Soft‐switching and sensorless voltage balancing modulation for unidirectional modular multilevel DC–DC converter

Abstract

DC–DC converters are important in medium-voltage direct current (MVDC) distribution networks. For the scenarios where energy flows unidirectionally from MVDC distribution networks to low-voltage direct current (LVDC) loads, the DC–DC converter, which composes of a modular multilevel converter (MMC) on the MVDC side, an uncontrolled rectifier on the LVDC side and a high-frequency transformer as the AC link, is the most cost-efficient candidate. In this study, a new modulation method with quasi-triangular AC-link current is presented to realise zero-voltage turning-on for all switches on the MVDC side, as well as zero-current-switching for the uncontrolled rectifier on the LVDC side. Besides, by adding simple and low-cost voltage-balancing D-L auxiliary circuits between adjacent submodules (SMs), all SM capacitor voltages can be well balanced without voltage sensors. The validity and effectiveness of the proposed method are verified by the simulation and experimental results.