Impacts of Reactive Power Control Dynamics on Transient Stability of Grid-forming Converters Considering Current Saturation: Analysis and Enhancement

The transient synchronization stability of grid-forming converters (GFMCs) is significantly challenged under grid voltage sags. Continuous efforts have been devoted to analyzing the GFMC transient stability, with limited attention paid to the impacts of control loop dynamics. However, the complex control dynamics, especially the interactions between the active/reactive power control loops and the current saturation process (CSP), are crucial for accurately describing the transient behavior and evaluating the stability. Thus, in this study, a new large-signal GFMC model is established, considering the reactive power control (RPC) with different kinds of controllers and the CSP simultaneously. It is revealed that GFMC does not switch to the current-limited mode immediately, and the dynamics of RPC further affect the transient behavior before the current limiting significantly. Hence, the complex control dynamics can alter the mode switching point of current saturation, thereby increasing the risk of loss of synchronization (LOS). Based on the above findings, comprehensive comparisons of typical RPC controllers are presented to facilitate practical engineering applications. A unified stability enhancement method is proposed for solving the problem of LOS. Finally, experiments validate the correctness of the analysis and the effectiveness of the proposed control strategy.