The isolated modular multilevel DC-DC (IM2DC) converter is a promising converter to serve as a DC transformer in DC grids. The design and analysis of the IM2DC converter in the literature always assume constant capacitor voltages for all sub-modules (SMs), so the modeling methods of the dual-active bridge (DAB) converter can be directly applied to the IM2DC converter. In this paper, the IM2DC converter is modeled based on the harmonic state-space (HSS) equations, so that the SM capacitor voltages ripple can be accurately captured. Then, the impact of the SM voltages ripple on power capability, zero-voltage switching (ZVS) region, and circulating current are revealed. It is found that the SM capacitor voltages ripple can be utilized to boost the power transfer capability of the converter. Although the SM capacitor voltages ripple will distort the AC voltages, such distortion is invisible on DC sides. The analysis is validated by both simulation and experiment.
