A DC-DC modular multilevel converter design for HVDC interconnection

High voltage direct current (HVDC) power transmission systems offer significant advantages over conventional high voltage alternating current (HVAC) power grids, including higher efficiency, enhanced long-distance bulk power transfer capability, improved controllability, and seamless integration of renewable energy sources such as offshore wind farms. HVDC systems also save operating costs, provide reliable power delivery to isolated or remote networks, and facilitate asynchronous power grid interconnections. To facilitate the integration of HVDC power systems with varying voltage levels while improving grid flexibility and reliability, advanced DC-DC power converter topologies are essential. This study presents a novel DC-DC modular multi-level converter (MMC) architecture that provides a suitable solution for integrating modern asynchronous HVDC power networks and these modern HVDC power systems with existing asynchronous HVAC power systems. The transformerless approach significantly reduces the proposed converter's volume, weight, price, and power loss. A detailed mathematical model and harmonic analysis of the proposed DC-DC MMC are derived, and simulation results demonstrate successful bi-directional power transfer between asynchronous grids. The control strategy ensures stable and efficient power flow regulation, confirming the effectiveness of the proposed DC-DC MMC for HVDC power network integration.