  In this paper we study the DC power flow equations of a purely resistive DC
power grid which consists of interconnected DC microgrids with constant-power
loads. We present a condition on the power grid which guarantees the existence
of a solution to the power flow equations. In addition, we present a condition
for any microgrid in island mode which guarantees that the power grid remains
feasible upon interconnection. These conditions provide a method to determine
if a power grid remains feasible after the interconnection with a specific
microgrid with constant-power loads. Although the presented condition are more
conservative than existing conditions in the literature, its novelty lies in
its plug-and-play property. That is, the condition gives a restriction on the
to-be-connected microgrid, but does not impose more restrictions on the rest of
the power grid.
