Abstract

Modular multilevel cascade converters (MMCCs) are considered a promising power electronics topology in industry. Their scalability allows to reach (ultra/very) high voltage levels with low harmonic content and high efficiency and makes MMCCs an ideal solution for high-power applications; such as electrical drives, solid-state transformers, and high-voltage direct-current (HVdc) transmission systems. However, the high levels of thermal, electrical, and mechanical stress on the power electronics devices and the large number of components (e.g., capacitors or semiconductors) make MMCCs prone to faults. Fault detection and diagnosis (FDD) in combination with fault isolation and system reconfiguration techniques, based on cell redundancy, can increase the reliability, availability, and safety of MMCCs, which is crucial for their utilization in critical energy applications. This second part of the article comprehensively surveys: 1) fault tolerance and FDD; e.g., expert system, model-, or hardware and data-based FDD methods, and 2) system reconfiguration strategies (e.g., cold- or hot-redundant) for MMCCs. Finally, the state of the art, challenges, and future research trends and opportunities toward reliable MMCC-based systems are revealed.