Research On Modeling And Control Strategy Of Quasi-Z Source Three-phase Photovoltaic Inverter In Weak Grid

With the depletion of fossil energy,the rise of new energy sources such as solar energy and wind energy,solar energy is inexhaustible,clean and pollution-free,so photovoltaic grid-connected inverters have received extensive attention from scholars.The distributed energy structure is accompanied by a large number of power electronic devices connected to the power grid,which causes the power grid to exhibit weak power grid characteristics;the resistance and inductance characteristics of the power grid interact with the grid-connected inverter,which may cause the system stability margin to decrease or even lose stability.The research object of this paper is the quasi-Z source three-phase photovoltaic grid-connected inverter under weak power grid,aiming at the modeling of photovoltaic quasi-Z source inverter,photovoltaic cell output power control,and grid-connected current distortion caused by grid-connected voltage harmonics.The content is mainly divided into the following parts:Firstly,the mathematical modeling and simulation model of the photovoltaic cell that converts solar energy into direct current are established,and the nonlinear characteristics of its output under the influence of temperature and light are analyzed.From the perspective of volt-second characteristic and charge conservation,the relationship between the input and output voltage of the quasi-Z source inverter topology is obtained,and the small signal model and state space model are established.Secondly,the space vector modulation(SVPWM)technology is used to realize the decoupling of the front and rear stages.The physical meaning of the sliding mode nonlinear controller is introduced.Based on the adaptive sliding mode control law,the DC side sliding mode controller of the photovoltaic quasi-Z source inverter is designed,and the existence,arrival and stability conditions that need to be satisfied are proved.Dynamic response characteristic of exponential powers,fast powers,and adaptive reaching laws.Under ideal power grid conditions,after coordinate transformation,the control analysis is simplified,and the current double closed-loop control loop is designed.Through the bode diagram of the open-loop transfer function of the control loop,it can be seen that meets the engineering design requirements.Then,on the basis of ideal grid conditions,the weak grid is simplified as impedance,and the effects of resistance and inductance on grid-connected system stability are analyzed separately.The system impedance model is established to analyze the instability principle of the system.The large amount of background harmonics of the grid voltage causes the distortion of the grid-connected current,and the grid voltage control loop is introduced into the grid-connected current double closed-loop control loop.Aiming at the problem of system instability caused by proportional feed-forward,a resonant feed-forward control strategy is proposed,which reduces the sensitivity of the control loop stability margin to the grid inductance and resistance value.Resonance feed-forward can effectively improve the stability of the system under weak grid.Finally,a simulation model of the system is built in Matlab/Simulink to verify the validity and rationality of the proposed control strategy;the hardware-in-the-loop simulation platform is used in the experiment.The feasibility of the control strategy is further verified.