Asymmetrical Reactive Power Capability of Modular Multilevel Cascade Converter Based STATCOMs for Offshore Wind Farm

Modular multilevel cascade converters (MMCCs) are becoming attractive solutions as high-voltage Static Synchronous Compensators (STATCOMs) for power plants in renewable energy generation, in order to satisfy the strict grid codes under both normal and grid fault conditions. This paper investigates the performances of four potentially used configurations of the MMCC family for the STATCOM in large-scale offshore wind power plants, with special focus on asymmetrical low-voltage ride through capability under grid faults. The specifications and the sizing of components of each type of practical 80-MVar/33-kV-scaled MMCC-STATCOM are carefully designed and compared. The total cost and volume are compared based on the total power semiconductor chip area and the total energy stored in the passive components. Asymmetrical reactive power delivering operation of the MMCC family considering the dc-link capacitor voltage-balancing method is solved mathematically in order to quantitatively understand the performance limitations and behaviors. The electrothermal stress of the power modules used in each type of the MMCC for a practical 80-MVar/33-kV-scaled STATCOM is analyzed. The asymmetrical reactive power capability of the MMCC solutions is compared under different scenarios of grid faults, while considering the device temperature limits as well as voltage saturation. It is found that the MMCC configuration with double-star bridge cells becomes the most attractive circuit configuration for the STATCOM application based on the obtained results.