  Conventional electric power systems are composed of different unidirectional
power flow stages of generation, transmission, and distribution, managed
independently by transmission system and distribution system operators.
However, as distribution systems increase in complexity due to the integration
of distributed energy resources, coordination between transmission and
distribution networks will be imperative for the optimal operation of the power
grid. However, coupling models and formulations between transmission and
distribution is non-trivial, in particular due to the common practice of
modeling transmission systems as single-phase, and distribution systems as
multi-conductor phase-unbalanced. To enable the rapid prototyping of power flow
formulations, in particular in the modeling of the boundary conditions between
these two seemingly incompatible data models, we introduce PowerModelsITD.jl, a
free, open-source toolkit written in Julia for integrated
transmission-distribution (ITD) optimization that leverages mature optimization
libraries from the InfrastructureModels.jl-ecosystem. The primary objective of
the proposed framework is to provide baseline implementations of steady-state
ITD optimization problems, while providing a common platform for the evaluation
of emerging formulations and optimization problems. In this work, we introduce
the nonlinear formulations currently supported in PowerModelsITD.jl, which
include AC-polar, AC-rectangular, current-voltage, and a linear network
transportation model. Results are validated using combinations of IEEE
transmission and distribution networks.
