Impact of phase-locked loop on grid-connected inverter stability under weak grid conditions and suppression measures

The growing portion of renewable energy in the energy mix has led to the gradual emergence of weak or very weak grid characteristics with high impedance. In this context, the phase-locked loop (PLL) and its interaction with other key control links present a significant challenge to the stable operation of grid-connected inverters. Recent studies have focused on PLL induced frequency coupling and negative impedance characteristics and their impact on system stability. However, there is a lack of comprehensive compilation and systematic summarization of these results, making subsequent research direction unclear. This paper comprehensively summarizes the existing literature and concludes that the structure of the Phase-Locked Loop (PLL) leads to frequency coupling within the system, potentially inducing harmonic oscillations. Specifically, when the PLL bandwidth is excessively wide, it enhances the dynamic response of the system, simultaneously broadening the range of influence where negative damping phenomena occur. Conversely, when the PLL bandwidth is overly narrow, the stability of the inverter is improved, albeit at the cost of compromised dynamic performance. Additionally, the paper examines the frequency coupling phenomenon generated by PLL and its negative impedance characteristics. Based on the analysis, the paper systematically summarizes and discusses methods to enhance system robustness through PLL parameter adjustment, filter design, and voltage feedforward control. Furthermore, it considers the PLL as an active disturbance channel in the grid system and explores how other potential disturbances affect overall system stability through the PLL. Lastly, the article highlights the shortcomings of current research, identifies key points and challenges, and provides valuable references for future research directions.