Research On Modular Multilevel Converter With Energy Storage System And Its Control Technology

Under the background of "carbon neutral" goal,more and more renewable energy is connected to the grid,which brings power fluctuation,lower inertia and damping loss to the grid.Modular multilevel converter(MMC)is widely used in medium and high voltage applications because it has lower harmonic and easy to expand the voltage level.On this basis,energy storage system can be added to suppress the power fluctuations effectively.What’s more,combined with the virtual synchronous machine control strategy,the output inertia and damping can be improved.However,the traditional MMC with centralized or distributed energy storage system show the disadvantages of inflexible energy storage power and complex system control.In this thesis,MMC with partial energy storage system(MMC-PESS)is proposed and the control strategy are developed.Firstly,the topology,operation principle and modulation method of MMC-PESS are analyzed and the power flow is summarized.Secondly,On the basis of the mathematical model of MMC-PESS,the relationship between the energy storage ratio and energy storage power is explored both in the rectifier and inverter operation.Then,the capacitor voltage balance and circulating current suppression strategies are given.Based on them,the virtual synchronous generator and virtual synchronous motor control strategy are proposed according to the rectifier or inverter operation,so that the inertia and damping of the system can be improved to enhance the stability of the grid.Aiming at the problem of the different state of charge(SOC)of energy storage sub modules,the SOC balance control strategy among phases,upper and lower arms and ESMs in the same arm are developed.Nextly,the simulation model of MMC-PESS is built in MATLAB/Simulink environment to verify the above control strategies and the relationship between energy storage ratio and the energy storage power.Finally,a three-phase five-level down-scaled MMC-PESS prototype is constructed.The device selection and design of the control system are analyzed in detail.And the related experiments are carried out on the prototype.The simulation and experimental results show that the proposed MMC-PESS and the corresponding control strategy are effective.In addition,MMC-PESS can effectively achieve peak shaving and valley filling in the grid.At the same time,the energy storage system can also help to the frequency regulation,which shows a broad prospect in the future energy Internet.