Key Technologies Of Integrated Hybrid Energy Storage System With MMC

With the rapid development of distributed generation technology,a large number of distributed power sources based on wind energy and solar energy are connected to the grid,which causes a lot of power quality problems.Energy storage is an effective way to improve the power quality.However,single kind of energy storage device cannot meet multiple requirements of the grid.In addition,energy storage systems are usually based on two-level converters,which limits the application in high voltage and power condition.Therefore,after researching hybrid energy storage technology and MMC-based energy storage system,this paper proposes a Modular Multilevel Converter-Hybrid Energy Storage System(MMC-HESS)and studies its key technologies.Firstly,the topology of MMC-HESS is introduced.It is composed of lithium battery energy storage system and supercapacitor energy storage system through MMC topology.Furthermore,the basic principle of MMC in supercapacitor energy storage system and the basic principle of bidirectional DC/DC converter in lithium battery energy storage system are introduced.In addition,the equivalent circuits of lithium battery and supercapacitor are selected,as well as their SOC calculation methods.Secondly,MMC-HESS system control strategy is studied,including the control strategy of MMC and bidirectional DC/DC converter.The control strategy of MMC includes a current inner loop decoupling control strategy,a power outer loop control strategy,and a sub-module voltage equalization control strategy.The control strategy of the bidirectional DC/DC converter includes an inner loop current control strategy and an outer power loop control strategy based on SOC equalization.The control strategy of MMC and bidirectional DC/DC converter are verified by Matlab/Simulink simulation.Thirdly,MMC-HESS energy management strategy is studied.The power distribution strategy divides power fluctuations into high-frequency components and low-frequency components,respectively as the supercapacitor energy storage system and the lithium battery energy storage system charging and discharging power command.The power regulation strategy independently adjusts the power command according to the SOC of the two energy storage components in order to prevent overcharge and overdischarge of the energy storage component.According to the charge and discharge state of the lithium battery energy storage system and the supercapacitor energy storage system and the power adjustment amount,the total power adjustment amount of MMC-HESS is coordinated among the two energy storage systems.The power limiting control limits the charging and discharging power of the supercapacitor energy storage system and the lithium battery energy storage system in a certain range,to ensure the safe operation of the MMC-HESS.The effectiveness and feasibility of the MMC-HESS energy management strategy is verified by Matlab/Simulink simulation.Finally,based on the dSPACE,a scaled-down three phases MMC-HESS experimental platform with 12 sub-modules is built.Experimental verification of MMC control strategy,bidirectional DC/DC converter control strategy and partial energy management strategy is completed under discharge and charge state of MMC-HESS.The experimental results are consistent with the theoretical research and simulation results,which proves the validity and feasibility of the proposed MMC-HESS and its control strategy.