| In recent years,the renewable energy power generation technologies of the wind power and photovoltaics have developed rapidly.However,the inherent randomness and intermittent nature of renewable energy power generation challenges the ability of the power grid to accept renewable energy power generation.Because most renewable energy sources have a DC output form and easily form a local DC microgrid and then connect with a large grid,experts pay attention and become a research hotspot.The energy storage unit is necessary for most DC microgrids and plays an important role in suppressing fluctuations in intermittent energy sources and maintaining energy balance,so as to improve the stability and scheduling flexibility of distributed generation and grid operation,and to maximize the efficiency.The research focus and difficulty of distributed energy storage in DC microgrids are DC/DC bidirectional converters of high-voltage and high-power and control algorithms suitable for distributed energy storage characteristics,as well as scheduling strategies for the energy storage system to access the DC bus,maintaining the DC bus voltage and power balance.In order to study the energy storage system,this paper studies the topology of bi-directional DC/DC converters,the l oad current distribution of energy storage systems,and the stability of energy storage systems.The specific research contents are as follows:(1)The DC/DC bidirectional converter is a bridge connecting the DC bus and the energy storage medium.The bidirectional flow path of the energy needs to have a very high dynamic response speed.Therefore,this paper studies a novel three-level DC/DC bi-directional converter topology with easy input and output modular operation.On the basis of traditional switched capacitor three-level DC/DC bidirectional converters,CLLC three-level DC/DC bidirectional converters are proposed to improve the voltage gain level and efficiency;In order to meet the requirements of high-voltage,high-power energy storage,the traditional flying capacitor type three-level DC/DC bidirectional converter is deeply analyzed,and its modular combination method is studied.(2)In order to meet the requirements of high-voltage and high-power energy storage,this paper focuses on the modular multiphase multi-level DC/DC bidirectional converter composed of flying capacitor type three-level converters and proposes modulation strategies and control schemes.To solve the module pressure and current sharing problems.The serial and parallel structu re between the modules is flexible,and the module combination mode is selected according to the voltage level and the capacity level.The parallel use of the modules can increase the current flowing through the converter and the use of the modules in seri es can increase the voltage level of the converter.(3)Because the energy storage medium is inconsistent with state-of-charge(SOC)due to process and external environmental factors,it exhibits randomness.This paper proposes an improved SOC exponential droop control strategy aiming at the randomness and slow change of the state of charge of the energy storage unit in the distributed energy storage system.The power function of SOC can improve the SOC resolution,the droop curve of SOC index function is s mooth and there is no discontinuity,and the improved SOC exponential droop control is conducive to finding the optimal droop curve to make the system quickly converge to the equilibrium state.Since droop control of the SOC index function and secondary recovery control of the bus voltage are based on low-bandwidth communication,it is necessary to study the solution when the communication line fails.The simulation model and experimental platform were used to verify the validity and correctness of the proposed method.(4)The distributed energy storage unit plays a key role in maintaining the stability of the DC bus voltage and the power balance.If there is no difference adjustment,the service life of the energy storage unit will be shortened.Minimizing the number of distributed energy storage units accessing bus bars and increasing the service life of energy storage units requires hierarchically coordinated control of each unit in the DC microgrid,focusing on the analysis of the main tasks of the energy storage units in hierarchical coordination and control.issues that need resolving. |
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