Virtual Impedance Based Stability Improvement for DC Microgrids with Constant Power Loads

DC microgrid provides an efficient way to integrate different kinds of renewable energy sources with DC couplings. 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. This negative impedance makes the system poorly damped and the stability is thereby degraded. To enhance the system stability, virtual impedance based stabilizer comprised of series-connected inductance and resistance is employed. In particular, two types of stabilizers are used. 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 here. To validate the stability with the above stabilizers in a DC microgrid with parallel interfacing converters and CPL, the impedance matching approach is employed. The output impedance of the source converter and input impedance of the load are calculated respectively, and the influence of droop control, negative incremental impedance of CPL, proposed stabilizers are considered in the calculation of the impedance. It is demonstrated that with the proposed stabilizers, the instable poles can be moved to the stable region in the frequency domain. Simulation model with three interfacing converters is implemented based on MATLAB/Simulink to verify the proposed method.