Research On Control Strategy Of Photovoltaic Grid - Connected Z - Source Inverter

With the growing environmental problems and consumed fossil energy, the use of clean energy attracts more and more attention. As a representative of clean energy, photovoltaic power generation has been developing rapidly in recent years, accordingly the study of grid inverter also entered a rapid development period. Z source inverter as a new inverter topology received widespread attention.Firstly, the current domestic and overseas development situation of photovoltaic power generation system has been introduced. On the basis of analyzing working principles and limitations of traditional voltage source and current source inverter, working principle and process of Z-source inverter have been studied. Compared with traditional inverter, Z-source inverter has solved the problem that traditional inverter can only be buck or boost inverter. Z-source inverter can realize both uplift and reduce the voltage by adjusting shoot-through duty ratio d and modulation m. At the same time, due to the introduction of Z-source network, inverter can realize the shoot-through of up and down switch tubes at the same time without introducing dead band time, which increases the reliability of the circuit. By intensive studying working principles and process of Z-source inverter, quantitative relation between variables in shoot-through state and non shoot-through state has been made clear. By contrastive analysis of constant direct modulation, maximum direct modulation and modified constant switching frequency modulation, in the case of the same inverter effect, modified constant switching frequency modulation can significantly reduce the operating frequency of the switch tubes.Secondly, Z-source inverter mathematical model has been established by state space average method and small signal perturbation method. On the basis of building mathematical models of Z-source network modeling and inverter side network respectively, the whole model of grid connected inverter has been gotten by coupling those two models. The whole grid control system has been designed based on model structure, which shows that the control system can be divided into three closed loops as grid current loop, Z-source capacitor voltage loop and PV array output voltage loop. Each PI parameter of the closed loops has been adjusted and their correctness has been verified by step disturbance experiment. Meanwhile, the simulink model of photovoltaic power generation system has been set up to analyze the properties of PV system and two MPPT control methods as P&O and incremental conductance have been analyzed. The effectiveness of the control method has been verified by MATLAB simulation.Finally, the prototype of the entire system has been designed, including the design and selection of Z-source capacitance, inductance, DC input diode and switch tube. Auxiliary circuits such as driver circuit, sampling circuit, zero capture circuit have been designed. Schematic drawing and the PCB circuit of the final Z-source inverter have been designed by Altium Designer software platform. The program based on DSP2812 has been written by CCS3.3 platform. Finally, the validity and correctness of the control method, modulation strategy and parameters design have been verified by MTALAB simulation and prototype test for getting simulation and experimental wave forms which fully meet the theoretical calculation value.