  In this paper, we provide an extension of the scalable algorithm proposed in
(Riverso et al., 2015) for the design of Plug-and-Play (PnP) controllers for AC
Islanded microGrids (ImGs). The method in (Riverso et al., 2015) assumes DGUs
are arranged in a load-connected topology, i.e. loads can appear only at the
output terminals of inverters. For handling totally general interconnections of
DGUs and loads, we describe an approach based on Kron Reduction (KR), a network
reduction method giving an equivalent load-connected model of the original ImG.
However, existing KR approaches can fail in preserving the structure of
transfer functions representing transmission lines. To avoid this drawback, we
introduce an approximate KR algorithm, still capable to represent exactly the
asymptotic periodic behavior of electric signals even if they are unbalanced.
Our results are backed up with simulations illustrating features of the new KR
approach as well as its use for designing PnP controllers in a 21-bus ImG
derived from an IEEE test feeder.
