traffic turnaround. As a result, the required charging capacities are increasing and thus also the battery and charging voltages. The joint project “HV-MELA-BAT”, coordinated by the Fraunhofer Institute ISE (Freiburg, Germany), therefore aims to develop the necessary power electronic converters and a contact system for high currents and voltages. A buffer storage is to ensure full charging power even at power-limited grid connection points. Other project partners are Motion Control & Power Electronics GmbH, STS Spezial-Transformatoren Stockach GmbH, Mercedes-Benz Energy and the Fraunhofer Institute for Transportation and Infrastructure Systems IVI.

“The project focuses on the further development of existing fast charging infrastructure based on the CCS standard towards the future Megawatt Charging System standard. This poses new challenges for both the power electronics and the contact system,” explains project coordinator Stefan Reichert from Fraunhofer ISE.

Central aspects of the research project are the power electronic converters, the grid-side rectifier and the modular interconnection of DC/DC converters for galvanic isolation and for adapting the charging voltages to the vehicle battery. The system is to be supplemented by a buffer storage from automotive second-life batteries so that future charging stations can also be used with low grid connection power.

The aim is to make the system capable of addressing the widest possible range of charging voltages and vehicles, thus ensuring downward compatibility. Conceptually, the interconnection of up to four 250 kW charging points and the integration of regenerative sources and sinks will also be investigated within the system.

The MCS charging system and the associated buffer storage will be set up and evaluated at the Centre for Power Electronics and Sustainable Grids at Fraunhofer ISE. To implement the power electronic converters and the contact systems for the future high-power charging infrastructure, these components must be able to process DC voltages of up to 1250 V. In addition, the engineers must develop efficient topologies as well as inductive transformers with very high clock frequencies as part of the project.

High charging power, high charging voltage, high requirements

Increasing the charging voltage up to 1250 V allows high charging power at moderate charging currents. However, increasing the charging voltage requires the use of new efficient circuit topologies as well as appropriate semiconductor switches. The central power electronic converter, which creates a galvanic isolation between the grid and the vehicle battery, is being developed and built by Fraunhofer ISE. It should be highly efficient and compact at the same time. By using semiconductor switches made of silicon carbide (SiC), high clock frequencies are to be achieved.

The demands on inductive components (e.g. transformers) also increase with the clock frequency. STS is developing a highly compact transformer for this purpose. The other power electronic converters, such as the active rectifier and a hard-switching buck converter, are provided by M&P. Mercedes-Benz Energy is developing a modular buffer storage system from automotive second-life batteries for this application. The Fraunhofer IVI is supplying a contact system that can contact currents of more than 1500 A.

The project is supported by the German Federal Ministry of Economics and Climate Protection (BMWK) and runs until July 2025.

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