Abstract
Direct current (DC) distribution systems and DC microgrids are becoming a reliable and efficient alternative energy system, compatible with the DC nature of most of the distributed energy resources (DERs), storage devices and loads. The challenging problem of redesigning an autonomous DC-grid system in view of using energy storage devices to balance the power produced and absorbed, by applying simple decentralized controllers on the electronic power interfaces, is investigated in this paper. To this end, a complete nonlinear DC-grid model has been deployed that includes different DC-DERs, two controlled parallel battery branches, and different varying DC loads. Since many loads in modern distribution systems are connected through power converters, both constant power loads and simple resistive loads are considered in parallel. Within this system, suitable cascaded controllers on the DC/DC power converter interfaces to the battery branches are proposed, in a manner that ensures stability and charge sharing between the two branches at the desired ratio. To achieve this task, inner-loop current controllers are combined with outer-loop voltage, droop-based controllers. The proportional-integral (PI) inner-loop current controllers include damping terms and are fully independent from the system parameters. The controller scheme is incorporated into the system model and a globally valid nonlinear stability analysis is conducted; this differs from small-signal linear methods that are valid only for specific systems, usually via eigenvalue investigations. In the present study, under the virtual cost of applying advanced Lyapunov techniques on the entire nonlinear system, a rigorous analysis is formulated to prove stability and convergence to the desired operation, regardless of the particular system characteristics. The theoretical results are evaluated by detailed simulations, with the system performance being very satisfactory.
Highlights
The traditional structure of power systems has dramatically changed to involve modern active distribution and microgrid-based schemes
All the loads have been considered only as constant power loads [17,18,19], a fact that is in contradiction with the results presented in [20], where it has been shown that a significant portion of Direct current (DC) loads should be modeled as constant impedance loads
The proposed control design and analysis is of great significance in real-world applications, and constitutes the main contribution of the present work, since stable operation of a DC distribution system can be guaranteed with the fast dynamics of the system to converge to the equilibrium
Summary
The traditional structure of power systems has dramatically changed to involve modern active distribution and microgrid-based schemes. The proposed control design and analysis is of great significance in real-world applications, and constitutes the main contribution of the present work, since stable operation of a DC distribution system can be guaranteed with the fast dynamics of the system to converge to the equilibrium. On this basis, the conventional analysis, as extensively mentioned in the literature (see for example [12]), with the droop-based outer controllers incorporated, seems to be further valid for the entire closed-loop DC-grid system. As indicated by the results, the desired power balance and battery charge sharing is well-achieved by the droop controllers
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