Research On Bi-directional DC/DC Converter Parallel Operation

:With the development of new energy generation technologies, battery energy storage system has become a basic supporting facilities.In recent years, due to the increasing of the installed capacity, photovoltaic solar power and wind power generation system has put forward higher demands on storage system. Large-scale battery storage technology is eager to be studied. In this paper, an energy storage system, which contains bi-directional DC/DC converters in parallel, has been proposed. The paper focuses on current distribution control strategy and power flow control strategy. Simulation and experimental results show that the method used in this paper is feasible and effective.Firstly, an energy storage system which contains bi-directional DC/DC converters parallel has been proposed, so as to expand the capacity of battery energy storage system. After analysis of the basic topology, bi-directional Buck-Boost converter is selected to apply to this paper. Two kinds structure of energy storage system are mentioned. In one structure, Bi-directional DC/DC converters run in parallel and then connect to the DC link; and in the other one, Bi-directional DC/DC converters run in parallel and then connect to DC/AC converter.Secondly, current distribution control and bi-directional power flow control are studied. Making it as classification criteria that whether there is interconnects or not, droop-adjustment method and double-loop current regulation method are discussed. Through simulation and experiments, the characteristics of two kinds of methods have been analyzed and compared. In regard to the system that bi-directional DC/DC converters run in parallel and then connect to the DC link, a new method is proposed by which power flow can be controled through adjusting the voltage reference value. In regard to the system that DC/DC converters run in parallel and then connect to the DC/AC inverter, the decoupling control of active and reactive power is discussed. In d-q coordinate system, the current reference value is determined by the active and reactive power. Through current closed-loop control of id and iq, the grid power can be effectively controled. The above methods have been simulated and experimented; the results show the good performance of the method.Thirdly, vanadium redox battery (VRB) is studied in this paper, which has been developed rapidly recently. A control strategy that current can be distributed according to the SOC of VRB has been proposed. This method uses a double-loop control, in which the reference of inner loop will be revised to achieve control anticipation. Vanadium redox battery (VRB) is studied in this paper, which has been developed rapidly recently. A control strategy that current can be distributed according to the SOC of VRB has been proposed. This method uses a double-loop control, in which the reference of inner loop will be revised to achieve control anticipation. A model combined with wind farm has been set up. The simulation results verify the feasibility of the method.Lastly, experimental platform in which bi-directional DC/DC converters run in paralle is set up. The main circuit and control circuit are designed, including the selection of the inductor-capacitor parameters, sampling circuit, pre-charging circuit, etc. At the same time software is designed. Experimental verification of part of the methods in this paper has been done.