Research On Photovoltaic Grid-connected System Based On Quasi-Z Source Cascaded Multilevel Inverter

With the energy crisis and environmental pollution problems become increasingly serious,solar energy stands out from many renewable energy sources because of its rich resource and no pollution to the environment.It has become the most promising new energy.In the photovoltaic power generation system,the inverter as the core device connecting the photovoltaic power supply and the power grid and its quality is related to the performance of photovoltaic power generation system and whether it can be promoted in the market The quasi-Z-source inverter has the characteristics of straight-through boost and the output voltage of cascaded multilevel inverter has ladder wave and low harmonic content.In this paper,we combine the advantages of the quasi-Z-source and cascaded multilevel inverter for the relevant research.First of all,this paper gives an overview of the existing photovoltaic grid-connected inverter.After analyzing the advantages and disadvantages of the traditional inverter,the final choice of quasi-Z-source inverter for the boost device is made.After analyzing the principle of Z-source inverter,a unified state space equation and small signal model are established,and the parameters are designed based on it.After analyzing the advantages and disadvantages of the traditional multilevel technology,the cascaded H-bridge multilevel technology is finally selected.In the process of inverting,the through-boost of the quasi-Z-source is considered,and the advantages of the two methods are used to determine the object of this paper which namely quasi-Z-source cascade multi-level inverter.Secondly,based on the quasi-Z-source cascaded multilevel photovoltaic grid-connected system,and after the research of the modulation method on the inverter,the improved carrier phase shift technology is proposed.This technology combines the through duty ratio in the traditional horizontal carrier phase shifting technology,which makes the system not only get the effect of contravariance but also the boost.In the study of grid-connected control strategy,the photovoltaic cell voltage loop,MPPT control,and grid-connected current loop are analyzed emphatically.After the relevant parameter design and controller selection,the master and slave control strategy which is suitable for the system is determined,and the grid connected system is realized.Finally,the simulation model of single-phase and three-phase system are built on MATLAB / SIMULINK simulation platform,and the simulation results are validated and analyzed.The results verify the correctness and feasibility of the control method used in the system.