Transient Stability Analysis of Grid-Tied Converters Considering PLL’s Nonlinearity

Transient instability events of grid-tied converters probably occur while riding through grid faults. During lowvoltage ride through (LVRT) period, seeing from the converter terminal towards the grid, the Thvenin equivalent grid impedance becomes pretty significant, accordingly making the converter terminal voltage highly sensitive to the output current. Under such circumstances, it is challenging for the converter to resynchronize with the grid via a phase-locked loop (PLL). This paper develops a reduced-order nonlinear model to elaborate on the dynamic synchronization characteristic of the converters. By considering the impact of grid impedance and analysing spatial vector tracking relation, resynchronization principle of the converters during LVRT is revealed. Besides, the impacts of circuit parameters and controller parameters, including residual grid voltage, grid impedance, current references, and PLL parameters, on the transient stability of the converters are investigated. The results are verified by simulation and experimental results.