Research On Control Strategy Of Two-stage Photovoltaic Grid-connected System

In recent years,the energy crisis has become increasingly serious.Based on the concepts of environmental protection and sustainable development,photovoltaic grid-connected power generation technology has ushered in development opportunities.With the widespread use of two-stage photovoltaic grid-connected inverters,the study of their control strategies has received widespread attention from scholars at home and abroad.This paper takes the two-stage three-phase photovoltaic grid-connected inverter as the research object.First,for the pre-stage DC / DC link,an improved maximum power point tracking(MPPT)algorithm is designed to realize the maximum power point tracking of the photovoltaic cell.Secondly,for the DC / AC inverter link,in the normal power grid environment,the inverter control strategy is designed.Under weak power grid conditions,the phase-locked loop will affect the stability of the system.The grid inverter control method improves system stability.The main work of the full text can be summarized as follows:First,this thesis establishes a mathematical model of photovoltaic cells suitable for practical engineering,and analyzes its output characteristics in conjunction with the mathematical model.It compares several commonly used MPPT algorithms.Combining the output characteristics of photovoltaic cells,a variable step size fuzzy MPPT algorithm is proposed.Compared with the simulation of the variable step size conductance increment method,it is verified that the proposed method has the advantages of small steady-state output power fluctuation and good dynamic and static performance.Secondly,the three-phase LCL photovoltaic grid-connected inverter is modeled and analyzed.The advantages and disadvantages of Proportional-Resonant Controller(PR)control strategy and Quasi-Proportional Resonance Control(QPR)control strategy are compared.In order to improve the quasi-PR control strategy,a fuzzy control strategy is introduced.Based on the theoretical analysis and control requirements,a fuzzy controller is designed,and the control parameters of the PR controller are fine-tuned in real time.The Matlab/Simulink simulation verifies that the fuzzy quasi-PR control has the advantages of low current harmonic content and high steady-state accuracy.Finally,the harmonic linearization modeling method is used to model the impedance of the three-phase LCL grid-connected inverter.By combining the Bode diagram and the impedance criterion,the effect of the phase-locked loop is considered under weak grid conditions Stability is analyzed.A method of cascading low-pass filters in series before the phase-locked loop is proposed to reduce the influence of the phase-locked loop,thereby improving the stability of the grid-connected system.The effectiveness of the proposed method is verified by Matlab / Simulink simulation.