Research On Control Strategy Of Energy Storage Device Based On Modular Multilevel Converter

Energy storage device based on Modular Multilevel Converter(MMC)is a type of bidirectional power device,which treats MMC as power interface and batteries as energy storage device.Energy storage device based on MMC can absorb electricity from or release electricity to the power grid to relieve power fluctuations in renewable energy system,improving the stability of power system.Compared to traditional energy storage device,the distributed energy storage is adopted which connects batteries to the sub module via energy storage converter,lowing the threshold voltage level and series-parallel complexity of batteries and improving the stability of energy storage system.Energy storage device based on MMC has a promising future in the application of renewable energy power generation and locomotive traction.We do research on control strategies of grid-connected via energy storage device based on MMC,energy storage converter,SOC equilibrium between different batteries and voltage stability of sub module in this thesis.The structure of sub module,main circuit and operating mechanism are researched,and the current control model of MMC is established.The current controller of grid-connected MMC designed in dq rotating coordinate with the target of stabilizing current to the gird,which realizes the current control of MMC.The control strategy of MMC current controller is validated in Matlab/Simulink.The structure and working mechansism of bidirectional energy storage converter is researched,and the current control model is established based on the average state model of bidirectional energy storage converter.The current controller of bidirectional energy storage converter is designed with the target of stabilizing charging and discharging current.The control strategy of bidirectional energy storage converter is validated in Matlab/Simulink.The SOC equilibrium control between different batteries is proposed,and the SOC equilibrium control strategy between phases,arms and sub modules is established based on the SOC disequilibrium mechanism.The proposed SOC hierarchical closed-loop control strategy is validated in Matlab/Simulink.The stability of sub module voltage is summarized,and the control strategy to relieve fluctuations of avergae phase vlotage,average arms voltage and sub module voltage is establishd based on the voltage fluctuation mechansism.The sub module voltage hierarchical closed loop control strategy is implemented and validated in Matlab/Simulink.Based on the control strategies proposed in the thesis,the simulation of active power fluctuation suppression between AC/DC power grid interconnection validates the effectiveness on the modular multilevel energy storage device.