  This paper presents a small-signal analysis of an islanded microgrid composed
of two or more voltage source inverters connected in parallel. The primary
control of each inverter is integrated through internal current and voltage
loops using PR compensators, a virtual impedance, and an external power
controller based on frequency and voltage droops. The frequency restoration
function is implemented at the secondary control level, which executes a
consensus algorithm that consists of a load-frequency control and a single time
delay communication network. The consensus network consists of a time-invariant
directed graph and the output power of each inverter is the information shared
among the units, which is affected by the time delay. The proposed small-signal
model is validated through simulation results and experimental results. A root
locus analysis is presented that shows the behavior of the system considering
control parameters and time delay variation.
