Small-Signal Stability of HVDC-Connected Offshore Wind Farms Under Weak-Grid Conditions

This paper investigates the small-signal stability of an offshore wind farm connected to a weak onshore AC network through voltage-source-converter-based high-voltage direct-current transmission. An eight-state reduced-order model represents the offshore grid-forming converter, onshore phase-locked loop (PLL), active-power control, DC-link voltage regulation, and battery energy storage system (BESS). Eigenvalue analysis is used to assess the effects of short-circuit ratio (SCR), PLL natural-frequency scaling, wind-power loading, and BESS inclusion. Reducing the SCR from 10 to 1 decreases the damping ratio of the PLL-related mode from 0.571 to 0.285. Increasing the PLL scaling factor from 0.5 to 2.0 raises the modal frequency from 7.41 Hz to 29.51 Hz while maintaining an approximately constant damping ratio. Wind-loading effects are comparatively small, and BESS inclusion provides supplementary reactive-power and voltage support without materially changing the principal PLL-related mode.