  Proper modeling of inverter-based microgrids is crucial for accurate
assessment of stability boundaries. It has been recently realized that the
stability conditions for such microgrids are significantly different from those
known for large- scale power systems. While detailed models are available, they
are both computationally expensive and can not provide the insight into the
instability mechanisms and factors. In this paper, a computationally efficient
and accurate reduced-order model is proposed for modeling the inverter-based
microgrids. The main factors affecting microgrid stability are analyzed using
the developed reduced-order model and are shown to be unique for the
microgrid-based network, which has no direct analogy to large-scale power
systems. Particularly, it has been discovered that the stability limits for the
conventional droop-based system (omega - P/V - Q) are determined by the ratio
of inverter rating to network capacity, leading to a smaller stability region
for microgrids with shorter lines. The theoretical derivation has been provided
to verify the above investigation based on both the simplified and generalized
network configurations. More impor- tantly, the proposed reduced-order model
not only maintains the modeling accuracy but also enhances the computation
efficiency. Finally, the results are verified with the detailed model via both
frequency and time domain analyses.
