Research On Control Strategy Of Grid-connected Inverter Under A Weak Grid

As the power conversion interface between distributed generation system(DGS)and the power grid,the performance of grid-connected inverter determines the quality of grid-connected current.However,the power grid,at the point of common coupling of DGS,exhibits the characteristics of a weak grid with this inevitable grid impedance due to the increasing long transmission lines and multiple isolated transformers employed to interconnect the high penetrated DGs and the utility.Therefore,grid-connected inverter may suffer from the poor power quality and even the instability due to the dynamic interaction between the grid impedance and the output impedance of the inverter.Focusing on the control strategies of improving the stability of grid-connected inverters under a weak grid,the main works of this thesis are as follows:1)The interaction mechanism between grid impedance and single/multiple inverters is analyzed based on the impedance model of single and multiple grid-connected inverters.According to the criterion of impedance stability,the control scheme to improve the stability of grid-connected inverters is summarized.2)In order to improve the stability of grid-connected inverter in a weak grid,a lead-lag compensated current controller is proposed to increase the phase of the output impedance after discussing the characteristics of output impedance in different frequency bands.Constrained by the phase margin of the impedance and the performance of the current tracking,a straightforward design procedure for this lead-lag network is presented to achieve an improved stability in a variety of gird impedance.3)In order to improve the stability of grid-connected inverter with grid voltage feedforward control,the influence of grid voltage feedforward controller on the output impedance of grid-connected inverter is discussed and grid impedance adaptive serial-parallel composite impedance controller is proposed.The optimal series impedance based on grid current derivative controller is obtained after analyzing the phase boosting capability of the components in the output impedance.The grid voltage derivative controlled parallel impedance method is introduced to improve the phase margin at the intersection of output impedance and grid impedance.Moreover,the effect of series impedance on the equivalent current source and the sensitivity of the differential coefficient to the incremental phase are discussed.Constrained by the phase margin at the intersection of the output impedance and the grid impedance,an algrithmn for adaptive tunning derivative coefficients is presented to further optimize the performance of the system.4)A 6kW single-phase LCL grid-connected inverter is built by Matlab/Simulink simulation software and RT-LAB real-time simulation platform to verify the above two control methods.Simulation and experimental results valid the effectiveness of the proposed the method and design methodology.