Research On Control Strategy And Stability Analysis Of Cascaded Three-level Buck-boost Converters

Under the background of "carbon peak and carbon neutral",people’s demand for green energy is increasing day by day.The nation calls for the vigorous development of new energy,solar energy,wind energy and other sustainable development of energy into electricity to alleviate the consumption of coal and other primary energy.However,these sustainable energy conversion has a wide range of fluctuations,cannot meet people’s increasing demand for power quality of electricity.In order to obtain stable and high-quality electric power,as an important access device of new energy grid,high-efficiency and high-reliability power electronics equipments have received more and more attention.As an important part of power electronics,DC/DC converter is widely used in renewable energy access because of its relatively mature technology.In DC/DC converter,the cascaded three-level Buck-Boost converter has the advantages of low switching stress and low switching loss,and is suitable for grid-connected photovoltaic and energy storage devices.In this paper,the working principle,controller design and large signal stability analysis of cascaded three-level buck-boost converter are studied.The main contents include the following aspects:(1)The working principle of the cascaded three-level buck-boost converter under three operating modes of buck,boost and buck-boost is analyzed.Combined with the concept of state average,the mathematical model of the system is obtained,which can be used for subsequent control.(2)In order to improve the dynamic response performance of the topology,a passive control strategy for cascaded three-level buck-boost converter based on PCHD model is proposed.Combined with the form of PCHD model,the state space equation of the cascaded three-level buck-boost converter is rewritten and the energy storage function is given.Verify the passivity of the converter,then use the IDA-PBC method to solve the passive control law of the converter and design the passive controller of the system.Finally,a cascaded three-level buck-boost converter system based on double closed-loop and passive control based on PSIM is built respectively.The simulation results of the two are compared and analyzed to verify the effectiveness and superiority of passive control.(3)Since this topology works in a wide range of fluctuations,its steady-state operating point will change abruptly when subjected to large external disturbances.It is necessary to analyze the large-signal stability of the cascaded three-level Buck-Boost converter with constant power load.Based on the T-S fuzzy model,the full-order model of the system with constant power load through the LC filter is obtained,and whether the LMI equation has a solution is used as the basis for judging whether the system is stable,the influence of the system main circuit parameters and controller coefficients on the critical power is analyzed,and the estimated The asymptotic stability domains of the system under different conditions are obtained,and the selection boundaries of the main circuit parameters and control parameters are obtained.(4)An experimental prototype of a cascaded three-level buck-boost converter is built.The design steps of main circuit,driving circuit and protection circuit and the selection of main circuit parameters are designed,and the control system of the prototype is designed with DSP as the microcontroller.The experimental results show the correctness of the research results in this paper.