A Combined Modular Multilevel Converters Topology with DC Fault Ride-Through Capability for Bulk Power Bipolar-HVDC Transmission Systems

In this paper, a novel bipolar combined modular multilevel converters (BC-MMCs) topology with DC fault ride through capability is proposed for bulk power HVDC transmission systems. The proposed BC-MMCs topology consists of two symmetric monopolar MMCs. Each of them is a combination of two types of converters: HBSM-MMC and the diagonal bridge sub-modules based MMC (DBSM-MMC). The two different MMCs are connected in series on their DC sides, and can be controlled independently. Replacing the HBSM-MMC, the proposed BC-MMCs is operated in current and reactive power control mode during normal conditions and is able to both clear the DC fault current and work in the static synchronous compensator (STATCOM) mode to provide reactive power to the AC grid under DC fault conditions. Compared with other MMC topologies with DC fault ride through capability, the proposed BC-MMCs is more economical for significantly decreasing the number of power devices. The operation states, control strategy and the DC fault ride-though capability of the proposed BC-MMCs are presented and simulation results using PSCAD/EMTDC validate the characteristics and advantages of the proposed BC-MMCs.