Novel modular multilevel converter against DC faults for HVDC applications

In view of the DC fault current isolation deficiency for the conventional half-bridge sub-module (HBSM) based modular multilevel converter (MMC), this paper presents an improved MMC topology. Both quasi reverse blocking submodules (QRBSMs) and current limit modules (CLMs) are employed to improve the DC fault handling capability for HVDC applications. This paper analyzes such a new converter configuration and operation principles. Then the DC pole-to-pole short circuit fault is taken into consideration for further study, as well as the fault current blocking mechanism and quantitative relationship between system electrical stress and key parameters. To validate the feasibility of the proposed topology and fault protection theory, extensive simulation results are demonstrated. It is concluded that the QRB-MMC can effectively block the fault current under DC fault condition. In addition, CLMs play an important role in further accelerating fault current attenuation. Moreover, QRB-MMC employs the original control and modulation strategies under normal operation conditions; thus, it further reduces the complexity of industry design.