Research On Control Strategy Of A T-type Three-level Energy Storage Converter

In recent years,renewable energy power generation systems,such as solar energy and wind energy,have achieved developed rapidly,but they have the disadvantages of instability such as large intermittentness and large fluctuation,which has brought severe challenges for safe access to the grid system.In order to solve the above problems,energy storage systems emerged as the times require.For energy storage systems,the most representative control strategy is virtual synchronous generator(VSG)control,which can simulate the droop characteristics,moment of inertia and damping characteristics of synchronous generators(SG),and provide the necessary voltage amplitude and frequency support for the power grid.Compared with the traditional two-level converter,the T-type three-level converter has the advantages of small harmonic content and low power loss.Therefore,this thesis takes the T-type three-level converter as the research object,focusing on the VSG control strategy.The main research contents are as follows:Firstly,the circuit topology and working principle of the T-type three-level converter are introduced,and the control strategy of the neutral point potential balance is explained.Secondly,the control strategies of voltage-type VSG and current-type VSG are studied in detail,and the voltage loop and current loop control of the system are analyzed.The online smooth switching strategy is discussed based on two VSG control strategies.The key parameters of the VSG control strategy are also analyzed and the impact of the key VSG parameters on the converter output frequency is studied.Then,the pre-synchronization control strategy with no phase-locked loop is studied on the basis of virtual impedance power,the system phase pre-synchronization is indirectly achieved by controlling the power flowing through the virtual impedance to zero.at the same time,the bidirectional energy flow strategy of the VSG is studied.Finally,a MATLAB/Simulink simulation model and an experimental platform of the T-type three-level energy storage converter with STM32F407ZG as the control core are built to verify the correctness and feasibility of the control strategy studied in this thesis.The system can work in the off-grid mode or the on-grid mode,and can realize the smooth switching between on-grid and off-grid modes and bidirectional energy flow...