Full Modular High-Current Variable-Voltage Rectifier Based on Simplified MMC-Topology with Resonant Conversion

• Applicant: Dr.-Ing. Frank Schafmeister
• Subject Area: Electrical Energy Systems, Power Management, Power Electronics, Electrical Machines and Drives ; Electronic Semiconductors, Components and Circuits, Integrated Systems, Sensor Technology, Theoretical Electrical Engineering
• Term: since 2021
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 456097802

State-of-the-art high-current rectifiers, as employed e. g. for hydrogen electrolysers, are characterized by a considerable physical volume, by a high harmonic distortion of their input current (THDi) and by a fixed, non-adjustable power factor. The aim of this project is to develop an electrically isolating, high-current variable-voltage (142-220V) rectifier of the 1 MW class based on the modular multi-level converter (MMC) concept and particularly employing full-bridge modules on the grid-side (here called “MMC-4”). Each module, equipped with SiC-devices, will be supplemented by a resonant tank and medium-frequency transformer (LLC converter) on the load side. There, soft switching of SiC devices will tremendously reduce losses and allow higher switching frequencies so that the passive components shrink in size. As final goal, the converter is expected to reach 98% average efficiency over a wide load range and about 30% size of state-of-the-art rectifiers while decreasing the input THDi of about 9 % to 3 % and moreover facilitating adjustable reactive power to the grid. Compared to recent MMC-based demonstrators set up by the applicants, the number of modules and therefore system complexity reduces with the proposed MMC-4 approach by up to 50 %. Consequently, the resonant (LLC) converters of the remaining modules need to be sophisticated high-power designs. Considering the apparent potential for future industrial implementation of the MMC-4 concept, special emphasis shall be directed to the analysis and improvement of robustness – both on physical component level as well as on a resilient operation strategy on system level. In this way, the project may contribute to an efficient and resource-saving hydrogen production in the future.

• DFG Programme: Research Grants