Modeling and Design Optimization of a Medium Voltage Rated Isolated DC-DC Converter for EV Charging Stations

• Applicant: Professor Dr.-Ing. Marco Liserre, Ph.D.
• 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 2022
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 492987385

The electrification of the transportation sector poses the challenge to develop a suitable fast electrical vehicle charging infrastructure, which serves the needs of the users and enables an efficient connection to the distribution grid. Today’s solution is the installation of an individual distribution transformer for the interconnection of the charging stations with the Medium Voltage (MV) distribution grid. This solution provides one galvanic isolation to the grid through the 50Hz transformer and a second galvanic isolation between the low voltage AC (LVAC) and the individual vehicles. Remarkably, with two isolation stages both architectures require a large footprint and cause significant losses when a vehicle is charged. Therefore, solutions are required to reduce the environmental impact of the charging infrastructure and the losses of the energy conversion, while offering scalability in the charging points.An MV-rated isolated DC-DC converter integrated into an MV distribution grid is proposed in this project as a promising solution to overcome the aforementioned issues. This consists of an MV-rated rectifier, which controls the MVDC link and MV-rated isolated DC-DC converters for the connection of the charging points. Thus, a single isolation stage is used instead of the two isolation stages as applied in actual commercial charging stations. The key component of the proposed solution is a multiwinding medium frequency transformer (MFT) , which provides not only the galvanic isolation but also multiple high-power charging points. A single-phase modular multilevel converter is employed in the primary side to supply the medium voltage to the multiwinding MFT and conventional H-bridges are used to configure the output ports. The topology sought to address the strategic objectives of the project i.e. high efficiency, power density and scalability. These goals can be achieved only through a system level modeling and optimization of the proposed high-power medium voltage DC-DC converter. The modeling and design optimization process of the proposed MV DC-DC converter is split in three work packages: (I) system level analysis software development, (II) ƞρ-Pareto optimization of the multiwinding MFT, and (III) hardware implementation and validation. The modulation of the MMC, power flow decoupling between the charging points, soft-switching boundaries, MMC capacitor size reduction, and efficiency calculation of the MMC are included in the system level analysis software. The design of the transformer core geometry, selecting core and insulation materials, winding topology, impact of modulation on the multiwinding MFT efficiency and power density are envisaged in the second work package. Finally, the optimum DC-DC converter design is experimentally validated with a hardware demonstrator in work package III.

• DFG Programme: Research Grants
• International Connection: China
• Cooperation Partner: Professor Giampaolo Buticchi, Ph.D.