Modeling the Output Impedance Negative Incremental Resistance Behavior of Grid-Tied Inverters

Small-signal stability in balanced three-phase systems can be analyzed using the Generalized Nyquist stability Criterion (GNC), which is based on source and load impedance in the synchronous rotating (d-q) frame. This paper models the d-q frame impedance of three-phase grid-tied inverters with current feedback control and Phase-Locked Loop (PLL). The result unveils a very interesting and important feature of three-phase grid-tied inverters, namely that its q-q channel impedance behaves as a negative incremental resistance. Further, the paper shows that this behavior is a consequence of grid synchronization, where the bandwidth of the PLL determines the frequency range of the negative incremental resistance behavior, and the power rating of the inverter determines the magnitude of the resistance. Simulation and experimental results verify the analysis.