Stability Analysis of a Remote DC Subgrid/Microgrid Connected to a Very Weak AC Grid

Instability issues can arise due to the high penetration of remote voltage source converter (VSC)-interfaced DC microgrid (MG) to the ac weak grid (WG), in which the grid impedance is large. This is due to the dynamic interaction between the VSCs and the WG impedance. In this work, a small-signal analysis is conducted to derive the full-order linearized model of the VSC-WG interconnection. Furthermore, a participation factor analysis is presented to identify the effect of varying the grid impedance on the VSC-WG dominant modes in the inversion and rectification modes of operation. It is found that although the system is initially stable in both modes, it tends to move toward the unstable region when the grid impedance increases. In this study, the initial locations of the corresponding dominant eigenvalues are fairly similar for both modes. However, unlike previous works, it is shown that the dominant modes in the rectification mode are much more sensitive to the grid impedance variation than the inversion mode. Time-domain simulations are conducted on a 7.25 MW dc MG which is interfaced to the ac grid via a VSC system to verify the validity of small-signal analysis in both modes.