A Simplified SISO Small-Signal Model for Analyzing Instability Mechanism of Grid-Forming Inverter under Stronger Grid

Different from grid-following inverters, grid-forming (GFM) inverters tend to be unstable under stronger grid conditions. However, the instability mechanism is still absent in existing research. To fill this gap, the full-order multiple-input multiple-output (MIMO) small-signal model of the GFM inverter is simplified as a single-input single-output (SISO) model on quadrature axis, so that the destabilizing factor can be clearly observed. Based on this SISO model, it is revealed that the power synchronization loop introducing an integral term into the voltage control loop is the main reason for instability. Decreasing the grid strength or the power droop gain can reduce the gain of the integral term and enhance stability. Finally, simulation and experimental results verify the effectiveness of theoretical analysis.