Research On The Technology Of Flying Capacitor Bi-Directional DC/DC Convert For DC Energy Storage

Non-isolated DC converter is suitable for photovoltaic DC energy storage system with wide voltage range due to its few devices,low cost and high voltage gain.Among them,fly-through capacitor three-level DC converter has the advantages of low switch voltage stress,which makes it widely used in high-power energy storage systems.The flying capacitor bi-directional DC/DC converter(FCBDC)solves the problem of limiting the voltage level in the direction of energy flow,and makes it have the function of boosting or reducing voltage in any direction of energy flow,which can adapt to different battery side voltage levels,greatly expanding the application conditions of the converter,and will have a wide application prospect in DC energy storage system.However,due to the symmetrical structure of the FCBDC,more switching devices are used in the converter,resulting in increased losses in the operation of the converter.Therefore,in order to reduce the operation loss of FCBDC and improve the operation efficiency of converter,this paper proposes an optimized modulation strategy based on duty cycle compensation.On the basis of phase-shifting modulation,this strategy reduces the average inductance current of the converter at the same output power level,increases the direct transmission power,and improves the operation efficiency of the converter by independently optimizing and compensating the duty cycle of the left and right bridge arms of the converter.At the same time,this strategy also has an obvious effect on reducing the inductance current ripple and further optimizing the operation performance of the converter.On this basis,this paper will analyze the inductance current and capacitance voltage of each working mode after compensation through the state variable analysis method,and design a two-way energy control closed-loop scheme under the energy storage mode,realizing the decoupling control of the system current and the two-side flying capacitor voltage.The paper also analyzes the control effect of the voltage outer loop,the current inner loop and the flyover capacitor voltage loop by using the Bode diagram method,and discusses the influence of the change compensation duty ratio on the control performance.Finally,a physical simulation experiment platform is built to verify the effectiveness and practicability of a series of control strategies for FCBDC.In practical application,the energy storage converter usually encounters abnormal conditions such as sudden increase of load and current,and the wave-by-wave current limiting strategy is an effective means to solve this problem.However,it is difficult for FCBDC to maintain the voltage balance of the flying capacitor under the traditional wave-by-wave current limiting strategy,which ultimately affects the operation of FCBDC under normal conditions.To solve this problem,this paper designs a wave-by-wave current limiting and pulse blocking strategy for FCBDC.By changing the frequency and initial phase of the switch pulse signal under the wave-by-wave current limiting,the strategy changes the operation mode of the converter under the wave-by-wave current limiting from three-level to two-level mode,thus ensuring the balance of the FCBDC to the flyover capacitor voltage under abnormal conditions,avoiding the use of hardware devices such as the flyover capacitor voltage clamp circuit,reducing the cost,easy to implement and easy to use in business.