  With the growing penetrations of distributed energy resources (DERs), it is
imperative to evaluate their impacts on transmission system operations. In this
paper, an iteratively coupled transmission and distribution (T&D) co-simulation
framework is employed to study the impacts of increasing penetrations of
distribution-connected photovoltaic (PV) systems on transmission system
voltages. The co-simulation framework introduces iterative coupling and
unbalanced transmission system analysis that help accurately replicate the
standalone T&D system results without resorting to the computational challenge
of developing large-scale standalone T\&D models. The integrated T&D systems
are evaluated for multiple PV deployment scenarios based on randomly generated
PV locations and sizes. A test system is simulated using the IEEE-9 bus
transmission system model integrated at each load point with three EPRI's
Ckt-24 distribution feeder models. The results are thoroughly validated using a
standalone T&D system model simulated in OpenDSS.
