Adaptive Transient Stability Control Strategy for GFL-GFM Hybrid Grid-Connected Systems Considering Power-Angle Difference Polarity

The increasing penetration of renewable energy has made the transient synchronization stability of grid-forming (GFM) and grid-following (GFL) converter hybrid systems an important concern. This paper proposes an adaptive coordinated transient stability control strategy based on the polarity of the power-angle difference between the two converters. First, a transient model is established to characterize the coupling between the power-angle dynamics of the GFM converter and the phase-locked-loop dynamics of the GFL converter. The analysis shows that the sign of the power-angle difference changes the direction of the cross-coupling terms, causing the interaction between the converters to exert either a stabilizing or destabilizing effect on their transient responses. The shift of the system equilibrium point under symmetrical grid-voltage sags is also analyzed. Based on this mechanism, the active-power reference of the GFM converter and the current-injection angle of the GFL converter are coordinately adjusted according to the voltage-sag severity and the identified coupling direction. MATLAB/Simulink R2024a simulations under the tested voltage-sag conditions show that, compared with the fixed-parameter control scheme, the proposed strategy reduces power-angle oscillations, improves transient energy balance, and maintains synchronization over a wider range of voltage-sag conditions. These results demonstrate improved transient synchronization performance of the GFL–GFM hybrid system under the investigated voltage-sag conditions and modeling assumptions.