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This paper investigates the stability of meshed DC micro-grids with constant power loads (CPLs), under decentralized primary and distributed secondary control schemes to achieve accurate power sharing and voltage restoration, with the latter being more significant to the case of parallel converters in a micro-grid architecture. The DC micro-grid consists of multiple DC/DC boost converters, which have nonlinear dynamics, feeding local CPLs, which also exhibit a nonlinear behaviour and introduce negative impedance characteristics that are well-known to yield instability. At the primary control layer, the droop control concept is suitably formulated and implemented using the recently proposed state-limiting PI controller to accomplish an inherent current limitation for each converter and simultaneously facilitate the stability analysis. Using limited information of the injected power from neighbouring converters and, depending on the micro-grid configuration, the load voltage, a distributed secondary controller is formulated to enhance the power sharing and accurately regulate the voltage to the rated value. By analysing for the first time both the dynamics of the converters with the CPLs and the two-layer control, singular perturbation theory is applied to analytically prove the stability of the entire DC micro-grid. The scalability of the system is also ensured through relevant passivity analysis. Simulation and experimental testings are performed to confirm the effectiveness and validity of the proposed method. |
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