An Isolated Modular Multilevel DC Converter with Unipolar-to-Bipolar Conversion

Deep-sea offshore wind power generation has gained increasing attention in the past decade. The low cost and high efficiency of the DC grid system make it more competitive when the transmission distance is over 100 km. As the key enabler of the DC grids, DC converters are necessitated to interconnect the DC lines with different voltage levels. Instead of using auxiliary circuits, this paper proposes a low-cost isolated modular multilevel DC converter (IMMDC) with unipolar-to-bipolar conversion topology to fit into the DC grids with different configurations. Moreover, the proposed DC-current-injection-based fault-tolerant scheme can maintain around 50% power transmission capability even under single pole open-circuit fault conditions for a certain period, enhancing the power supply continuity. The modularity and scalability of the proposed IMMDC topology can fit into different DC grid systems. The effectiveness and feasibility of the proposed topology and control strategy are verified using a 50 kV/±300 kV/100 MW MATLAB/Simulink 2022b simulation model.