Dynamic modeling of modular multilevel DC/DC converters for HVDC systems

This paper presents the first dynamic model of the DC-MMC, a new class of modular multilevel single-stage dc/dc converters aptly suited for HVDC systems. A state-space model for a single string of the DC-MMC is developed and validated by comparing with time-domain simulation results from a comprehensive switched converter model. The developed averaged model is utilized to investigate submodule capacitor voltage balancing characteristics of the DC-MMC. In particular it is demonstrated that, unlike the dc/ac modular multilevel converter, direct ac modulation of the DC-MMC is not sufficient to keep capacitor voltages balanced. The developed model is also used to propose and validate a supplemental controller that suppresses unwanted second and third harmonic circulating ac currents within the DC-MMC.