A Novel MMC Sub-Module Topology With DC Fault Clearance Capability

The sub-module of the modular multilevel converter (MMC) in practical engineering mainly adopt half-bridge sub-module (HBSM), which does not have the capability to clear dc fault current. Based on the analysis of the dc fault current path of traditional HBSM, this paper proposes a novel MMC sub-module with dc fault clearance capability. It adds a bidirectional switch and two diodes to the HBSM. When the dc fault occurs, the bypass absorption circuit is introduced into the arm through the diodes and the freewheeling effect of diodes is eliminated by the sub-module capacitor voltage. Thereby, the fault current can be extinguished rapidly and the requirement for the consistency of the trigger pulse is reduced. By comparing with the operation principle of the HBSM, it can quickly block the dc fault current without changing the original control and modulation strategy. Then, the fault clearance principle of the sub-module and the voltage stress of the key power devices are analyzed. The simulation results in MATLAB/SIMLINK have verified the validity and feasibility of the proposed sub-module topology in the dc fault current clearance.