  In this paper we present a framework to efficiently characterize the
available operational flexibility in a multi-area power system. We focus on the
available reserves and the tie-line flows. The proposed approach is an
alternative to the current calculation of the Available Transfer Capacity
(ATC), as it considers location and availability of reserves, transmission
constraints, and interdependencies of tie-line flows between different areas,
while it takes into account the N-1 security criterion. The method is based on
computational geometry using polytopic projections. It requires only a limited
amount of information exchange and does not need central coordination. The
method has two versions: a passive and an active approach, where neighboring
areas can share reserves. In that respect we also introduce the term "exported
flexibility", which could form the basis for a new trading product in
electricity markets. Case studies demonstrate the improved tie-line
utilization, especially if reserves are shared, and the visualization benefits.
