Synchronization Stability Analysis Under Ultra-Weak Grid Considering Reactive Current Dynamics

The synchronization stability of grid-following converter has attracted extensive attention. It has been reported that the transient synchronization behavior during a grid fault is dominated by the phase-locked loop (PLL), while the current loop is always approximated as a unity gain. Nevertheless, this letter notices that the reactive current dynamics will also influence the synchronization stability under ultra-weak grid, which increases the risk of losing synchronization, and causes the PLL output frequency to be outside the limitation. Accordingly, a refined fourth-order model is proposed to reveal the above underlying mechanism and quantify this coupling effect.