Grid-connected LCL-filtered inverters are commonly\nused for distributed power generators. The LCL\nresonance should be treated properly. Recently, many\nstrategies have been used to damp the resonance, but the\nrelationships between different damping strategies have not\nbeen thoroughly investigated. Thus, this study analyses the\nessential mechanisms of LCL-resonance damping and\nreviews state-of-the-art resonance damping strategies.\nExisting resonance damping strategies are classified into\nthose with single-state and multi-state feedback. Singlestate\nfeedback strategies damp the LCL resonance using\nfeedback of a voltage or current state at the resonance\nfrequency. Multi-state feedback strategies are summarized\nas zero-placement and pole-placement strategies, where the\nzero-placement strategy configures the zeros of a novel\nstate combined by multi-state feedback, while the poleplacement\nstrategy aims to assign the closed-loop poles\nfreely. Based on these mechanisms, an investigation of\nsingle-state and multi-state feedback is presented, including\ndetailed comparisons of the existing strategies. Finally,\nsome future research directions that can improve LCL-filtered\ninverter performance and minimize their implementation\ncosts are summarized.
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