Research On Control Strategy Of Energy-Stored Quasi-Z-Source Photovoltaic Grid-Connected Inverter System

With traditional fossil energy drying up day by day,solar energy as a clean energy will be one of the main sources of energy in the future,and photovoltaic power generation is an important way of solar energy utilization.Inverter is an important component connecting photovoltaic power generation system and power grid.The comprehensive performance and economic cost of inverter have great significance for photovoltaic power generation system.The energy storage quasi-z-source photovoltaic grid connected invertler system is taken as the research object.Aiming at the insufficiencies of the existing photovoltaic grid-connected control strategy,this thesis proposes an improved nonsingular fast terminal sliding mode control strategy,which can further improve the quality of grid-connected current,the speed of dynamic response,and the anti-interference ability of the system.The power feed-forward control was studied,and its output and voltage outer loop control were combined to ensure the coordinated operation of all power sources of the system.The main work of this thesis is summarized as follows:(1)The design and research of the overall scheme of the energy storage quasi-Z-source photovoltaic grid-connected inverter system are carried out.By studying of two typical working states of energy-stored quasi-Z-source network,i.e.shoot-through state and non-shoot-through state,the working principle is described in detail.The mathematical model of energy-stored quasi-Z-source network is established.Combined with the LCL three-phase grid-connected inverter mathematical model,the whole system is unified mathematical modeling,which lays the foundation for the subsequent controller design.The component parameters of LCL filter are calculated and selected.The characteristics of LCL filter in the system are analyzed,and the active damping method with capacitor current proportional feed-back is used as the resonance suppression method.(2)The overall control strategy of the system was analyzed and studied.The control of the system mainly includes four aspects:MPPT control,voltage outer ring control,power feed-forward control,current inner ring control.According to the mathematical model of the photovoltaic cell,the simulation model was constracted in MATLAB/Simulink.The output characteristics of the photovoltaic cell were analyzed.The MPPT control algorithm was analyzed and studied.The variable step-length conductivity increment method was used as the MPPT algorithm of the system,and the simulation verification was carried out.In view of the difference between PWM modulation mode and traditional inverter modulation mode in z-source inverter system,various modulation modes are analyzed and compared.The six-segment SVPWM modulation was adopted as the modulation mode of the system.Designed the energy storage unit in the system.(3)The thesis focuses on the grid-connected control of the system.To solve the problem that the dynamic response and anti-interference of the existing grid-connected inverter control mode are not ideal,the sliding mode control was adopted as the system control strategy.An improved nonsingular fast terminal sliding mode control strategy was proposed,in which the saturation function was introduced to replace the symbol function in the general sliding mode control to improve the chattering.The design process of voltage outer loop and current inner loop control for energy-stored quasi-Z-source system was given.On this basis,according to the situation that there are many power sources in the system and the general voltage outer loop is difficult to meet the control requirements,the power feed-forward control is studied to ensure the power balance of the system.The energy management mode of the system is designed and its output is used as power feed-forward after decoupled calculation.combined with the output of voltage outer loop as the input of current inner loop,so that all power sources of the system can operate coordinately.MATLAB/Simulink was used to simulate the control strategy of the system.Through comparative analysis,it is verified that the proposed power feed-forward control can make each power source operate coordinately and stably;the proposed grid-connected control strategy has better dynamic response characteristics and anti-interference ability,and has certain advantages.(4)Designed and produced a 4kW prototype and carry out experiments.The parameters of the main circuit are calculated and selected.TMS320F28335 was used as the main control chip,and the control of the main circuit was realized with the related control circuit.Designed and wrote the related software system,including the main program and interrupt program.Experiments were carried out on the prototype to verify the feasibility of the control strategy proposed by the system.