  The transmission grid is often comprised of several control areas that are
connected by multiple tie lines in a mesh structure for reliability. It is also
well-known that line failures can propagate non-locally and redundancy can
exacerbate cascading. In this paper, we propose an integrated approach to grid
reliability that (i) judiciously switches off a small number of tie lines so
that the control areas are connected in a tree structure; and (ii) leverages a
unified frequency control paradigm to provide congestion management in real
time. Even though the proposed topology reduces redundancy, the integration of
tree structure at regional level and real-time congestion management can
provide stronger guarantees on failure localization and mitigation. We
illustrate our approach on the IEEE 39-bus network and evaluate its performance
on the IEEE 118-bus, 179-bus, 200-bus and 240-bus networks with various network
congestion conditions. Simulations show that, compared with the traditional
approach, our approach not only prevents load shedding in more failure
scenarios, but also incurs smaller amounts of load loss in scenarios where load
shedding is inevitable. Moreover, generators under our approach adjust their
operations more actively and efficiently in a local manner.
