Harmonic Analysis And Suppression Of Micro-grid Inverters

Micro-grid system,which can organize distributed power supply,is a small-scale energy system,it can operate in isolation and can be connected to the traditional grid.Grid-connected operation can easily meet the load requirements and improve the efficiency of large-scale utilization of renewable energy.As an important interface between the distributed generation system and the grid,grid-connected inverter undertakes the task of inverting DC power to AC power which will be fed into the grid.The performance of inverter decides the stability of grid-connected system and power quality,harmonic current content is one of its important indicators.At present,one of the hot spots of the research is harmonic analysis and suppression of grid-connected systems,which has drawn a lot of attention of domestic and foreign scholars.The impedance model can be used for harmonic analysis and suppression of grid-connected systems,but it is difficult to analyze harmonic quantitatively with the existing impedance model.In order to solve this problem,single-phase LCL grid-connected inverter is taken as the research object in this thesis,the impedance model of the grid-connected system and the harmonic suppression are deeply studied.In this thesis,the design method of LCL filter is summarized.The good high-order harmonic suppression ability and the resonance problem of LCL filter are discussed.The principle of passive damping of capacitor series resistance and active damping of capacitive current feedback are analyzed.The comparison of these two schemes shows that the active damping control system with capacitive current feedback has the better performance.The simulation results show the validity of the two schemes.Through the impedance model of the grid-connected system,stability of the system can be analyzed,the harmonic suppression strategy can be studied and the harmonic interaction analysis of the system can be carried out qualitatively.In order to improve the accuracy of the impedance model and make it more realistic,this thesis proposed a method about introducing the harmonic current directly into the impedance model based on the study of the frequency characteristics of the impedance model,and established the harmonic impedance model of the grid-connected system.Harmonic impedance model does not consider the actual existence of a variety of non-linear factors,the modeling process is simple.Simulation verifies the accuracy of harmonic impedance model.The voltage feed-forward control strategy is deduced and analyzed based on the impedance model,and the mechanism of the multi-resonant control strategy is analyzed.The advantages and disadvantages of these two kinds of low harmonic current suppression strategies are compared through the theory deduction and simulation.With the progress of new energy power generation technology,more and more inverters are connected to the power grid,and it exists the obvious harmonic interaction in the system,which may affect the stable operation of the system.Based on the proposed harmonic impedance model,the impedance network of multi-inverter grid-connected system is established.The coupling matrix of grid-connected current is deduced,and the harmonic interaction mechanism is analyzed.Harmonic suppression strategy of multi-inverter grid-connected system is studied and the influence of digital delay on the system is analyzed.In order to quantitatively analyze the harmonic content of multi-inverter grid-connected system,based on the low-frequency characteristics of the impedance model,a harmonic prediction model of the multi-inverter grid-connected system is proposed.Simulation shows that the harmonic prediction model can be used to predict the amplitude of the low harmonics quickly and accurately,and it has high amplitude and phase angle accuracy especially in 3rd,5th and 7th harmonic frequencies.