Virtual impedance based stability improvement for DC microgrids with constant power loads

In this paper, in order to improve the stability of DC microgrids with constant power loads (CPLs), a virtual impedance based method is proposed. The CPLs have inherent instability issues induced by negative incremental impedances. To enhance the system stability, two types of virtual impedance based stabilizers comprised of series-connected inductance and resistance are employed. Type I stabilizer locates at the output capacitor branch, and Type II stabilizer locates at the output inductance branch. Meanwhile, considering that the parallel interfacing converters are commonly in parallel in a microgrid, droop control is taken into account. To validate the stability of the above stabilizers in a DC microgrid with parallel interfacing converters and CPLs, the impedance matching approach is employed. It is demonstrated that, the instable poles can be moved to the stable region in the frequency domain by the proposed stabilizers. Simulations with three interfacing converters are conducted by using MATLAB/Simulink, which verify the effectiveness of the proposed methods. It is shown that both of the proposed virtual impedance based stabilizers can effectively enhance the system stability with CPLs.