Research On Grid-connected Technology Of Three-phase Inverter Based On Repetitive Control

With the increasing demand for clean energy,distributed generation technologies based on various renewable energy sources have become the focus of attention in many countries in recent years.The grid-connected inverter acts as a bridge between the distributed generation unit and the grid,and its performance determines the quality of the grid-connected current.Improving and optimizing the control strategy of grid-connected inverters has become an important research topic today.In this thesis,taking the three-phase grid-connected inverter as the research object,and the grid voltage synchronous detection technology and grid-connected current control strategies o for grid-connected inverter was discussed.Fast and accurate acquisition of the frequency and phase information of the grid is a prerequisite for accurate control of the grid-connected current,which is also an important guarantee for achieving unit power factor operation of the inverter.To improve the frequency and phase detection accuracy of PLL under complex grid conditions,DMTOGI which can eliminate the interference of DC disturbance component and fundamental negative-sequence component of the grid voltage is proposed.Combined DMTOGI with MAF as a hybrid filter which can completely eliminate the influence of the main interference components of the grid voltage on the detection accuracy of the PLL at the rated frequency of the grid.In order to improve the dynamic response speed of PLL,combining the designed hybrid filter into "QT1PLL".And establish the mathematical model of the designed PLL and complete the controller parameter design.The simulation proves that the designed PLL has excellent detection accuracy and dynamic response speed under complex grid conditions.In order to improve the quality of grid-connected current,a frequency-adaptive shunt internal model repetitive control strategy was proposed.Aiming at the problem of insufficient dynamic performance of traditional repetitive controllers,shunt internal model structure composed of odd-order internal mode and even-order internal mode can reduce the inherent delay time of repetitive control by half,while the parallel proportional controller will further improve the dynamic performance of the composite controller.Fractional-order delay links which caused by changes of grid frequency approximated by Lagrange interpolation method.In this way,the resonant frequency of the internal model can approach the frequency of the grid in real time.The stability conditions,convergence and anti-interference ability of the composite controller are analyzed,and the parameters of the repetitive controller are designed reasonably in this thesis.The effectiveness of the proposed strategy is confirmed by simulation.Finally,a three-phase grid-connected inverter experimental platform was built,and the platform hardware circuit and software system was designed.The experimental platform is used to verify the steady-state performance and dynamic performance of the shunt internal model repetitive controller proposed in this thesis.