DC-DC modular multilevel converter with high frequency transformer for transportation applications

This paper proposes the design considerations of a high-voltage-high-power dc-dc modular multilevel converter (MMC), which is consist of the half-bridge submodules and an isolation transformer. To adapt the power density requirements of the transportation applications, a modulation method with low switching frequency for submodules but high-frequency ac voltage for transformer is proposed, aiming to reduce the size of the transformer, the inductors and the dc-capacitors of the submodules. The voltage fluctuation at the dc-capacitor in such a modulation is also estimated. Finally, a design example with 10kV to 20kV voltage and 1MW rating power is given. The analysis and simulation results are matched, which shows the validity of the analysis.