New Submodule Circuits for Modular Multilevel Current Source Converters with DC Fault Ride Through Capability

In this paper, two types of new submodule (SM) circuits are proposed for modular multilevel current source converter (MMCSC) with dc fault ride through capability, namely, type I diagonal bridge SM (DBSM) and type II DBSM. The topologies and possible operation states of the two types of DBSMs are presented. Each type of DBSM is capable of providing bipolar voltages in unidirectional current. Replacing the half bridge SMs (HBSMs) in the conventional modular multilevel converter (MMC) with either type of DBSMs, the MMC can operate as a current source converter during normal operation, and is capable of not only blocking the dc fault current, but also functioning as a static synchronous compensator (STATCOM) to support the ac grid to ride through dc faults. Compared with the other MMC topologies with dc fault ride through capability, the proposed DBSM-based MMCSC is cost-effective, as it saves on the use of a significant number of semiconductor devices. The operation principle, control strategy and dc fault ride-through scheme of the proposed DBSM-based MMCSCs are presented. Simulation results in the PSCAD/EMTDC environment demonstrate the validity and characteristics of the DBSM-based MMCSC.