Since they were first proposed, modular multilevel converters have been strongly studied in the literature. Due to their high component count, some concerns regarding their reliability arise, and several fault-tolerance schemes have been proposed in recent years. This article presents a comprehensive survey on the converter fault-tolerance strategies for modular multilevel converter (MMC) high-voltage direct current (HVdc) systems based on the available technical literature. Some MMC-HVdc solutions adopted in industry are reviewed. This work proposes the analytical expressions to determine the maximum number of submodule failures in the MMC-HVdc system. Thereafter, the limitations of the main converter fault-tolerance schemes under submodule failures are described and critically compared. A case study is conducted based on 1000-MVA Xiamen MMC-HVdc project with ±320-kV dc-link voltage, aiming to evaluate the effect of the faulty submodules on the reliability and power losses of the converter. Finally, the remaining challenges and opportunities in the converter fault tolerance in MMC-HVdc systems are stated as well as the future directions of this field.

Modular Multilevel Converters (MMC) are complex systems, composed of many elements, and exposed to critical load demands in some cases. Thereby, a detailed design of its components is of preeminent importance to achieve a high system-level reliability. However, the high number of devices challenges the trade-off between cost and reliability. This work, introduces a reliability-oriented design methodology, based on the cost to achieve a predefined unreliability level ( $U_x$ ). A flowchart presents the main steps of the process, including the mission profile definition, selection of power devices, thermal modeling, reliability modeling and the reliability-oriented selection. To evaluate the proposed methodology, a case study considering 17 MVA/13.8kV MMC-STATCOM with a real mission profile data is conducted. A $U_x$ — $cost$ map is introduced to compare various design solutions, based on power devices of different voltage classes and current capabilities.

The modular multilevel converter (MMC) has been widely adopted in different applications, such as high-voltage direct current (HVDC), electric drives and static synchronous compensators. Despite the application, MMC realization requires a high number of power electronic components, sensors, and communication cables. This paper proposes a sliding-mode observer for capacitor voltages of the MMC, using the measurements of arm currents and arm voltages. A linear corrector within the hysteresis band is proposed to reduce the chattering in the observer dynamics. Moreover, guidelines for tuning the observer gain are presented. The SM capacitance value is included as a parameter uncertainty of the observer. The proposal reduces the requirements of voltage sensors and optical links in MMC and allows fault-tolerant operation. The proposed observer performance is evaluated through a simulation of 100 MVA three-phase MMC-HVDC during initialization, steady-state, and submodule failures. The effect of the switching and sampling frequencies on the observer performance is also evaluated. Finally, experimental results validate the proposal in a downscale 3 kVA single-phase MMC prototype.

In recent years, medium-voltage variable-speed drives have become popular in the industry. However, in some applications, the use of long cables can lead to overvoltages at the motor terminals, affecting the motor lifespan. Under such conditions, the use of passive filters is recommended. However, the use of inductive capacitive (LC) filters results in resonances, leading to control stability issues. This issue can be mitigated by introducing a resonance damping strategy. In this work, four damping strategies are implemented and designed: Passive damping, capacitor current feedback, capacitor voltage feedback, and notch filter-based damping. The paper performs a comparative study on the current control phase margin, current and voltage harmonic spectra, and overall changes in the control structure. Then, the effect of the damping strategy on the VSD control is evaluated, creating guidelines to support the selection of the appropriate damping strategy. The results indicate that capacitor voltage feedback stands out, since this strategy presents an interesting dynamic behavior while allowing the elimination of the passive damping losses at a relatively low cost.