Optimal Energy Utilization of MMC-HVDC System Integrating Offshore Wind Farms for Onshore Weak Grid Inertia Support

Abstract

Modular multilevel converter-based high-voltage direct current (MMC-HVDC) systems have the potential to support grid frequency by utilizing the electrostatic energy stored in sub-module capacitors. However, existing energy control schemes cannot effectively use the MMC capacitor energy to provide sufficient inertia support. Additionally, those control schemes are based on grid following control, suffering from stability issues when the AC grid short circuit ratio (SCR) decreases. Thus, a grid-forming control scheme capable of active energy management for MMC-HVDC systems integrating offshore wind farms (OWFs) is developed, achieving flexible energy regulation and stable operation under weak grid conditions. To improve inertia support performance, an optimal two-stage energy utilization method is further proposed to overcome the drawbacks of existing grid-forming methods. Finally, comparative simulations are performed in a 2-area 4-machine weak grid to verify the effectiveness and advantages of the proposed method.