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Passivity-Based Design for the Plug-and-Play Single-Loop Controlled LCL-Filtered Inverter

The grid impedance variation poses a challenge to the stability of LCL-filtered inverter. Critical stable ranges have been revealed for the single-loop inverter-current-feedback (ICF) and grid-current-feedback (GCF) controlled ones. However, they are all derived under the assumption that the grid impedance is inductive, whereas the presence of parasitic grid capacitances will lead to different results. In this paper, considering the capacitive condition and based on passivity concept, it is found that the stability is hardly secured for either the ICF or GCF controlled ones. From perspective of passivity, the phase of inverter output impedance would better be shaped between -90° and 90°. In view of this, the feedforward of the voltage of the point of common coupling (PCC) is applied in this paper to achieve this target, and the proper feedforward solutions are given. As a result, the inverter could harvest the robust plug-and-play functionality regardless of grid impedance. The correctness of the theoretical analysis and effectiveness of the proposed methods are verified by simulation results.