1.State Key Laboratory of Power Transmission Equipment&System Security and New Technology，Chongqing University，Chongqing400044，China;2.Shenyang Power Supply Company，State Grid Liaoning Electric Power Co.Ltd.，Shenyang110000，China

In order to ensure safe charge-discharge of the vanadium redox flow battery（VRB）energy storage system，different charge-discharge control modes for VRB with DC/DC converter are proposed.Based on modeling of the VRB equivalent circuit and its validity test，a dual closed-loop strategy for the DC/DC converter is proposed，of which the inner loop is the average current control on VRB side inductance while the outer loop is a switch control by constant active power，constant voltage and trickle current.By resorting to the dual closed-loop strategy with the DC/AC grid side converter，the stability of DC bus voltage is maintained.Then，with VRB state of charge（SOC）and terminal voltage as constraint conditions，the corresponding three-stage safe control mode of charge and discharge is also proposed.Taking the5kW VRB energy storage system with DC/DC converter as the study case，the operational characteristics of VRB are simulated in the proposed switch control mode and compared with the traditional constant power charge-discharge control with no switch.The dynamic response of the control strategy is studied using the constant active power control mode.The results show that VRB is better enabled to operate in the safe region by the proposed charge-discharge switch modes for DC/DC converter control，which also ensure a quick charge-discharge dynamic response.