In this paper, dynamic performance of automatic generation control (AGC) of two area thermal–thermal power system is determined through the steam chest and reheater constant trajectories. In earlier AGC studies, steam chest and reheater time constants denoted as ${T_{\text{SC}}}$ and ${T_{\text{RH}}}$ are considered to be constant, irrespective of plant generation schedules. However, it is discovered that ${T_{\text{SC}}}$ and ${T_{\text{RH}}}$ follow the specific trajectories with the plant generation schedules. These time constant trajectories are determined by the power output control strategies being employed to change the plant generation schedules. These control strategies are the different combinations of steam pressure versus flow rate curves used to operate the plant at different generation schedules. In this paper, time constant versus generation schedule trajectories of ${T_{\text{SC}}}$ and ${T_{\text{RH}}}$ have been derived for different power output control strategies. Furthermore, these time constant trajectories have been utilized to investigate the dynamic performance of two area reheat thermal power system.