Research On Grid-Connected Current Control Scheme For Three-Phase LCL-Type Inverters Under Non-Ideal Grid Condition

The Distributed Generation System(DGS) is flexible, friendly to the environment and has the advantage of high efficiency. Grid-connected inverters play an important role in the DGS as the system energy exchange interface between renewable energy sources and grid. When the grid condition is non-ideal, the current quality of grid-connected inverters injected to Point of Common Coupling(PCC) will be getting worse, which brings the problem of unbalanced and harmonic components. Thus, it is very necessary to study the current control scheme for grid-connected inverters under non-ideal grid conditions.In this thesis, the current control scheme for three-phase LCL-type grid-connected inverters under non-ideal grid condition is studied. Firstly, the mathematical modeling of three-phase LCL-type grid-connected inverters has been built, the mathematical modeling of decoupling control under the d-q coordinate system is then derived, and the control loop and control parameters are designed. On this basis, the impacts of the unbalanced and harmonic grid voltages to the grid-connected current are analyzed. The grid voltage feedforward control scheme for the non-ideal grid conditions is studied, which is adopted to suppress the harmonic and unbalanced components of grid-connected current affected by non-ideal grid based on the Double Second Order Generalized Integrator Phase-Locked Loop(DSOGI-PLL) technique, and then the structure and parameter of grid voltage feedforward control are designed. Furthermore, the simulation model of the three-phase LCL-type grid-connected inverter is built by the Matlab/Simulink software to verify the feasibility of the current control scheme in this thesis. Finally, a digital control based experimental prototype of three-phase LCL-type grid-connected inverter is built according to the system requirements, the software and hardware design of the inverter is depicted. The experment results are provided to verify the feasibility of the current control scheme to suppress the harmonic and unbalanced component under the non-ideal grid conditions.