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.
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 (Ux). 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 Ux − cost map is introduced to compare various design solutions, based on power devices of different voltage classes and current capabilities.
Since they were first proposed, modular multilevel converters have been strongly studied in 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 paper presents a comprehensive survey on the converter fault-tolerance strategies for MMC-HVDC systems based on the available technical literature. Some MMC-HVDC solutions adopted in industry are reviewed. This work proposes 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.
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