The Impact of Virtual Impedance on System Stability in Grid-Forming Converters with Cascaded Control Structure

Virtual impedance (VI) is widely used in grid-forming (GFM) converters to improve adaptability to stiff grid. However, there are various forms and positions of VI, and the impact of different VI configurations on the impedance characteristics and stability of GFM converters remains underexplored. To address these issues, this paper investigates different types of VI implementations, establishes impedance models for the GFM converter with various VI types, and performs stability analysis. It is revealed that whether the converter output current or the grid current is used as the VI signal, both have identical impedance reshaping effects. Furthermore, compared to placing VI at the modulation stage, adding VI to the voltage loop reference yields the most significant improvement in interaction stability. Finally, simulation results validate the accuracy of the impedance model and the correctness of the theoretical analysis.