Phasor Domain Steady-State Modeling and Design of the DC-DC Modular Multilevel Converter

The dc-dc modular multilevel converter (MMC), which originated from the ac-dc MMC, is an attractive converter topology for the interconnection of medium-/high-voltage dc grids. This paper presents design considerations for the dc-dc MMC to achieve high efficiency with reduced component sizes. A steady-state mathematical model of the dc-dc MMC in the phasor domain is developed. Based on the developed model, a design approach is proposed to size the components and to select the operating frequency of the converter to satisfy a set of design constraints while achieving high efficiency. The design approach includes sizing the arm inductor, submodule capacitor, and phase filtering inductor along with selection of the ac operating frequency of the converter. The accuracy of the developed model and the effectiveness of the design approach are validated based on the simulation studies in the PSCAD/EMTDC software environment. The analysis and developments of this paper can be used as a guideline for designing the dc-dc MMC.