Research On Energy Coordinated Control Of Modular Three-port DC Converter

As a new type of multi-functional transformer for connecting medium and low voltage DC bus,the modular three-port DC converter is widely used in DC microgrid,new energy transportation,large-scale energy storage system,and naval power supply,providing an effective way to achieve the goal of"Double Carbon",thanks to the advantages of multi-ports,easy access and flexible energy flow.The modular three-port DC converter comprises a modular multilevel converter,a Dual Active Bridge converter(DAB),and a hybrid energy storage unit,each with multiple unit branches.According to how the load is operating,each unit branch transmits forward or backward energy.Each unit in the reverse transmission works in concert to control the direction of energy flow before dispersing the energy.Energy coordination control of the entire converter is more difficult due to the diversity of energy flow modes,and research on the issue of power distribution in the reverse transmission of energy is less extensive for the current modular three-port DC converter.Therefore,it is of great practical significance to study the energy coordination control strategy of modular three-port DC converter for its practical engineering application.(1)This thesis takes the modular three-port DC converter as the research object,describes the topology of the modular three-port DC converter,the control method of each unit,and the current status of domestic and international research on energy coordination control;gives the overall topology of the modular three-port DC converter,and specifically introduces the structure and working principle of the three component units;on this basis,analyzes the six energy flow modes of the modular three-port DC converter under the condition of the submodule capacitance voltage value interval,and divides each unit controller into three levels according to the hierarchy.(2)The problem of being unable to track the new state reference value when the DAB converter in a modular three-port DC converter is subjected to external disturbance is addressed.In this thesis,the DAB converter’s Euler-Lagrange(E-L)model is established based on the Passivity-Based Control(PBC),and the passive analysis is performed.After proving that the DAB converter is strictly passive,a passive controller for the DAB converter is designed to improve the converter’s energy dissipation rate and ensure the system’s global stability.To compensate for the loss of fast tracking performance during the design of the passive controller,the transformer secondary output current in the PBC is converted to the converter load output current to achieve the converter output voltage error close to 0.The Lyapunov function V is defined and derived with the Backstepping Control(BSC).The error energy storage of the associative PBC the derivative equation of the Lyapunov function H_e is obtained,and the linear quantity k is introduced to design a passive backstepping controller with the goal of shifting compared to the DAB converter’s.The feasibility of the passive backstepping control of the DAB converter is verified by Matlab/Simulink simulation software.(3)The problem of how to distribute the reverse transfer energy of the modular three-port DC converter in each subunit is considered.Based on double closed-loop control of the half-bridge submodule,passive backstepping control of the DAB converter,and fuzzy-droop control of the hybrid energy storage unit,a total-division control method is used to coordinate the control of the energy according to the forward and reverse transfer mode of the converter.The total control system in reverse transmission is based on the total value of reverse transmission energy and the capacity of submodule unit and hybrid energy storage unit,which are distributed to medium DC bus and hybrid energy storage unit respectively according to dynamic proportional values,and the energy in hybrid energy storage unit is further filtered to get the respective energy values of super capacitor and battery.The simulation model of modular three-port DC converter is built by Matlab/Simulink,and the typical modes of forward and reverse energy flow are simulated.The simulation results show that the energy in the reverse transfer mode can be distributed and stored between the medium DC bus and the hybrid energy storage unit,and the transfer efficiency is 89.2%,which proves the effectiveness of the proposed energy coordination control strategy.