Using virtual impedance to analyze the stability of LCL-filtered grid-connected inverters

This paper compares three current control methods for LCL-filtered grid-connected inverters, they are single-loop control based on inverter current feedback, that based on grid current feedback, and dual-loop control with capacitor current feedback active damping. The virtual impedance introduced by each method is identified, which comprises frequency-dependent resistance and reactance. The reactance shifts the LCL resonance frequency while a positive resistance provides damping for the resonance and hence stabilizes the system. Using the virtual impedance, the system stability is analyzed. The stable range of sampling frequency for different methods is deduced, as well as the gain boundaries of the controllers. The study therefore facilitates the analysis and design of control loops for grid-connected inverters with LCL filters, and it has been verified by experiments.