Instability of a DC microgrid with constant power loads caused by modal proximity

This paper investigates stability of a DC microgrid with integrated constant power loads (CPLs). The investigation is based on an interconnected model of the DC microgrid with two subsystems. One subsystem consists of a CPL under examination and the other subsystem is constituted by remainder of DC microgrid (ROM). Analysis is carried out to indicate when an oscillation mode of CPL subsystem is in the proximity of an oscillation mode of ROM subsystem on complex plane, stability of the DC microgrid decreases as caused by the CPL, leading to growing oscillations in the DC microgrid in the worst case. Hence, the investigation reveals the mechanism about why the CPL may cause instability of DC microgrid from a new perspective of modal proximity. An index is proposed to detect the instability risk by applying the modal analysis to subsystems. The proposed application extends the merit of widely used frequency-domain analysis to the modal analysis, because instability danger can be detected by the proposed modal analysis in practice without having to establish parametric model of the DC microgrid. Two example DC microgrids with CPLs are presented to demonstrate and evaluate the analysis and conclusions made in the paper.