  With the proliferation of distributed energy resources (DERs), located at the
Distribution System Operator (DSO) level, uncertainties propagate from
Transmission System Operator (TSO) to DSOs and vice versa. Therefore, to enable
interoperability, while ensuring higher flexibility and cost-efficiency, both
systems need to be efficiently coordinated to operate in sync. Moreover,
because of the intermittency of renewable generation, voltages exhibit
fluctuations thus necessitating the inclusion of AC power flow. Difficulties
behind creating such TSO-DSO coordination include the combinatorial nature of
the operational planning problem involved at the transmission level as well as
the nonlinearity of AC power flow within both systems. These considerations
significantly increase the complexity even under the deterministic setting. In
this paper, a deterministic TSO-DSO operational planning coordination problem
is considered and a novel decomposition and coordination approach is developed.
Within the new method, the problem is decomposed into TSO and DSO subproblems,
which are efficiently coordinated by updating Lagrangian multipliers. The
nonlinearities at the TSO level caused by AC power flow constraints are
resolved through a novel dynamic linearization. Numerical results based on the
coordination of 118-bus TSO system with 32 DSO 34-bus systems indicate that
both systems benefit from the coordination.
