Transient Synchronizing Stability Analysis of VSG Considering Voltage/Current Source Mode Switch Induced by Current Saturation Limit

The virtual synchronous generator (VSG) has been proven to be a promising control strategy for grid-forming (GFM) converters, which is applicable to power systems with a high penetration of converter interfaced generation. However, different from real synchronous generators (SGs) consisting of copper–iron, the VSG is essentially a power semiconductor device with poor overcurrent capability. Current saturation limiting is usually employed in the VSG to prevent overcurrent, which may induce transient voltage/current source switching mode (TVCSM) and deteriorate VSG’s transient synchronizing stability. In this paper, the transient stability of the VSG-based GFM converter is focused on with the effect of the TVCSM considered especially. First, concerning the VSG’s virtual rotor motion influenced by the TVCSM, a switching dynamic model is developed to depict the VSG’s synchronizing behavior. It is identified that the TVCSM introduces switching dynamics and reshapes the equivalent P-δ curve of the VSG’s virtual rotor motion. Then, the impact mechanism of the TVCSM on the transient synchronizing stability is clearly revealed based on the acceleration/deceleration area method. Analysis results indicate that the TVCSM increases the acceleration area and decreases the maximum deceleration area, worsening transient stability. To quantitatively assess the impact of the TVCSM on the transient stability further, a novel analysis idea combining the phase portrait method and inverse trajectory method is proposed to accurately estimate the critical clearing angle (CCA) and critical clearing time (CCT). Finally, the influence factors including the power reference, maximum allowable current, grid impedance, damping and grid voltage sag are investigated. MATLAB/Simulink simulations are presented to verify the theoretical analysis.