  As the installation of electronically interconnected renewable energy
resources grows rapidly in power systems, system frequency maintenance and
control become challenging problems to maintain the system reliability in bulk
power systems. As two of the most important frequency control actions in the
control centers of independent system operators (ISOs) and utilities, the
interaction between Economic Dispatch (ED) and Automatic Generation Control
(AGC) attracts more and more attention. In this paper, we propose a robust
optimization based framework to measure the system flexibility by considering
the interaction between two hierarchical processes (i.e., ED and AGC). We
propose a cutting plane algorithm with the reformulation technique to obtain
seven different indices of the system. In addition, we study the impacts of
several system factors (i.e., the budget of operational cost, ramping
capability, and transmission line capacity) and show numerically how these
factors can influence the system flexibility.
