On the stability of distributed secondary control for DC microgrids with grid-forming and grid-feeding converters

Abstract

This paper investigates the stability problem of a single-bus DC microgrid with mixed grid-forming/grid-feeding converters and the constant impedance, constant current, and constant power (ZIP) loads. Considering the different output properties of grid-forming and grid-feeding converters, distributed secondary controllers with mixed V–I droop and I–V droop controllers are designed to achieve accurate bidirectional current sharing and voltage regulation. As few converters have access to the information of the common bus, an observer is designed for each local controller to estimate the bus voltage. Then, the observer state is used to generate voltage correction term in the feedback loop, which gets potential faster regulation compared with the existing approaches that use pinning gains. Based on the rank-one perturbation analysis, a stability criterion of DC microgrid is given for the case of ZI load, where global voltage regulation and current sharing are guaranteed. Then, the stability criterion is extended to the case of ZIP load, where local stability of the DC microgrid is guaranteed. Both numerical examples and experimental tests are given to verify the effectiveness of the theoretical results.