Direct current (DC) distribution systems and DC microgrids are becoming a reliable and\nefficient alternative energy system, compatible with the DC nature of most of the distributed energy\nresources (DERs), storage devices and loads. The challenging problem of redesigning an autonomous\nDC-grid system in view of using energy storage devices to balance the power produced and absorbed,\nby applying simple decentralized controllers on the electronic power interfaces, is investigated in this\npaper. To this end, a complete nonlinear DC-grid model has been deployed that includes different\nDC-DERs, two controlled parallel battery branches, and different varying DC loads. Since many loads\nin modern distribution systems are connected through power converters, both constant power loads\nand simple resistive loads are considered in parallel. Within this system, suitable cascaded controllers\non the DC/DC power converter interfaces to the battery branches are proposed, in a manner that\nensures stability and charge sharing between the two branches at the desired ratio. To achieve this\ntask, inner-loop current controllers are combined with outer-loop voltage, droop-based controllers.\nThe proportional-integral (PI) inner-loop current controllers include damping terms and are fully\nindependent from the system parameters. The controller scheme is incorporated into the system\nmodel and a globally valid nonlinear stability analysis is conducted; this differs from small-signal\nlinear methods that are valid only for specific systems, usually via eigenvalue investigations. In the\npresent study, under the virtual cost of applying advanced Lyapunov techniques on the entire\nnonlinear system, a rigorous analysis is formulated to prove stability and convergence to the desired\noperation, regardless of the particular system characteristics. The theoretical results are evaluated by\ndetailed simulations, with the system performance being very satisfactory.
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