Small-Signal Modeling and Stability Mechanism Analysis of Grid-Connected Converter Based on PLL Synchronization-Dominated Loop

Recently, the instability of renewable energy generation connected to weak AC grid has significantly impeded the large-scale integration and utilization of renewable energy. To elucidate the above stability mechanism, the component connection method (CCM) can be used to develop a multivariate complex frequency domain small-signal model of grid-connected converter. Subsequently, a small-signal control block diagram based on the synchronization-dominated loop is formulated by analyzing small-signal propagation paths. Then, a stability criterion is proposed based on the feedback control theory, delving into the stability mechanisms of different feedback loops, system operating points and grid conditions. It is demonstrated that as the output power increases or the grid strength diminishes, the local positive feedback loop comprising the q-axis current control loop and PLL through grid impedance predominates the overall control loop, which is the underlying cause of system instability. Finally, the accuracy of the stability analysis is verified by time-domain simulation.