Droop coefficient correction control for power sharing and voltage restoration in hierarchical controlled DC microgrids

With the widely application of the DC microgrid (MG), the utilization of renewable resources is improved by interconnecting nearby DC MGs. Since MGs are connected by different line impedances, the intuitive tradeoff exists between the conflicting goals of current sharing and voltage regulation in the conventional droop control scheme. A hierarchical control structure with a dynamic droop coefficient correction control (DCCC) is presented to achieve better power sharing in DC MGs. In the proposed hierarchical control scheme, the droop coefficient correction method is employed in the primary level, which corrects the droop coefficient automatically and the proportional current has been shared. Moreover, the droop coefficient of each converter is also dynamically adjusted to deal with the impact of uncertain line impedances. The robustness of performance to system uncertainties is also demonstrated. Meanwhile, the different current ratio of the microgrids can be provided to the DC bus by using the tertiary control level, which achieves an arbitrary power sharing among the microgrids. In addition, a small-signal model is developed to investigate the effects of coupling relationship in the control parameters in the voltage loop, voltage restoration controller and droop coefficients. Finally, the experimental results are presented to evaluate the effectiveness of droop coefficient correction control approach and hierarchical control scheme for the DC MG under resistive and constant power load (CPL) conditions.