Research On Coordinated Control Of Box-Layout Vanadium Redox Flow Battery Pack

Large capacity storage systems which use energy storage battery as reserve capacity can realize filling valley and modulating peak and improve the stability of power system. As a new type of energy storage batteries, Vanadium Redox Flow Battery (VRB) has advantages of large capacity, low self-discharge rate, fast response and high conversion efficiency. It’s suitable for large-scale energy storage. In recent years, people pay close attention to the VRB. VRB compose large capacity energy storage systems by series-parallel connection. Due to the inconsistency of the battery, the phenomenon of over-charging and over-discharging appears during the process of charging and discharging, which affects the service life of VRB. In order to improve the inconsistency of the battery, the equalization circuit is put in the energy storage systems, which aims at realizing the coordinated control of box-layout Redox Flow Battery Pack (BL-VRB). In the large capacity energy storage systems, the Bidirectional DC converter (BDC) is to control charging and discharging of VRB. But coordinated control is worked in the charge and discharge process of VRB. So the coordinated control of multiple BL-VRB can not influence the normal work of BDC. The research on coordinated control of BL-VRB is of engineering significance.Firstly, the working principle, equivalent model, charging and discharging method of VRB are introduced in this article. The model of VRB is built in Simulink simulation platform to study the charge and discharge characteristics of VRB, which lay the theoretical foundation. Secondly, through the analysis and comparison of the main circuit of the commonly used equilibrium, the active equalization circuit such as Flying-capacitor method is chosen for coordinated control of single BL-VRB. And the equilibrium principle of Flying-capacitor method is analyzed. Due to the high price of the super capacitor, in order to reduce the cost, the equilibrium speed is analyzed. Several major factors that affect the equilibrium speed and the selection rule of the super-capacitor parameters and the switching frequency parameters are obtained. Then the equilibrium strategy by changing switching frequency is proposed to achieve the coordinated control of BL-VRB. In order to verify the validity and accuracy of the variable frequency equilibrium strategy, the model of the single BL-VRB coordinated control system is built in the Simulink simulation platform. Finally, on the basis of single BL-VRB coordinated control, the two level of equilibrium main circuit is proposed to achieve coordinated control between multiple BL-VRB. Then the model of multiple BL-VRB coordinated control system is built in Simulink simulation platform. This equilibrium strategy is proved more effective and faster.There are some researches on BDC during the preliminary experiment. Based on the previous research, the software of BDC is designed to realize the double closed loop charging control of BDC in this article. Through a lot of simulations and experiments, it is proved that the coordination control does not influence the normal work of BDC.As a result, the equilibrium strategy and the two level of equilibrium main circuit which are studied in the article can be applied in large capacity storage systems. It can not only reduce system cost, but also achieve maximum utilization of the energy storage systems. And the selection rule of super capacitor and switching frequency of VRB has a reference value in the application of large capacity storage systems.