Uncertainties and wide range variation characteristics of grid impedance in a weak grid can seriously affect control performance of grid-connected inverter system, and even cause the system unstable. At present, impedance-based analysis using inverter output impedance and grid impedance is adopted to study the inverter-grid system stability. However, using this method to represent system phase margin (PM) is inaccurate. What’s worse, stability criteria based on traditional impedance may be inapplicable when the inverter control parameters are changed. In order to overcome above shortcomings of traditional impedance analysis method, its stability criterion is reconstructed. The impedance-based stability criterion after reconstructing can not only accurately represent system stability margin, but also apply to variable-parameter-controlled grid-connected inverter system. Finally, by establishing grid-connected inverter model with LCL filter under weak grid condition, correctness of theoretical analysis and validity of the reconstructing method are verified with simulations.
KEY WORDS :weak grid;grid-connected inverter;grid impedance;impedance analysis;reconstructing of stability criteria;
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