A Self-Sustained and Flexible Control Strategy for Islanded DC Nanogrids without Communication Links

This paper proposes a self-sustained and flexible control strategy for autonomous dc nanogrids (NGs) in remote and rural areas without the need for a communication system. The proposed control strategy of NGs is based upon a hierarchical control, in which the primary control manages the power balance inside the NG and the secondary control is responsible for removing deviation of the dc bus voltage caused by the droop operation. The state of charge of the battery and the external dc bus signal are taken into account in the proposed control strategy in order to avoid the overcharge/deep discharge of the battery as well as the collapse of the external dc bus. Bidirectional power flow among multiple NGs is implemented through a dedicated interconnected bidirectional dual-active-bridge dc/dc converter installed inside the NG to ensure a galvanic isolation among multiple interconnected NGs. Finally, the small-signal model is developed, in which the small-signal transfer function of an entire NG is derived from the small-signal transfer functions of every single converters of the system. From the attained transfer function, the appropriate secondary controller is designed, and the system stability is analyzed. The proposed control strategy is validated through simulations and experiments.