Stability Analysis and Damping Enhancement Based on Frequency-Dependent Virtual Impedance for DC Microgrids

This paper focuses on the stability analysis and damping performance improvement of dc microgrids. The small-signal model of a dc microgrid has been derived. Eigenvalue analysis results reveal the relationship between the system stability and different factors of dc microgrids, including types of dc load, the droop coefficient, line parameters, etc. It shows that the poorly damped LC circuits in dc microgrids reduce the system damping and bring in high frequency oscillations. To improve the damping performance, a frequency-dependent virtual impedance approach is proposed, which can effectively shape the high frequency impedance of the dc bus voltage control units, improve the system stability, and mitigate the oscillations. Detailed design guidelines on virtual impedance to achieve good stability and transient performance are also provided. Simulation and experimental results are obtained to confirm the validity of the proposed approach.