A Multi-Functional Virtual Impedance for Droop-Controlled Inverter-Based Microgrids

In isolated microgrids, conventional droop control was introduced to attain equal power sharing while maintaining voltage and system frequency. However, the conventional droop control leads to poor reactive power sharing because of the microgrid feeders mismatch. To tackle this issue, the virtual impedance was proposed to improve the accuracy of reactive power sharing by eliminating the effect of the mismatch in microgrid feeders. On the other side, the virtual impedance was employed for other purposes such as limiting the DG output current during faults to increase the sensitivity of the protection system. Utilizing the virtual impedance for achieving the reactive power sharing enhancement and increasing the protection system sensitivity in one control system has not been explored. Therefore, this paper proposes a decentralized multi-functional virtual impedance (MFVI) scheme for AC islanded microgrids that can enhance the accuracy of reactive power sharing during normal conditions and limit the fault current of inverter-based DGs during faults using adaptive multi-functional characteristics depending on the output current and voltage of the DGs. In addition, the control scheme is provided with negative sequence (NS) inner control loops to mitigate the voltage unbalance in unbalanced microgrids during normal conditions. The MFVI control scheme was tested on the IEEE 13-bus system. The results show that the proposed scheme enhances the reactive power sharing during normal loading conditions and provides DG fault current limitation.