Research On Dual-Model Control Strategy Of Cascaded-type Energy Storage System Considering SOC Balancing

In order to effectively solve the problems of intermittency and uncertainty of renewable energy sources generation,battery energy storage systems(BESS)are widely deployed in grids with high penetration rate of renewable energy sources.Aiming at the unbalanced state of charge(SOC)of BESS battery modules in these scenarios,power quality control and modes switching in grid-connected and islanded modes,this thesis focuses on the SOC equalization control,control methods of grid-connected and islanded modes,switching method of cascaded-type BESS,the details are as follows:(1)Modeling and power control research of cascaded-type BESS.Analyzing the topology characteristics of the cascaded-type BESS,the basic models of the energy storage system are established in islanded and grid-connected modes.According to the grid-connected and islanded system models,SOC information estimation,power control principle,voltage and current control methods are expounded.(2)Aiming at the cascaded-type BESS in islanded mode,a distributed SOC equalization control method based on local information is proposed,which can ensure the SOC equalization of battery modules with different capacities through power adjustment without communication.This method avoids the influence of battery modules capacity on SOC balancing by adjusting droop coefficient,and analyzes the influence of SOC adjustment index on SOC balancing speed.In the view of large time scale of SOC changes,the proposed strategy is used for steady-state analysis and stability analysis based on the singular theory,which theoretically proves the effectiveness of the method.(3)For the cascaded-type BESS in grid-connected mode,a low-bandwidth distributed SOC equalization control method is proposed.The method obtains SOC information of adjacent battery modules through low-bandwidth communication,and dynamically adjusts the active power and voltage reference of each sub-module based on the SOC information,thereby adjusting the SOC imbalance.By introducing the distribution correction coefficient,the speed of SOC equalization is adjusted.Theoretical analysis and simulation results prove the correctness of the method.(4)A unified distributed control method for on-grid and off-grid mode is proposed.This method reduces the impact of switching from islanded mode to grid-connected mode through the time sequence design of mode switching.Pre-synchronization control and secondary frequency adjustment are introduced to reduce the difference between the output voltage of the sub-module and the grid waveform at the time of grid connection.Combined with the SOC equalization strategies in islanded and grid-connected modes,a multi-layer unified distributed control strategy is proposed to achieve SOC equalization in various operating modes.While achieving SOC balancing in various operating modes,it also solves the problems of inrush current and grid-connection synchronization during mode switching.(5)The experimental platform of the cascaded BESS is built,and the experimental results verify the feasibility and effectiveness of SOC balancing control strategy and modes switching strategy of grid-connected and islanded modes.