Influence of PLL Dynamics of Grid-Connected Inverters on Voltage Flicker Frequency

Grid-connected inverters for photovoltaic systems widely use the active islanding detection method with step injection of reactive power. Although 6Hz voltage flicker caused by this method is well understood, a new flicker oscillating at approximately 3Hz has recently been observed. Conventional theory cannot explain this frequency shift. This paper clarifies the mechanism of the 3Hz flicker by modeling the dynamics of the Phase-Locked Loop (PLL). We derive and analyze a transfer function that incorporates the PLL's dynamic characteristics. Frequency analysis and transient simulations demonstrate that the PLL's natural frequency is the dominant factor determining whether the flicker occurs at 3Hz or 6Hz. Furthermore, we investigate the reason for the recent emergence of 3Hz flicker. The analysis reveals that the extension of the frequency measurement moving average window to 320ms, which was implemented to mitigate 6Hz flicker, altered the phase characteristics of the feedback loop. Under this condition, the phase lag induced by the PLL shifts the system's instability point to 3Hz. Thus, PLL dynamics are the primary cause of the frequency variation.