A Novel Open-Circuit Fault-Tolerant Operation for Modular Multilevel Converters

Modular multilevel converters (MMCs) have attracted much interest from researcher worldwide. It is a desirable solution due to important factors like low harmonic content at high output voltages, modularity, avoiding the use of large capacitors and separate DC sources, easy scaling to any voltage level, and reduced voltage stress on switches. Also, for high and medium power applications, such as HVDC and motor drives, high-voltage applications require a series cascade of hundreds of sub-modules. Depending on the type of sub-module selected for the application, the sub-module of the MMC may contain several switches. Depending on the MMC-based application, the converter typically needs to operate for a long period of time, two or three years without interruption. MMCs can experience any number of electrical problems, including single line-to-ground faults, DC bus short-circuit faults, and switch short- and open-circuit faults. A malfunction like this can damage the MMC and cause the system voltage to drop. Therefore, a robust fault-tolerant technique needs to be developed. Any fault-tolerant method can be divided into three phases: fault detection and localization, fault bypass mechanism, and post-fault control. A major emphasis of this work is post-fault control measures to quickly restore pre-fault voltage levels.