Research On The Robustness Against Grid Impedance Of LCL-type Grid-connected Inverter

Utilization of fossil fuels such as coal, oil and natural gas leads to environment pollutionand resource exhaustion. Renewable energy such as solar energy and wind energy, has beendrawing more and more attentions. Distributed power generation systems (DPGSs) is oneof the effective utilization approaches of the renewable energy. The LCL-typegrid-connected inverter is the popular interface device between the DPGSs and gird. Thecapacitor current feedback active damping is an effective approach of damping the LCLfilter resonance so as to ensure the system stability. When digital control is employed, thecontrol delay will reduce the robustness of LCL-type grid-connected inverter against thevariation of the grid impedance. This thesis investigates the design of the control-parameterfor improving the robustness of the LCL-type grid-connected inverter with capacitorcurrent feedback active damping.The average switching model of the digital controlled LCL-type grid-connected inverterwith capacitor current feedback active damping is built in this thesis. Through theequivalent transformation of the control block diagram, the effect of the capacitor currentfeedback active damping with the control delay is analyzed, and the stability constraintsconditions of the digital control system are discussed. Afterwards, two criterions ofimpedance intersection and pole trajectory plot for determining the system stability arepresented when considering the variation of grid impedance.Through the analysis of the relationship between the grid impedance and the constraintsof system stability, the grid impedance variation range, within which the system is unstablewithout considering the grid impedance when designing the parameters of the inverter, arecalculated in this thesis. An design method of control parameters is proposedcorrespondingly. With this designed control parameters, if the resonant frequency of LCLfilter doesn’t cross one-sixth the switching frequency when the grid impedance varies, thesystem will always be stable; and if the resonant frequency crosses one-sixth the switchingfrequency when the grid impedance varies, the system will also be stable except a criticalpoint.In order to alleviate the stability problem resulted by the one-sample delay of digital control, this thesis discusses the effect of capacitor current real-time feedback activedamping on the improvement of the robustness of grid-connected inverter. Meanwhile, thisthesis investigates the equivalent physical models of two capacitive current feedback activedamping control strategies and robustness against the grid impedance.A prototype of6-kW digital controlled LCL-type single-phase grid-connected inverterwith capacitor current feedback active damping has been constructed in the Lab. Theexperimental results demonstrate the correctness of the theoretical analysis.