Stability Improvement Without External Communication for DC Microgrid Equipped With Positive Feedback Islanding Detection

Unplanned islanding events in dc microgrids bring severe safety hazards to distributed generators (DG) and consumers. The positive feedback islanding detection method (IDM) provides guaranteed protection for consumers due to its small non-detection zone and high detection speed. However, the positive feedback loop continuously injects disturbances into DG and thus will result in a reduction in the stability margin of the grid-connected dc microgrid. Conventional solutions usually provide stability compensation based on grid current in the dc/dc converter to avoid conflicts with IDM, but it requires external communication and thus reduces system reliability. To address this issue, the cascaded impedance model of the grid-connected dc system is developed first in this paper, which reveals the instability mechanism caused by the IDM. On top of that, the phase stability region of the voltage source converter (VSC) is explored and described to guide the damper design. Furthermore, an active damping method is proposed to realize phase compensation for the VSC impedance. In this manner, the negative effect of the islanding detection loop on the stability margin is effectively mitigated without external communication links. Finally, the proposed method is validated in hardware-in-loop experiments.