Fault detection and tolerant control of open-circuit failure in MMC with full-bridge sub-modules

Modular multilevel converter (MMC) has drawn tremendous attention in the academia and industry since its invention, and it is considered as a very suitable solution for the high-power applications. It is usually composited by a large number of half-bridge or full-bridge sub-modules (SMs), which makes the reliability as one of the most important challenges for MMC. Besides, due to the DC side short-circuit blocking and ride-through capability, MMC with full-bridge SMs is very competitive and will be widely used in the future. Considering the IGBT open-circuit failure in MMC with full-bridge SMs, the fault features are analyzed in detail in this paper. Then a complete set of fault detection and tolerant control strategy is proposed. Firstly, the IGBT open-circuit fault is detected based on two sliding mode observers for the circulating current and the output current, respectively. Secondly, a tolerance period is inserted immediately to make the transient process smooth. Thirdly, the failed SM is localized through comparing all the capacitors' voltages. Finally, the failed SM is bypassed and the other healthy SMs are reconfigured to realize the continuous operation. The feasibility and effectiveness of the proposed fault detection and tolerant control strategy have been confirmed through the simulations in MATLAB/Simulink.