-This paper presents the coordinated control of distributed energy storage systems (DESSs) in DC micro-grids. In order to balance the state-of-charge (SoC) of each energy storage unit (ESU), an SoC-based adaptive droop control method is proposed. In this decentralized control method, the droop coefficient is inversely proportional to the n th order of SoC. By using SoC-based droop method, the ESUs with higher SoC deliver more power, while the ones with lower SoC deliver less power. Therefore, the energy stored in the ESU with higher SoC decreases faster than that with lower SoC. The SoC difference between each ESU gradually becomes smaller and finally the load power is equally shared between the distributed ESUs. Meanwhile, the load sharing speed can be adjusted by changing the exponent of SoC in the adaptive droop control. The model of SoC-based adaptive droop control system is established and the system stability is thereby analyzed by using this model. Simulation and experimental results from a 2×2.2 kW parallel converter system are presented in order to validate the proposed approach.

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In this paper, a hierarchical control system for parallel power electronics interfaces between ac bus and dc bus in a hybrid microgrid is presented. Both standalone and gridconnected operation modes in the dc side of the microgrid are analyzed. Concretely, a three-level hierarchical control system is implemented. In the primary control level, the decentralized control is realized by using the droop method. Local ac current proportional-resonant controller and dc voltage proportionalintegral controller are employed. When the local load is connected to the dc bus, dc droop control is applied to obtain equal or proportional dc load current sharing. The common secondary control level is designed to eliminate the dc bus voltage deviation produced by the droop control, with dc bus voltage in the hybrid microgrid boosted to an acceptable range. After guaranteeing the performance of the dc side standalone operation by means of the primary and secondary control levels, the tertiary control level is thereafter employed to perform the connection to an external dc system. Meanwhile, the impact of the bandwidth of the secondary and tertiary control levels is discussed. The closed-loop model including all the three control levels is developed in order to adjust the main control parameters and study the system stability. Experimental results of a 2×2.2 kW parallel ac-dc converter system have shown satisfactory realization of the designed system.

Double-quadrant state-of-charge based droop control method for distributed energy storage systems in autonomous dc microgrids," IEEE Trans. Smart Grid, 2014.