Mitigation of dynamic coupling and sub-synchronous oscillations in AC–DC coupled multi-terminal MMC–HVDC networks

This paper investigates the dynamic coupling mechanisms and sub-synchronous oscillation (SSO) phenomena in multi-terminal modular multilevel converter–based high-voltage direct current (MMC–HVDC) networks and proposes an effective mitigation strategy. A detailed small-signal model is developed to capture internal MMC dynamics, converter control interactions, and the dynamic coupling between offshore and onshore converters through the common DC link. Eigenvalue- and impedancebased analyses reveal that arm capacitor voltages, zero-sequence component of the circulating current, and DC-voltage controller collectively shape the oscillatory behavior and inter-converter coupling within the 0–50 Hz range. Mode sensitivity analysis further demonstrates that droop gain, DC-voltage PI parameters, and damping coefficients significantly affect modal damping. To mitigate coupling-induced instabilities, a modified circulatingcurrent controller is introduced to regulate the zero-sequence current component and suppress its participation in SSO modes. Frequency- and time-domain simulations confirm that the proposed controller effectively mitigates oscillations at SSO range, and 150 Hz, limits disturbance propagation through the DC link, and improves both transient and steady-state damping. The frequency-domain responses analysis establishes a comprehensive framework for understanding and mitigating dynamic coupling in MMC–HVDC networks, supporting the stable integration of large-scale offshore renewable energy systems.