A Control Strategy for Self-Sustained and Flexible DC Nanogrids

Microgrids are becoming a potential solution for combining distributed generation units, such as photovoltaic panels, wind turbines and energy storage systems. As a simple and small version of a microgrid, a nanogrid is a power distribution system that is suitable for a single node, such as a small building or a private house. The nanogrid can be flexibly connected to or disconnected from other power entities through a gateway. In most cases, the nanogrid is connected to the utility grid to avoid the power outage and to increase the operational efficiency. However, the current standalone nanogrid model is not suitable because an imbalance between the generated and consumed electrical power might occur. The main objective of this research work is to develop a self-sustained and flexible control strategy for autonomous direct current (DC) nanogrids in remote and rural areas without the need for a communication system. The proposed control strategy for the nanogrids is based upon a hierarchical control, in which the primary control manages the power balance inside the nanogrids and the secondary control is responsible for removing deviation of the DC bus voltage caused by droop operation. The state of charge (SoC) of the battery and the external DC bus voltage 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. The control algorithm also ensures a flexible exchange of power inside a nanogrid as well as among multiple nanogrids without any extra digital communication link. Bidirectional power flow among multiple nanogrids is implemented through a dedicated interconnected bidirectional Dual Active Bridge (DAB) DC/DC converter installed inside each nanogrid to ensure a galvanic isolation among multiple, interconnected nanogrids. The proposed control strategy is validated through both simulations and experiments. Simulation and experimental results are used to validate the operation of the proposed control algorithm and prove the resemblance between theory and experiments. However, in order to implement the proposed control strategy, a model of the DC nanogrid has to be developed. For that reason, modeling of every single converter in the system should be conducted. The second important contribution of this research is modeling and control for converters independently, including a bidirectional buck converter and a dual active bridge converter. A small-signal model based on the state-space averaging technique for the bidirectional buck converter is developed, in which only the mean value (i.e. “zeroth” harmonic) of the state variables is taken into account. On the other hand, the generalized state-space averaging-based modeling method is used to obtain the state-space representation of the DAB converter, in which the direct current (DC) component and the fundamental harmonics in the Fourier series expansion of state variables are considered. Transfer functions from control-to-output are determined, which will be used to define two controllers for the current and voltage loops in a cascaded control structure. Simulations and experiments will be used to validate the operation of the proposed method. As aforementioned, modeling and control for each converter in the DC nanogrid is performed separately. Nevertheless, when these converters are connected to form a complete DC nanogrid, they will affect each other and the stability of the entire system is influenced as well. To overcome this problem, a model of the entire system has to be developed and the system stability has to be analyzed. For this purpose, the small-signal transfer function of a DC nanogrid is synthesized from the small-signal transfer functions of every single converter of the system. Using this transfer function, the system stability is analyzed and the secondary controller is designed. Simulation and experimental results are used to verify a stable operation of the DC nanogrid system.