Improved hybrid modular multilevel converter with enhanced fault ride-through capability and fast pre-charging strategies

Abstract

This study proposes an improved hybrid modular multilevel converter (MMC) topology with enhanced fault ride-through capability and fast pre-charging strategies. This improved MMC topology mainly consists of the half-bridge sub-modules (HBSMs), the modified series connected double sub-modules (MSDSMs), and the adjustable damping sub-module (ADSM). In the improved topology, in addition to being configured on the MMC AC-side, resistors are also configured in MSDSMs and ADSM, respectively, in the MMC. This type of resistance configuration based on the improved MMC topology benefits adjusting the value of the total equivalent current limiting resistance during the pre-charging process, leading to a higher pre-charging speed and a lower SM voltage drop risk. Three different pre-charging strategies can cope with changes in resistance configuration caused by the increasing number of SMs. Furthermore, the resistances in MSDSM and ADSM also help dissipate the energy caused by the DC short-circuit fault so that the fault ride-through capacity is enhanced. The simulation and experimental results validate the effectiveness of the proposed topology and the corresponding pre-charging strategies.