  Proliferation of smart grid technologies has enhanced observability and
controllability of distribution systems. If coordinated with the transmission
system, resources of both systems can be used more efficiently. This paper
proposes a model to operate transmission and distribution systems in a
coordinated manner. The proposed model is solved using a Surrogate Lagrangian
Relaxation (SLR) approach. The computational performance of this approach is
compared against existing methods (e.g. subgradient method). Finally, the
usefulness of the proposed model and solution approach is demonstrated via
numerical experiments on the illustrative example and IEEE benchmarks.
