A Virtual Complex Impedance Based $P-\dot{V}$ Droop Method for Parallel-Connected Inverters in Low-Voltage AC Microgrids

Due to the high R/X ratio and mismatched feeder impedance of low-voltage microgrids, conventional droop method is no longer able to decouple the active and reactive power of distributed generators and the powersharing accuracy is degraded. In this article, a virtual complex impedance based P - V̇ droop method is proposed to decouple the powers and improve the power-sharing accuracy among DGs. With the virtual impedance method, the equivalent impedance between virtual power source and point of common coupling is shaped to be purely resistive. Then, a P - V̇ strategy is adopted to alleviate the effect of mismatched line impedance, where the virtual powers rather than the ordinary P/Q are used in the droop equation. In case the output voltage violates the operation code, a restoration mechanism is proposed to reset V̇ to zero. Compared with existing virtual impedance and Q - V̇ droop methods, the proposed method combines the advantages of both. Besides, a modified P - V̇ strategy is also presented to accelerate the restoration process and improve the active power-sharing accuracy at the same time. Simulation results validate the effectiveness of the proposed method.