Impedance-Based Stability Analysis of Droop-Controlled AC Microgrids with the Generalized Nyquist Criterion

An impedance model of a droop-controlled inverter is derived analytically. The cross-coupling between a distortion frequency and its mirrored frequency component is considered. This requires the impedance to be modeled as a MIMO system. An impedance-based stability analysis of a microgrid with two grid-forming inverters is performed. The results are validated by a laboratory test. This shows how the P-ω droop coefficient affects the stability of a microgrid and how the instability can be predicted with the analytical model.