在含多个分布式储能单元(distributed energy storage units,DESUs)的孤岛直流微电网中,针对初始荷电状态(state of charge,SOC)不一致、线路电阻不匹配导致DESUs的SOC不均衡以及直流母线电压偏移的问题,提出一种改进SOC均衡控制策略.首先,将DESUs的SOC与指数函数、反正切函数相结合,自适应调整P-U下垂系数,保证系统稳定运行并提高DESUs的SOC均衡速度;其次,基于动态一致性算法设计单一补偿项,恢复母线电压的同时消除线路电阻不匹配引起的功率分配偏差;然后,建立系统特征方程分析系统稳定性,为所提策略提供理论支撑;最后,基于Matlab/Simulink软件和RT-LAB实时仿真平台搭建含多个DESUs的直流微电网模型,在不同运行工况下验证了所提SOC均衡控制策略的有效性.
SOC balancing control strategy for distributed multiple energy storage in isolated DC microgrid
With the growing popularity of renewable energy,DC microgrids have garnered great academic interest from both home and abroad due to their effectiveness in tackling such issues as reactive power and harmonics.However,intermittent and fluctuating characteristics exhibited by renewable energy sources like wind and solar power may cause substantial fluctuations in system power and bus voltage,thereby posing serious challenges to the safe and stable operation of DC microgrids.To address the problem,multiple distributed energy storage units(DESUs)have emerged as prominent control strategies.Nevertheless,when multiple distributed energy storage systems operate in parallel,discrepancies in initial state of charge(SOC)among DESUs along with mismatched line resistances may induce voltage deviations on the DC bus and uneven charge distribution among units,leading to excessive charging/discharging for certain storage units that ultimately shortens their lifespan while adversely affecting the overall safety and stability of the system.We propose an improved SOC balancing control strategy to address the issues of initial SOC inconsistency,mismatched line resistance leading to SOC imbalance,and DC bus voltage offset in DESUs.First,the SOC of distributed energy storage units is combined with exponential and inverse tangent functions to adaptively adjust the P-U droop coefficient,ensuring stable operation of the system and improving the speed of SOC balancing for distributed energy storage units.Second,a state factor related to converter output voltage,power and droop coefficient information is designed.The average values are estimated based on a dynamic consensus algorithm and transmitted between adjacent converters through low-bandwidth communication lines.A PI controller compensates for the generated voltage offset to achieve bus voltage restoration and precise power allocation.Then,a system characteristic equation is built to analyze system stability and provide theoretical support for the proposed strategy.Finally,a DC microgrid model containing multiple distributed energy storage units is built by using Matlab/Simulink software and RT-LAB real-time simulation platform to verify the effectiveness of the proposed SOC balancing control strategy under different operating conditions.Our main conclusions are:① Compared to the exponential droop control,our strategy introduces an arctangent function to adaptively adjust the P-U droop coefficient based on SOC changes.Thus,it ensures the droop coefficient operates within allowable limits and guarantees system stability.Moreover,it improves the SOC balancing speed of DESUs and achieves fast SOC balancing.② By considering the influence of droop coefficients and line resistance,a single compensation term is designed to solve power distribution deviation caused by mismatched line resistances in DESUs,and maintain bus voltage stability.Compared to traditional strategies,our solution only requires one compensation loop to achieve both bus voltage restoration and power distribution deviation objectives,thereby reducing overall system design complexity.③ Our strategy is not affected by the fluctuations in photovoltaic output power,load variations or DESU switching operations,demonstrating its effectiveness and feasibility.
DC microgriddistributed secondary controlSOC balancing controlvoltage compensationpower distribution
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