Small-signal analysis and stability enhancement of offshore wind farms connected to DRU-HVDC

Diode rectifier unit (DRU) based offshore wind power integration system possess promising application prospects owing to their economic and reliability advantages. However, due to the DRU's lack of active control capability and inability to provide AC voltage support, such converter-dominated systems exhibit weak damping and face risks of oscillatory instability. This paper investigates the small-signal stability of DRU-connected offshore wind farms employing P-V/Q-f grid-forming control strategies and proposes improvement measures. First, a small-signal state-space model of the system is established, and its accuracy is validated by comparing it with time-domain simulation results. Analysis based on this model reveals the limitations of conventional control approaches in terms of stability. To address this issue, two supplementary damping control strategies for enhancing small-signal stability are proposed, namely supplementary active power damping control and supplementary reactive power damping control. Unlike conventional methods, the proposed strategies are designed to effectively increase the system damping ratio without affecting the steady-state operating point. The effectiveness of the proposed damping control strategies is validated through frequency-domain analysis and time-domain simulations. Finally, control parameters are optimized to further improve the system's small-signal stability margin.