A Capacitor Voltage Balancing Method for a Buck Modular Multilevel DC Transformer Under Light Load Conditions

Non-isolated modular multilevel DC transformer (MMDCT) is favored in interconnecting DC grids with different voltage levels due to the elimination of the transformers. To maintain the internal energy balance of the MMDCT, its submodule (SM) capacitor voltage is generally regulated by a control scheme based on the sorting algorithm. Moreover, the undesirable large dv/dt issue caused by the device's switching operation can be effectively addressed by the stepped two-level modulation. However, the sorting algorithm cannot operate properly under light-load conditions, deteriorating the normal operation of the MMDCT. This paper proposes an advanced stepped 2-level modulation strategy, maintaining the SM energy balance under all working conditions. The effectiveness and feasibility of the proposed scheme are verified by the MATLAB/Simulink simulation.