Physical Interpretation of Synchronization Stability of Grid-Following Inverters from Two Reference Frames Perspective

As the penetration of wind power plants increases in the power system, the grid condition becomes increasingly weak, which may make grid-following (GFL) inverters lose synchronization stability under large grid disturbances. In the existing literature, generally the phase-locked loop (PLL) is mentioned as the main reason for instability. However, the above statement may not be accurate. This paper reveals that the interaction between the PLL and the weak grid is the root cause of instability. To support this claim, two comparative study cases are provided. 1) The GFL inverter tends to be unstable under weak grid conditions, but it is stable under strong grid conditions; 2) When an ideal angle is used for Park transformation rather than the PLL’s angle, the GFL inverter is stable under weak grid conditions. Moreover, a physical interpretation of the synchronization stability of GFL inverters is illustrated and possible improved control solutions are proposed accordingly.