Modular-Multilevel-Cascade Converters are the preferable choice for high-voltage and high-power applications. The price of simple scalability, modularity, and flexibility is paid by a large number of components, especially semiconductors and capacitors. Such components are prone to failure, so fault detection and diagnosis, fault-tolerant control concepts, and reliability analysis are research fields within this topology. This paper presents several fault-tolerant approaches for the class of Modular-Multilevel Converters under the severe failure of a whole cluster and compares them in detail. Furthermore, the different fault modes are emphasized, and operation recommendations are given. All approaches are extensively derived and verified by simulation and experimental results in a steady-state. Additionally, experimental results of transients, i.e., the transition from normal-to fault-operation and power steps, are provided for the most suitable proposed approach. The results show the proper functionality of the control strategies. The simple implementation enables easy integration into (existing) systems and thus provides better fault-tolerance.

A generic current control system for modular multilevel cascade converters (MMCCs) is proposed. It is based on (i) input/output-linearization (I/O-linearization), (ii) feedforward disturbance compensation and (iii) parallelized proportional-integral-resonant controllers to compensate for arbitrarily many harmonics (including positive and negative sequence). The controller design is performed in state space. It is generic and flexible and combines pole placement and LQR tuning. Moreover, it allows to take all system parameters such as cluster or output (grid) inductances or resistances individually into account during controller design and tuning. Moreover, the typically imposed simplifying assumptions on system dynamics (e.g., identical parameters) or operating conditions (e.g., balanced loading) can be overcome. The proposed control system is validated for a dual-star bridge-cell MMCC. The simulation results illustrate its excellent control performance under ideal but also non-ideal conditions, where conventional approaches usually fail.

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 several kinds of 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 paper comprehensive surveys: (i) reliability including failure rates, fault modes, failure mechanisms and fault impact analysis, (ii) concepts of fault tolerance and FDD (e.g. expert system, model- or hardware and data-based FDD methods) and (iii) system reconfiguration strategies (e.g. cold- or hot-redundant) for MMCCs. All discussed aspects are related to generic MMCC models and operation principles, unifying notation of over 250 papers and, therefore, simplifying the understanding and comparability. The state-of-the-art, challenges and future research trends and opportunities towards reliable MMCC-based systems are thoroughly collected.

Modular Multilevel Cascade Converter (MMCC) have achieved a great importance in high voltage converter applications. As they require a high amount of capacitors whose voltages must be known for the control of the MMCC, many voltage sensors are needed. This paper presents a nonlinear Lyapunov-based observer for cell voltage estimation. The proposed observer does solely rely on the knowledge of the phase currents i.e. no voltage sensors are required and the convergence of the estimator is shown with a stability proof by Lyapunov. The performance of the observer is validated simulatively using MATLAB/Simulink.