The potential of modular multilevel converters for charging large electric vehicles
Ygor Pereira Marca defended his PhD thesis at the Department of Electrical Engineering on November 20th.
As electric transport surges forward, the demand for ultrafast chargers capable of handling the needs of large electric vehicles becomes ever more critical. Traditional ultrafast chargers, reliant on bulky 50Hz transformers for voltage conversion and isolation, face challenges in scaling up efficiently. The PhD research of Ygor Pereira Marca, conducted as part of the NEON (New Energy and Mobility Outlook for the Netherlands) project, presents an alternative: modular multilevel converters (MMCs). By leveraging advanced power electronics, this innovative approach offers a compact, efficient solution to revolutionize ultrafast charging.
Current ultrafast chargers use line-frequency transformers to step down voltage and provide electrical isolation. While effective, these transformers are large, heavy, and limit the system's scalability. As electric transport expands, especially for heavy-duty applications like buses and trucks, the physical and operational constraints of these transformers pose significant challenges.
The MMC-based solution
To overcome these limitations, this research investigates the use of direct three-phase to single-phase MMCs with full-bridge submodules. These converters can replace the traditional line-frequency transformers with compact medium-frequency transformers, significantly reducing the size and weight of ultrafast chargers. Unlike other MMC topologies, the direct ac/ac MMC is uniquely suited for this application. It enables direct conversion from a three-phase medium-voltage grid to a single-phase medium-frequency voltage. The system then connects to a medium-frequency transformer and an ac/dc converter, providing the necessary voltage and current to charge or discharge a battery.
Analytical modeling and control strategies
A core aspect of the research involved developing an analytical model to analyze the converter's circuit states. This model was used to design a control scheme that effectively regulates the currents at both the three-phase and single-phase terminals and the capacitor voltage within the submodules. Two voltage operation modes were examined at the single-phase terminal: sinusoidal and square-wave. Notably, square-wave operation showed superior performance due to its reduced pulsating power and the fewer submodules required. This approach also minimized voltage ripple and current stress on the submodule capacitors.
Designing for stability and efficiency
Under both balanced and unbalanced grid voltage conditions, the submodule capacitor's rms current and voltage ripple were studied to ensure reliable operation. These insights allowed Pereira Marca to calculate the necessary capacitance for energy storage, optimizing the system's performance while maintaining stability.
A step toward sustainable mobility
The proposed model and control strategies were tested through simulations of a full-scale medium-voltage-connected charger. Additionally, a scaled-down prototype was developed to experimentally verify the system's effectiveness. Both simulation and experimental results confirmed the potential of MMCs to serve as a transformative technology for ultrafast charging of large electric vehicles. This research highlights the promise of MMCs in addressing the limitations of traditional ultrafast chargers. By reducing size, improving efficiency, and enabling scalable designs, modular multilevel converters pave the way for widespread adoption of electric transport, supporting a greener, more sustainable future.
Title of PhD thesis: . Promotor: Dr. Henk Huisman. Co-promotor: Dr. Maurice Roes.