Harmonic Transfer-Function-Based alpha,beta-Frame SISO Impedance Modeling of Droop Inverters-Based Islanded Microgrid With Unbalanced Loads

Impedance-based approach is generally studied for stability analysis of the power electronic converter-based system. Considering multiple-frequency coupling effects, in this article, we establish the harmonic transfer function (HTF) based alpha beta-frame impedance models of droop inverters based islanded microgrid with unbalanced loads. Therein, the HTF-based alpha beta-impedance modeling methods of unbalanced loads, as well as droop inverters in parallel, are proposed. Afterward, the HTF-based alpha beta-frame impedance models are equivalent to single-input single-output (SISO) impedances for simpler impedance measurement and stability analysis, preserving the alpha beta-frame coupling and multiple-frequency coupling characteristics. Finally, the experimental measurement results verify the accuracy of the alpha beta-frame SISO impedances. Besides, the stability analysis based on the SISO impedance models accurately reveals low-frequency oscillations of the islanded microgrid with unbalanced loads, further verifying the effectiveness of the SISO impedance models.