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SummaryWith the rapid development of renewable energy power generation technologies in high‐voltage direct‐current (HVDC) system, the high step‐up ratio dc‐dc conversion plays an important role to connect dc grids with different voltage levels. This paper presents a multicell nonisolated resonant modular multilevel dc–dc converter (MMDC) for medium‐ and high‐voltage applications. The converter exhibits expandability without using transformer. Moreover, by adding additional switched‐inductor (SL) cells, switched‐capacitor (SC) cells, and the upper arm submodules (SMs), the proposed converter can obtain high gain with low voltage stresses on switches and diodes. The operating states of SMs is regulated by the carrier phase‐shifted pulse width modulation (CPS‐PWM) to guarantee the number of inserted SMs is constant to support the high‐side load at any moment. The converter is operated in resonant mode with resonance between SM capacitors and arm inductor. The step‐up ratio could be regulated by adjusting the number of SL cells, SC cells, and upper arm SMs; thus, more flexible voltage regulation can be realized. In addition, the proposed converter has the voltage self‐balancing capability of capacitors in SMs and SC‐cells without additional balancing control strategy. The topology configuration, parameter design guideline, operating principle, and comparison with other MMDCs are presented. Finally, the theoretical analysis is verified by the simulation and bench‐scaled experimental prototype results.

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