Analysis on Instability Mechanism of Grid-Connected VSG Under Strong Grid Condition

Grid-forming (GFM) converters have been widely concerned because they can provide voltage and frequency support and can operate under very weak grid condition. However, some researches reveal that GFM converters may cause system instability under a strong grid condition. In order to explore the instability mechanism, the complete state space model of the system is established based on virtual synchronous generator (VSG) control strategy. It is indicated that the voltage control loop and the power control loop may interact under the strong grid condition, and the impedance between GFM converter and the grid is too small, which causes the system instability. Furthermore, the measure to improve the system stability is to change the bandwidth of the voltage loop and the range of the parameter design are given. Finally, the theoretical analysis is verified by simulation results.