ABSTRACT This paper investigates the small‐signal stability of offshore energy islands interconnected via multiple modular multilevel converter‐based HVDC (MMC‐HVDC) links. A linearised state‐space model incorporating detailed MMC internal dynamics and grid‐forming frequency and voltage droop control is developed to analyse inter‐converter interactions. The analysis reveals a previously unreported sub‐synchronous oscillation (SSO) mechanism arising from multi‐converter coupling through the offshore AC network. To mitigate this instability, a control‐oriented strategy based on inner‐current‐loop tuning is proposed. Eigenvalue analysis and time‐domain simulations demonstrate that the proposed method effectively suppresses SSO and significantly enhances stability margins. In the studied system, the stable frequency droop gain range is extended from 0.00165 p.u. to 0.9 p.u., enabling an increase in the maximum allowable active power imbalance between HVDC links from approximately 160 MW to 480 MW in a 1 GW offshore energy island. These results confirm the effectiveness of the proposed approach in improving operational flexibility and stability of multi‐terminal offshore HVDC systems. The practical feasibility of the proposed control strategy is further assessed using Speedgoat‐based real‐time simulations under diverse operating scenarios.
July et al. (2026) studied this question.