Frequency support scheme of grid-forming based hybrid cascaded HVDC integrated wind farms

The grid-following control-based hybrid cascaded high-voltage direct current (HC-HVDC) links may contribute to the worsening of inertia decline and instability issues, especially in situations characterized by a weak grid. To address the challenge, this paper proposes a grid-forming (GFM) based optimal frequency support scheme (OFSS) for HC-HVDC integrated wind farms. Firstly, a GFM control for hybrid converter is proposed and its stability under weak grid conditions is evaluated through a small-signal model. Then, DC current-voltage droop control is introduced to the line commutated converter (LCC). This control approach ensures the consistency of reactive power in the LCC by regulating the DC voltage and current in the same direction, thereby mitigating AC voltage fluctuations. It also facilitates active power control in the LCC in response to grid frequency variations. Furthermore, the OFSS can convey the frequency of receiving-end grid to the rectifier and wind farms without communication for precise power control. Finally, to validate the effectiveness of OFSS, a point-to-point HC-HVDC and a 4-machine test model are constructed on PSCAD/EMTDC. The OFSS significantly enhances the frequency support capability of HC-HVDC links and improves the robustness in weak grid. A grid-forming based optimal frequency support scheme (OFSS) for hybrid cascaded high-voltage direct current (HC-HVDC) integrated wind farms is proposed. The OFSS significantly enhances the frequency support capability of HC-HVDC links and improves the robustness in weak grid. image