Research On Improved Fixed-frequncy Control Strategy Of Two-stage Photovolatic Grid-connected System

In recent years,energy exhaustion and environmental degradation have brought great challenges to human society.The new energy power generation technology represented by photovoltaic power generation technology has attracted extensive attention from scholars at home and abroad.As an important device of new energy power generation,the performance of PV grid-connected system affects the quality of power generation.Therefore,its control strategy has been widely studied.This paper studies the control strategy of the two-stage PV grid-connected system,mainly improve the control algorithm of the pre-stage MPPT and the control algorithm of the post-stage grid-connected.The specific work contents are as follows:Firstly,according to the mathematical model of PV cells,it is modeled by simulink,analyze their lighting and temperature characteristics through the simulation,which leads to the necessity of MPPT control of photovoltaic cells.Secondly,the two working principles of MPPT are analyzed in detail,the advantages and disadvantages of the three conventional MPPT control algorithms are briefly summarized and the algorithm flows are given.The conventional MPPT algorithm cannot give consideration to the steady state accuracy and tracking speed at the same time,an improved variable-step INC is given based on the variable-step INC,and modeled in Simulink for general INC,variable-step INC and improved variable-step INC.Simulation results show that the improved variable-step INC has faster tracking speed and higher steady-state accuracy compared with the other two algorithms.Thirdly,the four-quadrant working principle of the grid-connected inverter is simply analyzed,the detailed design process of the main circuit parameters is given,and the output voltage vector of the grid-connected inverter is obtained by considering the finite switching state of the grid-connected inverter,and the forward Euler method is used to model the grid-connected inverter.Aiming at the traditional MPCC algorithm of the three-phase grid-connected inverter has high harmonic content,switching frequency is not fixed and the fixed-frequency effect of fixed-frequency MPCC algorithm is not good,an improved fixed-frequency MPCC algorithm applied to the three-phase grid-connected inverter is proposed.Its core is to adopt no beat control to distribute the action time of three voltage vectors.Then switch signals are obtained by DPWM0 modulation.Analyze three kinds of MPCC algorithm simulation and experiment,results show that the proposed improved fixed-frequency MPCC algorithm can effectively realize to reduce the output current harmonic content and fixed frequency control,and the dynamic performance is good.Finally,the improved variable-step INC algorithm and the fixed-frequency MPCC algorithm,namely the improved fixed-frequency control strategy,are applied to the two-stage PV grid-connected system compared with the traditional control strategy(conventional INC algorithm is used in the former stage and traditional MPCC algorithm is used in the latter stage).The simulation results show that the harmonics of a-phase current is 2.69% and the fixed-frequency control is realized under the improved fixed-frequency control strategy.However,the harmonics content of a-phase current is as high as 9.67% and the spectrum is scattered under the traditional control strategy,which verifies the superiority of the improved fixed-frequency control strategy.The paper has 58 pictures,9 tables,and 56 references.