Novel Half-bridge MMC Sub-module Topology with Four Capacitors

With the wide application of modular multilevel converter (MMC) in flexible DC transmission project, the DC side short circuit faults of overhead line occurs frequently. How to clear the DC faults quickly becomes one of the key problems to be solved in flexible DC transmission project. The traditional half-bridge sub-module topology lacks the ability to eliminate DC side faults. The full-bridge sub-module topology eliminates the fault current by outputting a negative level, but the investment cost is high. Based on this, this paper proposes a novel four-capacitor combined half-bridge sub-module (FCC-HBSM) topology based on the half-bridge sub-module. The FCC-HBSM consists of a four-capacitor sub-module by cascading four half-bridges, which can not only effectively and quickly remove the DC fault current, but also self-balance the capacitor voltage before and after the fault. Moreover, compared with some other improved sub-module topologies, it has a simpler control method and reduces the number of power devices, which can further reduce the high construction cost of converter stations. The simulation results based on the MATLAB/Simulink show that the FCC-HBSM has a good ability to clear DC faults and maintain capacitor voltage balance before and after faults. © 2022 Science Press. All rights reserved.