Abstract
<title>Abstract</title> <p>To address the transient instability issue of grid-forming (GFM) control in LCC-MMC ultra-high-voltage hybrid DC transmission systems with hierarchical connection at the receiving end—where current limiting tends to lock the converter into current-source mode during AC faults—this paper proposes a coordinated optimization control strategy based on the virtual power angle. The virtual power angle characteristic equation of the GFM converter is first established, and the evolution mechanism of the virtual power angle before and after current saturation is analyzed. Based on the equal-area criterion, the analytical relationship between the critical clearing angle and the active power setpoint is derived. On this basis, a dynamic power reduction control strategy based on surplus power is proposed, which detects the unbalanced power of the converter station during faults and adaptively adjusts the power setpoint of the P-f control, thereby actively increasing the critical clearing angle. Meanwhile, for scenarios where the virtual power angle exceeds the critical value, a feedforward compensation-based smooth switching control strategy between grid-following (GFL) and GFM modes is designed to ensure the continuity of the inner-loop current reference at the switching instant. PSCAD/EMTDC simulation results demonstrate that the proposed dynamic power reduction strategy exhibits smaller transient disturbances and shorter recovery time compared with the fixed power reduction scheme, while the smooth switching strategy effectively prevents the converter from being locked into current-source mode, significantly enhancing the system's transient stability under severe faults.</p>