Application Of Z-source Inverter In Photovoltaic Power Generation System

Due to the shortage of traditional fossil energy and the serious environmental pollution caused by the use of fossil energy,countries all over the world are constantly increasing the development and utilization of new energy.Solar energy is widely used all over the world because of its clean and green characteristics.As the main application occasion of solar power generation,photovoltaic grid-connected power generation has attracted a lot of attention and research from scholars at home and abroad.Because of its unique characteristics,Z-source inverters make up for the disadvantages of traditional grid-connected inverters,such as poor reliability,high distortion rate of output waveform and low efficiency,and have been applied in the field of photovoltaic power generation in recent years.Quasi-Z source inverter is a topology derived from the traditional Z source.It inherits all the advantages of the traditional Z source and makes the output current continuous.Therefore,based on the Quasi-Z source network as the front step-up structure,the photovoltaic grid-connected inverters are studied in this paper.Firstly,this paper analyzing the basic principle of Quasi-Z source inverters.The mathematical model of Quasi-Z source inverters is established by using state space averaging method.Based on zero-pole diagram and Bode diagram,analyzing the influence of parameters on the dynamic performance of the system.The calculation formulas of system parameters such as Z-source capacitance and inductance are deduced,which provide a basis for subsequent simulation analysis.The SPWM modulation strategies of several common Z-source inverters are compared,and the different configurations of two shoot-through zero vectors based on space vector modulation(SVPWM)strategy and their effects on the overall performance of the system are emphatically studied.Based on the vector control strategy of grid voltage orientation in rotating coordinate system,the latter grid side can realize the grid connection with unit power factor and ensure the stability of DC link voltage by using double closed-loop control structure.The control parameters of voltage outer loop and current inner loop are tuned by using classical control theory.The shoot-through zero vector loop is introduced to adjust the duty cycle.In order to obtain better dynamic and static characteristics of grid connection,comparing and analyzing two control strategies of maximum power tracking(MPPT)with variable step size and fixed step size.Finally,a simulation model of three-phase photovoltaic grid-connected system based on Quasi-Z source is established in Simulink of MATLAB.The grid-connected control strategy and two MPPT control algorithms are simulated.The simulation results show that the two-stage coordinated control of the former step-up and the latter step-inverters is good.The improved variable step-size MPPT control has faster response and smaller steady-state power oscillation than the fixed step-size scheme.From the performance point of view,the improved variable step-size scheme is superior to the traditional variable step-size scheme,which verifies the correctness of theoretical analysis and the feasibility of grid-connected control strategy.