Research On DC-DC Cascaded Energy Storage Converter Based On Droop Control

With the rapid development of the electric vehicle industry,a large number of power batteries will be returned in the next few years.Although these returmed batteries cannot meet the requirements of electric vehicles,they can be reused in energy storage systems.However,the inconsistency of the returned batteries is the key factor that restricts the energy utilization rate of the energy storage system.In the traditional energy storage converter,all energy storage batteries are grouped in series and parallel,so the independent control of battery can not be realized.Modular cascaded converters have attracted wide attention in high voltage and high power applications due to their advantages of easy expansion,flexibility and reliability,and high quality output waveform.In this thesis,the principle and control strategy of modular independent control for cascaded DC-DC converters are studied in depth.First,this thesis introduces and analyzes the structure of cascaded DC-DC energy storage system,working principle of converter,modulation strategy and modular independent control principle.Aiming at the problem of unbalance of module voltage and power in distributed control,the droop control method of series converter is put forward combined with the principle of traditional parallel droop control,and the improved droop control method is put forward to solve the problem of deviation in traditional droop control,and the effectiveness of droop control is verified by simulation.Secondly,considering that the inconsistency of the returned batteries is serious,the SOC imbalance becomes more and more serious with the accumulation of time in the operation process.To solve this problem,the SOC equalization strategy is added on the basis of droop control.Considering the inconsistency of the battery capacity further,capacity correlation coefficient is added to realize the SOC equalization control under different initial SOC conditions and different battery capacities.The effectiveness of the method is verified by simulation.Finally,based on the four-module DC-DC cascaded converter,the power balance and dynamic response under droop control are verified experimentally.On this basis,the SOC equalization control strategy is experimented,and the experimental results verify the effectiveness of the proposed control strategy.