Research On Load Sharing Method Of Energy Storage System Considering SoC Balancing Error In DC Microgrids

In the DC microgrid energy storage system,to meet the stable operation of the system and increase the capacity configuration,the structure of distributed parallel connection of multiple energy storage units to the DC bus is usually adopted.And The cooperative distribution of load energy by multiple storage units is realized through SoC-balancing control,which alleviates the system instability caused by overcharging,overdischarging or frequent switching of some storage units.The core idea of SoC-balancing control is to coordinate the load sharing of the energy storage system,causing the energy storage units with different initial SoC to operate toward the trend of SoC convergence consistency.In this paper,the process of SoC balancing operation is simulated by mathematical model and variation curve,and the possible equalization error problem in this operation process is quantitatively analyzed.Based on the analysis,the two improved SoC-balancing control methods are proposed for different application situations to optimize the load sharing of the energy storage system,respectively.Firstly,the working mechanism of droop control for load distribution in DC microgrid is analyzed,as well as the operating principle of the SoC balancing in energy storage system by droop control.Moreover,the process of SoC balancing is mathematically modeled based on the equivalent physical structure of the energy storage system and the droop expression.And the impact of relevant uncertainties is analyzed through the simulation of the operating curve,focusing on the SoC balancing error that caused by the difference in line resistance of each unit causes.Secondly,for the user-side low-voltage DC microgrid,a simplified SoC balancing control method is proposed for this user-side low-voltage DC microgrid.For the case where the line resistance difference between energy storage units is small,the droop control is designed by the voltage regulation function using SoC as the independent variable without the additional bus voltage regulation,and can be used for both the charging and discharging processes.The control method is free of the hardware sampling of output current and interconnection communication,and keeps the bus voltage deviation within a small range without additional bus voltage regulation control,simplifing the control and hardware parts.Besides,the function curve analysis is proposed to analyze the dynamic operation characteristics of the system to give the selection basis and constraints.The stability analysis of the system is conducted based on the small signal model.The correctness of the relevant theories in this simplified SoC balancing control is verified by simulation and experiment.Finally,a distributed nonlinear SoC-balancing method is proposed for energy storage systems with non-negligible line resistance differences in DC microgrids.The I-V droop control is firstly carried out by using the successive approximation method to compensate the difference between line resistances through dynamic virtual resistances,and then switched to the SoC balancing control stage through the action of switching buffer and global trigger signal.The whole process is carried out under the distributed control structure,and the global consensus objective of the system is achieved by the dynamic consistency algorithm,which effectively obtains the global variables and synchronous enable signals of the energy storage system without the central controller.The effectiveness and feasibility of this improved nonlinear SoC balancing control is verified based on the DC microgrid model built in Matlab/Simulink and controller hardware-in-the-loop platform.