A multi-objective hierarchical control framework for battery units in DC microgrids using improved adaptive droop control

We investigate the primary/secondary control problem for battery units with adaptive droop control in DC microgrids. A majority of existing control methods for state-of-charge (SoC) balancing among battery units require that the terminal voltage of each battery unit remains constant. To remove such a requirement, at the primary control level, we propose an improved adaptive droop control method by introducing an auxiliary battery state in terms of the battery parameters, the terminal voltage, and a power function of the SoC. The designed droop coefficient is inversely proportional to the auxiliary battery state. It is shown that the proposed adaptive droop control achieves proportional power sharing and SoC balancing, albeit at a cost of compromised voltage regulation. A salient feature of the proposed adaptive droop control method is that the battery’s terminal voltage is allowed to be time-varying. Moreover, to compensate for the voltage deviation caused by the improved adaptive droop control, we propose a consensus-based distributed control scheme at the secondary control level. It is shown that the resulting hierarchical control framework, consisting of the primary and secondary control levels, guarantees that the control objectives of voltage regulation, proportional power sharing, and SoC balancing are all achieved. Simulation studies based on an accurate electrical battery model validate the effectiveness of our multi-objective hierarchical control framework.