Transient Stability Analysis Between SG and Grid-forming VSCs with Current Saturation Control Considering Backward-swing Dynamics

This paper analyzes the transient angle stability between synchronous generators (SGs) and grid-forming voltage source converters (GFM-VSCs), considering backward-swing dynamics. The impact mechanism of current saturation control (CSC) on the backward-swing and forward-swing stability is addressed. Based on the second-order rotor motion equation, the conventional energy-based method for transient stability evaluation becomes rough driven by the interactions between SG and GFM-VSC. To overcome the barrier, a forward-time integral (FTI) method is proposed for calculating the critical clearing time (CCT). We discover that the CSC can affect the stable and unstable equilibrium points of the post-fault system, leading to both the backward-swing and forward-swing synchronism instability. Based on the mechanism, an enhanced synchronization control (ESC) is proposed. It combines a modified frequency control with a current-limit strategy to avoid backward-swing instability while enhancing forward-swing stability. Time-domain simulation verifies the theoretical analysis as well as the effectiveness and robustness of the proposed ESC.