A Resonant Modular Multilevel DC Transformer with Quasi-Square-Wave Modulation Scheme

DC grid is essential to efficiently integrating renewable energy and flexible access to DC loads. DC transformer is the key equipment to interconnect DC systems with different voltage levels, establishing DC grids. This paper proposes a resonant modular multilevel DC transformer with half-bridge submodule series connected at the medium-voltage side and full bridges parallel connected at the low-voltage side, which significantly reduces the voltage and current stress along with the number of power devices and transformers. Quasi-square-wave modulation and phase-shifted control increase the AC link frequency and achieve fixed frequency output voltage control. The transmission power characteristics are analyzed based on the fundamental harmonic approximation analysis method. And the key parameter design procedure and output voltage control strategy are presented on this basis. Finally, the feasibility of the proposed topology and theoretical analysis is validated by a 4kW/600V/100V prototype.