A Submodule Fault-Tolerant Operation Scheme of Modular Multilevel Converters through Novel Cold and Hot Mixed Topology

The fault-tolerant operation for the submodule (SM) faults is an important issue in the reliability and sustainability of modular multilevel converters (MMCs). Aiming to overcome the defects of heavy losses, complex algorithms and initiation interfer- ences in the existing methods, a fault-tolerant scheme based on the novel SM is proposed in this paper. The proposed cold and hot mixed SM (CHSM) contains two ordinary SMs, one of which operates and the other is reserved. In normal operation, the capacitor of the reserved SM can be charged by the operating SM. Based on that, there will be no extra operating loss in the normal mode of the CHSM and no charging interference when the CHSM initiates the fault-tolerant mode. In the postfault case, only the switching signals for the CHSM are adjusted to realize the fault tolerance. No complex algorithm is needed, which simplifies the fault-tolerant scheme. Moreover, with CHSMs, the fault-tolerant capability of MMC can further be improved. The performance analysis presents the superiorities of the MMC with the CHSMs in detail. Then, the operation of the CHSM and the effectiveness of the fault-tolerant scheme for MMC are verified by the hardware- in-loop experimental results.