A Fault-Tolerant Operation Method for Medium Voltage Modular Multilevel Converters With Phase-Shifted Carrier Modulation

This paper proposes a method for fault-tolerant operation of medium voltage modular multilevel converters (MMCs) with phase-shifted carrier (PSC) modulation. The existing methods need to recalculate and reconfigure all the carriers every time a submodule (SM) fails. However, the proposed method combines the sorting voltage balancing control and the PSC modulation to avoid breaking the symmetry and balance of the carriers. Therefore, the modulation stage in the proposed method can be kept the same and the output voltage of the MMC can remain unchanged without causing voltage mismatch. Moreover, considering the modulation margin and the physical redundancy configuration, two fault SM replacing strategies, i.e., optimal dynamic performance and optimal electrical stress are proposed as well as their respective control structures. Compared with the normal operation of the MMC, the proposed fault-tolerant operation method has only slightly changed the control system and thus the added calculation burden and control complexity can be ignored, which is extremely beneficial for the practical implementation. The effectiveness and performance of the proposed operation method are carefully verified by both simulation and experimental results.