Research On Control Strategy Of Photovoltaic Grid-connected System Based On Multi-peak MPPT

As an effective way to develop and utilize solar energy,photovoltaic power generation has been vigorously developed.However,in practical engineering,the variable and uneven irradiance of light source will cause the mismatch of photovoltaic array,and make its output characteristics present multi-peak phenomenon,which brings many problems to the stable output of array and grid connection.In this paper,the non-isolated two-stage three-phase photovoltaic grid-connected power generation system is taken as the object of study,focusing on the research and analysis of the maximum power point tracking of photovoltaic arrays under shadow conditions,suppression measures of common-mode current in non-isolated photovoltaic grid-connected system,and the control strategy of grid-connected current.The simulation and experimental testing system are carried out.Maximization of photovoltaic array output power and optimization of power quality in grid-connected system are achieved.The main work of the paper is as follows:(1)After analyzing and comparing the installation structure of photovoltaic array,DC-DC topology and DC-AC topology,the overall design scheme of the main circuit topology of the non-isolated two-stage three-phase photovoltaic grid-connected power generation system is determined,and the system-related performance indicators are clarified.(2)The output characteristics of photovoltaic cells are simulated in MATLAB/Simulink,based on this,the MPPT tracking principle and control method are analyzed.The mismatch phenomenon of photovoltaic arrays under shadow conditions is analyzed,and the multi-peak output characteristics of the array are simulated and analyzed.Aiming at the problem that traditional MPPT algorithm may fall into local extremum when tracking multi-peak global maximum power point,a compound MPPT algorithm based on improvedParticle Swarm Optimization(PSO)& improved Perturbation and Observation is adopted.The improved PSO algorithm is used to track the global maximum power point in the early stage of tracking,and when tracking tends to steady state the improved P&O is used.Finally,the advantages of the compound algorithm are verified by simulation.(3)The generation principle and suppression conditions of common-mode current in non-isolated photovoltaic grid-connected system are analyzed.A new FB7 grid-connected inverter topology is used to solve the problem of large common mode current in traditional H6 grid-connected inverter topology,and the principle of suppressing common mode current in the new FB7 combined with NEW PWM modulation mode is analyzed.Finally,a simulation model is built to verified four schemes of traditional H6 combined with three modulation mode such as SVPWM,DPWM,AZSPWM and new FB7 combined with NEW PWM modulation mode.The simulation results show the superiority of the new FB7 grid-connected inverter topology combined with NEW PWM modulation mode in suppressing common-mode current.(4)The principle and relationship of PI,PR and QPR control algorithms are analyzed.Aiming at the problem that tedious coordinate transformation and decoupling calculation of PI control algorithm,QPR control algorithm can not suppress DC component,PI+QPR compound control algorithm in two-phase static coordinate system is adopted to control grid-connected current.Finally,the simulation models are built to simulate PI control algorithm,QPR control algorithm and PI+QPR compound control algorithm respectively,the performances of three schemes for no static tracking of AC signal and DC component suppression are compared,the validity and correctness of PI+QPR compound control algorithm are verified.A 4.5kW experimental testing system is built,of which the corresponding hardware design and software programming are carried out with the main control chip—TMS320F28335.The experimental results are analyzed to verify the rationality of the system and the effectiveness of the control algorithm.