A Flying-capacitor Modular Multilevel Converter

With the increase of HVDC transmission, HVDC grids are the inevitable trend of the future. HVDC DC-DC converters act as the indispensable components to transmit energy and connect HVDC grids with different voltage levels. This paper proposes a flying-capacitor modular multilevel DC-DC converter for HVDC grid connection and offshore wind farm collection system. It consists of a number of modular multilevel converters (MMC) and a series of flying capacitors. Due to the mature technology of MMC, it is easy to control the power, circulating currents, submodule voltage balancing of the internal MMCs. A simple power control arithmetic of this converter is presented. The proposed converter inherits the merits of MMC and achieves a high efficiency high step-up ratio DC-DC conversion without using heavy transformers. Simulation results in PSCAD validates the feasibility of the converter.

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