Rotor-Angle Stability Enhancement Control for MMC-MTDC Systems Without Sending-End Power Reserves

High-voltage direct current (HVDC) systems are increasingly integrated into AC grids, making rotor-angle stability more critical. Improper HVDC dynamics during disturbances may weaken synchronizing torque and damping, thereby aggravating oscillations. This paper investigates the offshore wind farm (OWF)-based multi-terminal DC (MTDC) system and analyzes the impact of its power injections on grid rotor-angle stability. On this basis, a frequency-based power control logic is first proposed to enhance rotor-angle stability. Building on this logic, Lyapunov-based control laws are derived to coordinate active and reactive power both among multiple terminals (inter-terminal level) and within each converter under capacity constraints (intra-terminal level). To further optimize the coordinated control, an optimization framework is then designed, ensuring effective and secure support. Finally, considering the absence of power reserves at the sending-end OWF, a capacitor energy-based control strategy is proposed for MTDC systems to utilize all MMCs' capacitor energy at the same time for stability support (device level), thereby expanding the available support energy and further improving rotor-angle stability. The effectiveness and robustness of the proposed control are validated on the IEEE 39-bus system via PSCAD/EMTDC and MATLAB co-simulation.