Capacity Design of Cascaded Diode Rectifier-MMC Based HVDC for Offshore Wind Farm Integration

The cascaded diode rectifier (DR)- MMC based HVDC system is recognized as a promising topology for integrating offshore wind farms. However, the capacity design of DR-MMC remains an unsolved challenge. This paper proposes a capacity design method based on operating region (OR) analysis. First, the analytical models of DR-MMC with V-F grid-forming (GFM) control are established, revealing its operating characteristics. Then, considering multiple constraints, the OR of DR-MMC is calculated. The impact of different MMC capacities on OR is analyzed, highlighting that a system with a low MMC capacity ratio may be incapable of covering its full active power range. Hence, a three-layer GFM control utilizing MMC DC voltage is proposed, enabling operation under a lower MMC capacity ratio. Then, an optimal MMC capacity ratio is determined by balancing the system stability margin and economic considerations. Finally, the effectiveness of the designed MMC capacity with the proposed three-layer GFM control is validated by stability analysis and simulations in PSCAD/EMTDC.