  We propose an online framework to detect cyber attacks on Automatic
Generation Control (AGC). A cyber at- tack detection algorithm is designed
based on the approach of Dynamic Watermarking. The detection algorithm provides
a theoretical guarantee of detection of cyber attacks launched by sophisticated
attackers possessing extensive knowledge of the physical and statistical models
of targeted power systems. The proposed framework is practically implementable,
as it needs no hardware update on generation units. The efficacy of the
proposed framework is validated in both four-area system and 140-bus system.
