Research On Control Of Quasi-Z Source Inverter In Photovoltaic Grid-connected

The construction of new power system dominated by new energy is an important way to achieve “carbon peak,carbon neutralization”.Because the solar energy in the new energy is inexhaustible,it has the advantages of cleanness,environmental protection,no pollution,flexible configuration,and low maintenance costs,Photovoltaic(PV)power generation has been widely concerned and applied.PV grid-connected inverter is the key link to realize energy conversion and efficient use of PV power generation system,which is related to the safety,stability and efficient operation of the whole system.By adding impedance network in the front end of the inverter bridge and subtly introducing through state,Z-source inverter can achieve the purpose of both voltage boost and voltage drop without taking additional means to modify the switching signal.The improved quasi-Z-source inverter inherits all the advantages of the traditional Z-source inverter and has more advantages at the same time.Therefore,this thesis takes the photovoltaic grid-connected system of quasi-Z-source inverter as the research object and uses the special attributes of quasi-Z-source inverter to solve the problems existing in photovoltaic grid-connected control system.(1)The working principle and circuit of traditional Z-source inverter,quasi-Z-source inverter and switching inductor quasi-Z-source inverter in through and non-through state are analyzed.On this basis,according to the topological structure of each Z-source inverter,mathematical models are established for each impedance network component and power switch tube,and the analytical expressions of the mean value and effective value of each component voltage are listed.The relationship curves between each voltage and duty ratio are analyzed,providing theoretical basis for the parameter setting of impedance source network,the design of system controller and the design for unbalanced grid connection current/power fluctuations coordinated controller.(2)In order to solve the problem of unsatisfactory grid voltage caused by illumination intensity and external disturbance changes,an integral compensation sliding mode control strategy containing state variables is adopted for the outer loop of capacitor voltage of Z-source network in grid-connected system,so as to ensure the relative stability of capacitor voltage,no over and no loss of truth.An improved switching function is used to design a sliding mode controller for the grid-connected current inner loop,which can reduce chattering and improve the grid-connected effect.The simulation model is built on the MATLAB/Simulink platform.The simulation results show that the control strategy enhances the global robustness of the gridconnected system and can suppress the harmonic signal of the grid voltage,which improves the dynamic response speed and the grid-connected power quality of the system.(3)Aiming at the problem of how to eliminate negative sequence current,active power fluctuation and reactive power fluctuation and how to eliminate both active power fluctuation and reactive power fluctuation when the voltage is unbalanced,the system is modeled and the causes of grid-connected current distortion and output power fluctuation are analyzed respectively under the current balance and constant power control mode,and a reduced order resonance controller is proposed.Harmonic component and power fluctuation in gridconnected current are adjusted by adjusting coefficient to realize coordinated control of eliminating power fluctuation and negative sequence grid-connected current.The system simulation model is built on MATLAB/Simulink platform to verify the feasibility of the control strategy.