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Distributed control strategy of hybrid energy storage system in the DC microgrid

In a DC microgrid, because the output of renewable energy such as photovoltaic is intermittent, hybrid energy storage system (HESS) combining ultracapacitors and batteries is usually used to solve this issue. According to the characteristics of ultracapacitors and batteries, the low-frequency power fluctuations are assigned to batteries, while the high-frequency power fluctuations are assigned to ultracapacitors. When considering an ultracapacitor group, state-of-charge (SoC) of the ultracapacitors will be unbalanced between each other, on account of the differences of initial states or attributes. A novel distributed control method is proposed in this study. In general, ultracapacitors control the DC bus voltage and batteries control SoC of the ultracapacitors. Additionally, the distributed control adds the difference between one ultracapacitor's SoC and the average SoC of the ultracapacitor group to its current control loop. As a result, the dynamic stability of SoC among the ultracapacitors can be achieved, and the issues of overcharging and/or over discharging of the ultracapacitors can be avoided. At last, the effectiveness of the proposed control strategy is validated by the simulation model built in MATLAB/Simulink.