An Adaptive Virtual Impedance Control Scheme Based on Small-AC-Signal Injection for Unbalanced and Harmonic Power Sharing in Islanded Microgrids

To address the unbalanced and harmonic power sharing issue among parallel inverters caused by feeder impedance mismatch in islanded microgrids, an adaptive virtual impedance control method is proposed based on the injection of an extra small ac signal (SACS) in the output voltage of each inverter. Similar to the principle of active power–frequency droop, the frequency of the injected signal droops with the output unbalanced and harmonic power, while the active power produced by the injected SACS is detected to adjust the virtual impedance at the fundamental negative sequence and selected harmonic frequencies, which will tune the distribution of unbalanced and harmonic power in the system. When the injected SACSs of each inverter synchronize with each other and reach a common frequency in steady state, the virtual impedance of each inverter will be matched to each other for evenly sharing the unbalanced and harmonic power. This proposed method requires neither communication links among parallel inverters nor feeder impedance information. Furthermore, the control parameter design method based on modeling and stability analysis of the proposed control structure is discussed in detail. Finally, simulation and experimental results are provided to validate the effectiveness of the proposed scheme.