Successive-Approximation-Based Virtual Impedance Tuning Method for Accurate Reactive Power Sharing in Islanded Microgrids

Due to the mismatched line impedances in islanded microgrids, distributed generation units with conventional droop tend to have barely satisfactory reactive power sharing accuracy. To handle this problem, this article proposes a virtual impedance tuning method based on successive approximation to accurately compensate the mismatch among line impedances. First, in each distributed generation (DG) unit, an adaptive virtual impedance regulation based on the error between the actual output reactive power and its reference is established and combined with the conventional Q-V droop control. Then, these two control schemes operate alternately for several cycles, and in each cycle, the reactive power reference of the virtual impedance regulation is updated by the latest estimate from the Q-V droop control. Therefore, the virtual impedance of each DG unit will be tuned successively to an appropriate value to achieve accurate power sharing among DG units based on local information. Meanwhile, to ensure all the DG units switching between these two control schemes synchronously, a common triggering signal from the microgrid central controller is needed to start the internal time sequence of each DG unit. Furthermore, a comprehensive design procedure for key parameters is presented based on convergence analysis and small-signal modeling of the tuning process. Finally, the effectiveness of the proposed method is verified by simulation and experimental results. This article is accompanied by a video demonstrating the virtual impedance tuning process.