Accurate Active and Unbalanced Harmonic Power Sharing of Islanded Microgrid Based on Small-AC-Signal Injection

The structure of islanded microgrids is complex and has many problems. For this problem of equalizing the fundamental active power, many articles use the traditional droop control, which cannot simulate the physical characteristics of inertia and damping in the power generation process, which are considered by the virtual synchronous generator (VSG) technology. The mismatch of the feeder impedance can lead to the inequality of unbalanced harmonic power for the parallel inverters. To address this problem, a control method is proposed that injects an additional small AC signal (SACS)into the output voltage of each inverter and adds an adaptive virtual impedance. This method is similar to the conventional active-frequency droop control, where the output frequency of the injected SACS is linearly related to the output unbalanced harmonic power. When the unbalanced harmonic power rises, the output frequency of the injected signal also rises. And the virtual impedance on the negative sequence series is adjusted by detecting the active power generated by the injected SACS. When the system is stable, the frequency of the injected SACS in each inverter will be the same, and the sum of the virtual inductance and the feeder inductance of each inverter will be the same. The active power and the unbalanced harmonic power of the parallel distributed generator set can be equally shared without communication.