| Abstract |
In this paper, a full-bridge modular multilevel converter (MMC) and two half-bridge-based MMCs areevaluated for high-current low-voltage DC-applications such as electrolysis, arc welding or datacenterswith DC-power distribution. Usually, modular multilevel converters are used in high-voltage DC-applications (HVDC) in the multiple kV-range, but to meet the needs of a high-current demand atlow output voltage levels, the modular converter concept requires adaptations. In the proposed concept, the MMC is used to step-down the three-phase medium-voltage of 10kV , and provide up to 1MW to the load. Therefore, each module is extended by an LLC resonant converter to adapt to the specific electrolyzers DC-voltage range of 142-220V and to provide galvanic isolation.The six-arm MMC converter with half-bridge modules can be optimized by removing three arms, and thus halving the number of modules. In addition, the module voltage ripple and capacitor losses are decreased by 22\% and 30\% respectively. By rearranging the components of the half-bridge MMC to build a MMC consisting of full-bridge modules, the voltage ripple is further reduced by 78\% and capacitor losses by 64\%, while ensuring identical costs and volume for all MMCs.Finally, the LLC resonant converter is designed for the most efficient full-bridge MMC. The LLC cannot operate at resonance with a fixed nominal module voltage of 770V because the output voltage is142-220V . By decreasing the module voltage down to 600V , additional points of operation can beoperated in resonance, and the remaining are closer to resonance. The option to decrease the modulevoltage down to 600V , increases the number of required modules per arm from 12 to 15 , which requires to balance the losses of the LLCs and the first stage. |
