Research On Control Strategy Of Hybrid Sub-Module Modular Multilevel Energy Storage Converter

With the development of renewable energy generation technology,the penetration rate of renewable energy is increasing.However,due to the impact of climate change,day night alternation and other random factors,renewable energy has intermittence and volatility,and grid integration of renewable energy will affect the stable operation of the grid.In the process of studying how to increase the proportion of renewable energy generation,solve the problem of consumption and optimize the energy structure,large capacity energy storage is proposed as a feasible solution.High capacity energy storage puts forward higher requirements for energy storage converter.Hybrid sub-module modular multilevel converter(HMMC)has the advantages of high power and voltage level,and can block DC fault.It is suitable for the topology of high capacity energy storage converter.In this paper,the control strategy of HMMC energy storage converter is studied in the case of flexible DC transmission with large-scale renewable energy.The main research contents are as follows:1.The working principle and operation state of HMMC energy storage converter are analyzed.On this basis,the topology,mathematical model and modulation strategy of the converter are studied,which provides the theoretical basis for the control strategy research of HMMC energy storage converter.2.The control strategy of HMMC energy storage converter under normal operation is studied.The Grid-Connected control strategy and the state of charge(SOC)balance control strategy are studied respectively.The Grid-Connected control strategy includes the control strategies of AC side,DC side and battery side.For the AC side,the AC active power and reactive power control strategies are obtained based on the Grid-Connected model;for the DC side,the mechanism of DC circulation is analyzed,the transfer function model of DC power controlled by DC circulation is established,and the DC power control strategy is obtained;for the battery side,the dynamic mathematical model of Buck-Boost is established by using the small signal analysis method,and the sub-module average voltage control is proposed which realizes the balance of the capacitance voltage of the sub-module,and filters out the basic frequency and the second frequency components in the battery current.For the SOC balance control strategy of batteries,based on the three-layer SOC balance methods,the SOC balance control between phase,SOC balance between upper and lower bridge arms,and SOC balance control in arm are studied respectively.3.The control strategy of HMMC energy storage converter in DC fault state is studied,the various synthesis methods of phase voltage are analyzed,and the traditional zero voltage fault ride-through control strategy is studied.Aiming at its shortcomings,an improved zero voltage fault ride-through control strategy is proposed,which is realized by using the state switching of full bridge sub-module,so that the system can run continuously under DC fault state.In addition,based on the characteristics of DC fault,the methods of SOC balance between phases,SOC balance between upper and lower bridge arms are improved,and the SOC balance control is studied again.4.Aiming at the control strategy mentioned above,a simulation model is built on MATLAB/Simulink platform,and the simulation results verify the correctness and effectiveness of the above control strategy.