This paper analyzes the small-signal impedance ofthree-phase grid-tied inverters with feedback control and phaselockedloop (PLL) in the synchronous reference (d-q) frame. Theresult unveils an interesting and important feature of three-phasegrid-tied inverters – namely, that its q–q channel impedance behavesas a negative incremental resistor. Moreover, this papershows that this behavior is a consequence of grid synchronization,where the bandwidth of the PLL determines the frequencyrange of the resistor behavior, and the power rating of the inverterdetermines the magnitude of the resistor. Advanced PLL, current,and power control strategies do not change this feature. An exampleshows that under weak grid conditions, a change of the PLLbandwidth could lead the inverter system to unstable conditions asa result of this behavior. Harmonic resonance and instability issuescan be analyzed using the proposed impedance model. Simulationand experimental measurements verify the analysis.
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