Among the various topologies proposed in literature, the Modular Multilevel Converter (MMC) is considered the next generation of converters for medium and high voltage applications. This paper presents a comparison between two variants of modular multilevel converters for STATCOM applications: The Double-Star Chopper Cell (DSCC-MMC) and the Single-Delta Bridge Cell (SDBC-MMC). These converter topologies are compared and benchmarked during positive and negative sequence compensation. The comparisons are supported by analytical results regarding minimum effective dc-link voltage, number of cells, current rating, energy storage requirements and operation during unbalanced conditions. The dynamic behavior, power losses and cost of both solutions are evaluated through simulation model of a 15 MVA MMC STATCOM.

The modular multilevel converters (MMCs) have become an attractive topology in recent years. The MMC has been employed in several applications as HVdc, energy storage, renewable energy, electrical drives, and static synchronous compensators (STATCOMs). One advantage of MMC-based topologies is the inherited fault-tolerant operation associated with the high number of submodules (SMs). This work proposes the study of four redundancy strategies, which can be employed in modular multilevel converters: standard redundancy, redundancy strategy based on additional submodules (RAS), redundancy strategy based on additional submodules optimized (RASO), and redundancy strategy based on spare submodules. These strategies are compared through a case study of a 15 MVA MMC STATCOM. A new approach for the SM capacitance design is proposed, including the effect of the negative sequence current in the converter storage energy variations. The comparisons of the redundancy strategies are accomplished based on dynamic behavior, capacitor voltage balancing, control complexity, and power losses. Finally, the advantages and drawbacks of each redundancy strategy are presented.

The power produced in a photovoltaic (PV) system is highly dependent on meteorological conditions and the features of the connected load. Therefore, maximum power point tracking (MPPT) methods are crucial to optimize the power delivered. An MPPT method needs a DC-DC converter for its implementation. The proper selection of both the MPPT technique and the power converter for a given scenario is one of the main challenges since they directly influence the overall efficiency of the PV system. This paper presents an exhaustive study of the performance of four step-down/step-up DC-DC converter topologies: Buck-Boost, SEPIC, Zeta and Cuk, using three of the most commonly implemented MPPT techniques: incremental conductance (IncCond), perturb and observe (P&O) and fuzzy logic controller (FLC). Unlike other works available in the literature, this study compares and discusses the performance of each MPPT/converter combination in terms of settling time and tracking efficiency of MPPT algorithms, and the conversion efficiency of power converters. Furthermore, this work jointly considers the effects of incident radiation variations, the temperature of the PV panel and the connected load. The main contribution of this work, other than selecting the best combination of converter and MPPT strategy applied to typical PV systems with DC-DC power converters, is to formulate a methodology of analysis to support the design of efficient PV systems. The results obtained from simulations performed in Simulink/MATLAB show that the FLC/Cuk set consistently achieved the highest levels of efficiency, and the FLC/Zeta combination presents the best transient behavior. The findings can be used as a valuable reference for the decision to implement a particular MPPT/converter configuration among those included in this study.

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.

Due to the high integration of renewable energy, the power systems may become more susceptible to unbalanced grid voltages, grid frequency disturbances, harmonic resonances and others. Therefore, several works have pointed out that the ancillary services provided by the energy storage systems can minimize these issues. The combination of energy storage and static synchronous compensators (ES-STATCOM) has been proposed to reduce the cost. Modular multilevel converters (MMC) are featured as a good solution for ES-STATCOM realization, due to their high efficiency, modularity and inherent fault-tolerant structure. However, the study of the integration of the MMC based ES-STATCOM in a complex power system composed of power electronic converters, transmission system, renewable plants, transformers, etc, leads to a high computational burden. Therefore, this work presents a detailed design and implementation of three efficient simulation models of the MMC ES-STATCOM. The performance of these models compared during a grid frequency support case study by a 100 MVA/33 kV ES-STATCOM. In addition, the performance of these models is evaluated during a battery charging process and a grid symmetrical fault. The models are compared through the following aspects: processing time, normalized integral of absolute error, state-of-charge estimation and active and reactive power dynamic response.

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.