Hierarchical Control Strategy of Hybrid Slack Terminals in DC Micro-grid

In order to regulate the voltage fluctuation in DC micro-grid through different types of slack terminals, a hierarchical control strategy for hybrid slack terminals is proposed. Firstly, based on the idea of the best cycle depth of charging/discharging, the dual lithium batteries(A and B) are applied as the basic energy storage units. Super-capacitor (SC) is adopted to operate as a buffer device taking advantage of its high ramp rate performances. And the superior main DC bus is taken as the backup support with its high power. Then, the output power of the dual lithium batteries is controlled according to the super-capacitor voltage signal. In dynamic balance, the super-capacitor voltage can characterize the magnitude of the low frequency components of the DC bus voltage. Thus, there are four different hierarchies on the basis of the super-capacitor voltage signal. Four corresponding operation modes are designed to make every slack terminal obtain energy allocation efficiently. Finally, the effectiveness of the proposed control strategy is verified by the experimental results.