Stability Analysis of the Virtual Inductance for LCL Filtered Droop-Controlled Grid-Connected Inverters

The grid-connected inverters (GCIs) that use droop control can avoid instability issues caused by the phase lock loops (PLLs) when connected to weak grids. Thus, the droop-controlled GCIs (DGCIs) have attracted more and more attentions. However, one of the limitations of the droop control is that its performance is highly influenced by the line impedances. Virtual inductances (VIs) can be used to overcome this limitation. However, if LCL filters are used by the GCIs to attenuate current harmonics, the interference between the VIs and the LCL filters will cause a new type of system oscillations. In this article, the effects of the VI on the stability of the LCL filter have been explored when the grid-side current (GC) or the inverter-side current (IC) of the LCL filter is used as the feedback for the VI. It has been revealed that the stabilities of the system when using these two currents by the VI are different. The feasible ranges of the parameter values of the VI have been derived, with which the VI can be used without affecting the stability of the DGCIs. Simulation and experimental results have been provided, which verify the correctness of the analysis in this article.