Design considerations of a full bridge modular multilevel converter under variable DC link voltage

A full bridge modular multilevel converter (FBMMC) can work in buck and boost modes. It is thus suitable for grid-connected applications in which the dc link voltage varies over a wide range. In this work the emphasis is set on the steady-state analysis of the grid-connected FBMMC under different levels of the dc link voltage. Two control strategies for the dc capacitor voltage are considered and compared. The first approach regulates the dc capacitor voltage at the nominal value defined by the grid voltage, while in the second approach the dc capacitor voltage varies according to the variations in the dc link voltage. The study reveals that the circulating current, capacitor voltage ripple and output voltage THD are highly dependent on the level of the dc link voltage. It identifies that the level of the dc link voltage can play a significant role in minimizing the circulating current and capacitor voltage ripple. The study is based on the switching model of the single-phase FBMMC developed in Matlab Simulink environment.