Research On MPPT And Grid Connection Control Strategy Of Photovoltaic Power Generation System Considering The Influence Of Large Disturbances

In recent years,the grid-connected scale of photovoltaic power generation has increased year by year.Due to the randomness,intermittence and volatility of the output power of photovoltaic systems,the power system is increasingly impacted by the output power of photovoltaic systems.When the temperature or light intensity changes suddenly,the power generation efficiency and stability of the photovoltaic power generation system will be reduced.The sudden increase of the output power of the photovoltaic system and the disturbance of the parasitic capacitance circulation of multiple inverters will lead to the over-voltage of the DC bus of the inverter,so that the THD value of the output current increases sharply,which seriously affects the quality of the output power of the photovoltaic system.In order to improve the power generation efficiency and output power quality of photovoltaic power generation system under large disturbance,the maximum power point tracking technology and grid-connected control strategy of photovoltaic power generation system are studied in this thesis.Firstly,in order to obtain the law that the output characteristics of photovoltaic power generation system are affected by the environment,the influence of temperature and light intensity on the output characteristics of photovoltaic cells is simulated by MATLAB /Simulink platform.After analyzing the peak power and maximum power point voltage of the P-U curve at different temperatures and light intensities,the law that the output characteristics of photovoltaic cells are affected by temperature and light intensity is obtained.The DC-DC structure and DC-AC structure of the photovoltaic power generation system are analyzed,and the theoretical preparation for the subsequent control method improvement and model building is made.Secondly,the MPPT control method of photovoltaic power generation system under large disturbance is studied.Aiming at the defect that the tracking speed and tracking stability of the traditional and improved MPPT control will deteriorate when the temperature and light intensity change abruptly,this thesis proposes an improved perturbation and observation method based on multi-parameter.The control method uses the Mcquart method to fit the temperature,light intensity and duty cycle into the perturbation and observation method,so that the duty cycle can reach the maximum power point voltage directly when the external environment is greatly disturbed.The improved control method can improve the tracking speed and reduce the step size value.Finally,research is conducted on the grid connection control strategy of photovoltaic power generation systems under large disturbances.This thesis proposes a photovoltaic grid connected three loop control strategy based on PI and quasi PR to address the shortcomings of poor tracking speed and accuracy of grid connected current in single and double loop grid connected control strategies when overvoltage occurs on the DC side of the inverter.In this control strategy,the square wave voltage on the inverter side is feedback controlled to improve the tracking speed of DC over-voltage.At the same time,the inverter voltage loop and grid connected Current loop form a cascade structure through PI and quasi PR control to overcome the shortcomings of PI control that cannot track without static error and PR control that has small gain near the fundamental frequency.The third loop capacitor Current loop is set to suppress LC branch resonance and reduce system power consumption.Through the Simulink platform,the three loop control strategy based on PI and quasi PR was compared with the control group(double loop control strategy)for simulation.The results showed that the control strategy proposed in this thesis can improve the suppression effect on the output current THD value under general conditions and DC overvoltage conditions,and the grid connected current is always maintained near the rated value,verifying the feasibility and effectiveness of the control strategy proposed in this thesis.