New Submodule Topology Based Fault Tolerance for Modular Multilevel Converters

This article proposes a new submodule (SM) capable of fault tolerance. This SM integrates an operating half-bridge SM (HBSM) and a backup HBSM inside. The two HBSMs are connected by an intermediate unit, which contains a power switch and a bypass switch. In normal operation, the two HBSMs are conditionally connected in parallel. The structure can well charge the capacitor in the backup HBSM, which avoids the tedious charging before initiating fault tolerance. Different faults are considered with the proposed SM. The faults inside the proposed SM are tolerated by the backup HBSM. To tolerate the faults outside the proposed SM, the two HBSMs become cascaded. After the fault tolerance starts, enough output voltage levels and the symmetry of two arms in the same phase-leg can be ensured in the post-fault operation. With the proposed SM, the fault-tolerant method (FTM) is very simple and handles the drawbacks of the existing FTMs based on hot-reserved SMs and cold-reserved SMs. The performances of the proposed SM and related method are verified by the simulation and experimental results.