Adaptive Virtual Flux Droop Control Based on Virtual Impedance in Islanded AC Microgrids

Abstract

Virtual flux droop (VFD) method is a scheme, in which, instead of frequency and voltage amplitude, the phase angle and amplitude of the virtual flux are, respectively, adjusted to active and reactive power sharing. However, mismatched line impedances lead to improper power sharing. An adaptive VFD (AVFD) control strategy is proposed in this article to solve this problem. First, the effect of the mismatched line impedances on the VFD method is analyzed. Then, the idea of virtual impedance is employed to develop the AVFD control strategy. In addition, a small-signal model is presented which is used to tune the control parameters. Furthermore, the microgrid control approach is made based on the direct flux control (DFC) method to apply the proposed strategy. The resultant control scheme is simple without needing multiple feedback loops and complex transformations. In order to validate feasibility and effectiveness of the proposed control strategy, both simulation and experimental studies are implemented. The results show that by the proposed method, accurate power sharing proportional to the ratings of the sources can be achieved in spite of mismatched line impedances.