Active Symmetry Support Control for Grid-Following/Grid-Forming Hybrid Renewable Energy Stations Considering Transient Synchronization Stability

An active symmetry support control method based on model predictive control and incorporating transient synchronization security boundaries is developed for a grid-following (GFL)/grid-gorming (GFM) hybrid renewable-energy station comprising GFL wind energy and GFM energy storage. The control objective is to suppress disturbance-induced departures from the dynamic balance between generation-side support and grid/load demand, which are referred to here as power asymmetry rather than phase-domain three-phase imbalance, and to restore a power-symmetric operating state through coordinated active- and reactive-power support. Reduced-order transient synchronization models are established separately for the GFL phase-locked loop and the GFM virtual synchronous generator without explicitly constructing a transient coupling model between the two converter types. For the GFM converter, damping energy is incorporated into the fault-stage transient-energy balance. The resulting transient-energy functions and critical-energy criteria are used to determine the synchronization-stability boundaries, which are then mapped into dynamically updated P-Q feasible regions. A frequency–voltage prediction model combining a system frequency-response model and a point-of-common-coupling voltage-sensitivity model is then established. The GFL/GFM transient-energy constraints, dynamic P-Q security regions, power-regulation capability limits, and command-direction constraints are embedded in the receding-horizon optimization to coordinate active- and reactive-power support while preserving transient synchronization security. Local linearization of the transient-energy constraints and second-order-cone reformulation of the dynamic P-Q regions are adopted to improve online computational efficiency. Simulation results show that, compared with MPC without transient synchronization security boundaries, the proposed strategy limits power-angle excursions, enhances active- and reactive-power support, and mitigates frequency and voltage deviations. A sensitivity analysis is further conducted to examine how uncertainties in grid impedance, inertia, damping, and filter inductance affect the calculated critical energies and P-Q security regions. The results highlight the distinct effects of these parameters on the critical-energy thresholds and dynamic energy margins.