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This paper concerns the problem of regulating frequency in the changing electric energy systems with large presence of renewable energy resources. We start by observing that currently implemented frequency regulation approach, well-known as the Automatic Generation Control (AGC), is based on equilibria (steady-state) assumptions. However, frequency regulation in systems with fast and persistent fluctuations caused by wind and solar power, in particular, can no longer be viewed as equilibria problems. In order to better understand and overcome these limitations, we formulate the frequency regulation problem by viewing the future electric energy systems as a general dynamical system driven by disturbances. We derive a dynamical model of the system and further utilize its structure to systematically design an Enhanced-AGC (E-AGC). System performance is defined to consider tradeoff between quality of frequency regulation and its cost. Our proposed E-AGC generally supports coordination of several interconnected control areas for achieving the best performance at the system level. The proposed E-AGC is illustrated by simulating a small two-control area system and a 2000 bus, 15 generator real-world power system of Sao Miguel Island in the Azores Archipelago of Portugal. A comparison with system response of today's AGC is discussed.