A Power Boost Technique for the Isolated Modular Multilevel DC-DC Converter Based on Sub-module Capacitor Voltages Ripple

The isolated modular multilevel DC-DC (IM2DC) converter is promising to serve as a DC transformer in medium voltage (MV) or high voltage (HV) DC grids. The design and analysis of the IM2DC converter in the literature always assume constant capacitor voltages and neglect the voltage ripples 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 impacts of the SM capacitor voltages ripple on power transfer capability 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. The analysis is validated by both simulation and experimental results.