  Hierarchical optimization architectures are used in power systems to manage
disturbances and phenomena that arise at multiple spatial and temporal scales.
We present a graph modeling abstraction for representing such architectures and
an implementation in the ${\tt Julia}$ package ${\tt Plasmo.jl}$. We apply this
framework to a tri-level hierarchical framework arising in wholesale market
operations that involves day-ahead unit commitment, short-term unit commitment,
and economic dispatch. We show that graph abstractions facilitate the
construction, visualization, and solution of these complex problems.
