Analysis and Suppression of Capacitor Voltage Ripple for Hybrid MMCs Under Boosted AC Voltage Conditions

Abstract

Capacitor voltage ripple suppression is significant for hybrid modular multilevel converters (MMCs) to reduce the capacitance, footprint and cost. However, the existing voltage ripple suppression methods are mainly intended for half-bridge submodule (HBSM)-based MMCs, and not applicable to the hybrid MMC due to its special configuration of mixed half-bridge and full-bridge submodules (FBSMs). Therefore, this article aims to reduce the capacitor voltage ripple for hybrid MMCs under boosted ac voltage conditions. The analysis of charging and discharging behaviors of FBSMs and HBSMs with boosted ac voltage is conducted first to obtain the maximum energy ripple in SM capacitors, which is equivalent to peak-to-peak value of voltage ripple. Then, the effect of modulation index and power factor angle on the capacitor voltage ripple is investigated in detail to find the possible ways to suppress the fluctuation. Consequently, the optimal modulation index that minimizes the capacitor voltage ripple is derived. Furthermore, this article proposes a circulating current injection method to suppress the capacitor voltage ripple under various operating points. In this method, the capacitor voltage ripple amplitude is taken as the direct suppression object to ensure the effect of voltage ripple minimization. Finally, the validity of the proposed method is proved under different operating conditions by simulation and experimental results.