Research On Control Strategy For Two-stage PV Grid-connected System

With the advancement of science and technology,the shortage of non-renewable energy and the problem of pollution have increasingly become one of the important problems to be solved by the world.In addition to studying the efficiency optimization method in the process of non-renewable energy transfer,the research of renewable energy has gradually become a hot point in energy research.Solar energy benefits from its wide distribution of resources,and stands out among a wide variety of renewable energy sources.Therefore,it is of great research value to study how to optimize solar power generation systems.This paper takes the common two-stage grid-connected power generation system in the solar power generation system as the research object,explores its main structure and control method,and improves the control method of the two-stage photovoltaic system commonly used today.This article first analyzes the classification and development of the two-stage photovoltaic system.Starting with the topology and control methods,the structural characteristics of the commonly used two-stage photovoltaic system are described,and a topology of the stacked boost converter is introduced.At the same time,it focuses on the application of model predictive control method in power electronic converter.Secondly,the mathematical model of the photovoltaic panel and the commonly used maximum power point tracking control method are constructed,and the fixed-step conductance increment method is optimized.The structural characteristics and working principle of the full-bridge LLC-Buck stacked boost converter adopted in this paper are mainly introduced.The advantages of the improved conductance increment method and the resonance characteristics of the stacked boost converter topology are verified by simulation.Then,after establishing and detailing the mathematical model and working principle of the single-phase three-level NPC inverter topology of the subsequent stage,the SVPWM modulation algorithm is analyzed and the single-phase three-level NPC inverter is inverted by using the SVPWM modulation algorithm.The control objectives such as the balance of the midpoint potential voltage are achieved.The control effect of SVPWM modulation algorithm under steady state and dynamic state is verified through simulation.Finally,the application of FCS-MPC algorithm on single-phase three-level NPC is mainly introduced.The principle and characteristics of FCS-MPC algorithm are introduced first.Then start from the leakage current problem of the two-stage photovoltaic system,and the shortcoming that the FCS-MPC algorithm cannot strictly limit the switching frequency.In addition to the neutral voltage control and dv/dt control,the common mode voltage constraint added,and a method of observing the switching frequency through a sliding window is proposed to control and optimize the switching frequency of the system,and the selection of weight factors in FCS-MPC algorithm is explained.It is verified by simulation that the improved FCS-MPC algorithm has a faster dynamic response,effectively suppresses the effect of common mode voltage and overcomes the shortcoming that the traditional FCS-MPC algorithm cannot constrain the average switching frequency of the system.In this paper,a multi-level joint experiment platform is built,and on the experiment platform,the grid connection process of the two algorithms is experimentally verified and the superiority of the improved FCS-MPC algorithm is explained.There are 82 figures,10 tables and 132 references in this thesis.