Distributed Secondary Control for DC Microgrids With Near-Infinite Constant Power Load Accommodation

In DC microgrids, constant power loads (CPLs) inherently exhibit negative impedance characteristics, which are widely believed to degrade system stability as their penetration level increases. Consequently, extensive research has aimed to establish safe upper bounds for CPL penetration. However, these upper bounds are typically derived as sufficient conditions, making them overly conservative. Moreover, when multiple DC sources are connected in parallel to a common DC bus, the simultaneous need for current sharing and DC bus voltage regulation further complicates system control. To address these challenges, this paper proposes a novel distributed secondary control method based on the dynamic averaging of virtual voltage drops (VVDs). The proposed method offers two key advantages: 1) It ensures both precise current sharing and voltage regulation in single-bus DC microgrids, even in the presence of mixed ZIP loads, i.e., constant resistive loads (Z), constant current loads (I), and constant power loads (P). 2) Unlike existing approaches that impose conservative limits on CPL penetration, the proposed method theoretically demonstrates that the safe upper bound for CPLs can be arbitrarily large, enabling the DC microgrid to accommodate an almost infinite number of CPLs without compromising stability. Both Simulation and experiment studies are conducted to validate the effectiveness of the proposed method.