State-of-Charge Balance Using Adaptive Droop Control for Distributed Energy Storage Systems in DC Microgrid Applications

This paper presents the coordinated control of distributed energy storage systems in dc microgrids. 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 nth order of SoC. By using a SoC-based droop method, the ESUs with higher SoC deliver more power, whereas 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 the 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.

I. Introduction

With the objective to electrify remote areas and energy islands, the microgrid concept is gaining more and more popularity [1]. Nowadays, dc microgrids are becoming more attractive with the raise of dc power sources, storages, and the loads with natural dc coupling, e.g., photovoltaic modules, batteries, fuel cells, light-emitting diodes, and so on. With the comparison of the overall efficiency, it can be found that the efficiency of dc system is higher than the ac system [2]. At the same time, dc does not require for synchronization, and the problems caused by the reactive power and harmonics disappear. For these reasons, recently, there has been an increasing awareness on dc microgrids [3]–[10]. A typical configuration of a dc microgrid is shown in Fig. 1 [9]. Fig. 1.

Typical configuration of a dc microgrid.