Analysis Of Frequency Characteristics Of Impedance And Stability Of Three-Phase Grid-Connected Inverter Under Different Damping Methods

With the development of distributed generation system,grid-connected inverters have been widely used.However,because distributed generation systems are mostly located in remote areas,long-distance transmission lines make the impedance of power grid not negligible,and the power grid presents the characteristics of weak power grid.The interaction between grid-connected inverters and weak power grids may cause harmonic oscillation of grid-connected current and reduce power quality and system stability of grid-connected system.Traditional research on grid-connected inverters mainly focuses on changing the characteristics of controller loop to improve the robust stability of system.However,this kind of damping control method bases on the single inverter,which does not consider the frequency interaction between the inverters and the power grid.This method has great limitations and is no longer applicable.In this thesis,the impedance analysis method is applied to discuss the influences of damping method on the impedance characteristics of inverters under conventional control.The frequency stability of grid-connected system is studied by analyzing the interaction between the inverters and the grid.Firstly,the impedance model of LCL grid-connected inverters based on passive damping control is presented.Secondly,the different active damping methods of LCL grid-connected inverters are analyzed.Starting with the active damping form of current feedback,the impedance models of two active damping control inverters,capacitive current and grid-connected current,are introduced.The impedance models under different damping methods are derived and compared.Thirdly,the interaction between grid-connected inverters and power grid is analyzed based on the impedance analysis method,and the impedance boundary range of grid-connected system when harmonic oscillation occurs is considered by the stability criterion based on phase margin.In this thesis,the active impedance shaping control strategy of grid-connected inverters is studied.The robust stability of grid-connected inverters under different grid conditions can be enhanced by improving the system impedance phase margin.The derivation of the control strategy and the process of parameter design are depicted.Finally,a 10 kW experimental platform for three-phase grid-connected inverters is built.Based on this platform,the experimental verifications of the proposed scheme are completed.