Research On Multilevel Inverter Topology And Active Equalization Strategy For Energy Storage Systems

The development of new energy represented by wind power and photovoltaic is the inevitable choice to achieve the goal of "carbon peak" and "carbon neutral".At present,the development of onshore wind power resources has become saturated,and the development of offshore wind power will become the main development trend.In the construction and operation of offshore wind farms,energy storage system is needed as backup power supply to supply important loads in case of power failure and ensure power quality.The traditional hybrid cascaded multilevel inverter has the problems of high cost and large volume,and the power imbalance between the cascaded units will cause the SOC difference between the battery modules,resulting in the reduction of the effective capacity of the energy storage system.In this thesis,the offshore wind turbine backup power developed by mingyang Intelligent Energy Group Co.,Ltd.as the research object,proposes a new inverter to meet the power quality requirements of important loads in offshore wind turbines,at the same time,the SOC estimation and active equalization strategy is designed to solve the SOC difference in the energy storage system caused by the unbalanced output power between the cascaded cells.The specific research content is as follows:（1）In order to meet the requirements of power quality,cost and volume of offshore wind turbine backup power supply,a new hybrid cascade 13-level inverter topology was proposed.Compared with the existing 13-level inverter,fewer switching devices were used.The inverter realizes thirteen level output by adjusting switch tube on and off,and reduces switching loss by making all switch tubes work at fundamental frequency.The feasibility of the inverter is verified by building an experimental prototype.（2）To achieve accurate estimate for energy storage battery SOC in the system,based on the analysis of the lithium ion battery characteristics and on the basis of equivalent circuit model,by mixing pulse power battery parameters identification performance test experiment,on the basis of the design based on extended kalman filtering of lithium ion battery SOC estimation method,to active balancing control to provide accurate information about the battery SOC.（3）Aiming at the characteristics of unbalance of output power between cascaded units of hybrid cascaded multilevel inverter,a control strategy is designed for double-layer switching inductor active equalization circuit.Based on the analysis of the working principle and mathematical model of the active equalization circuit,the fuzzy equalization strategy based on range and the fuzzy equalization strategy based on mean-difference are designed to realize the control of the active equalization circuit.The results show that compared with maximumminimum method,the balancing speed and efficiency of maximum-minimum based fuzzy control strategy are improved by 3.4% and 0.8% respectively.Compared with averagedifference method,the equalization speed of fuzzy equalization strategy based on averagedifference method is improved by 65%.The research in this thesis provides a backup power design scheme for offshore wind turbines,which helps to reduce the volume and cost of backup power and overcome the SOC differences within energy storage systems,and has a certain application prospect.