Which of the following is NOT a mode of AGC?

Isochronous control is characterized by maintaining the frequency of a power system constant regardless of load variations, which is not typically categorized as a mode of Automatic Generation Control (AGC). In AGC systems, there are standard modes of operation focused on managing power generation based on system requirements—specifically, the balancing of generation with load and maintaining system reliability.

Flat tie-line control, typically used in interconnected systems, involves managing flows across interconnections, effectively maintaining the power transfer at specified levels. Tie-line bias control uses measured flows on tie lines to adjust generation to manage frequency, ensuring that the system stays within operational limits. Flat frequency control typically focuses on maintaining a nominal frequency across the system through adjustment of generation levels.

Thus, the correct answer stems from the nature of isochronous control, which prioritizes maintaining frequency in a stand-alone manner over the typical needs of AGC, making it distinct from the modes listed as standard operations within AGC frameworks.