  Modeling power transmission networks is an important area of research with
applications such as vulnerability analysis, study of cascading failures, and
location of measurement devices. Graph-theoretic approaches have been widely
used to solve these problems, but are subject to several limitations. One of
the limitations is the ability to model a heterogeneous system in a consistent
manner using the standard graph-theoretic formulation. In this paper, we
propose a {\em network-of-networks} approach for modeling power transmission
networks in order to explicitly incorporate heterogeneity in the model. This
model distinguishes between different components of the network that operate at
different voltage ratings, and also captures the intra and inter-network
connectivity patterns. By building the graph in this fashion we present a
novel, and fundamentally different, perspective of power transmission networks.
Consequently, this novel approach will have a significant impact on the
graph-theoretic modeling of power grids that we believe will lead to a better
understanding of transmission networks.
