This research paper describes the automatic generation control of a three thermal interconnected system where each area is provided with a reheat thermal unit. The dynamic stability of this multi-area system is regulated by PD-PID cascaded controller optimized by grey wolf optimization algorithm. Each area is provided by proper generation rate constraints (GRC) in order to consider system nonlinearities. The controller parameters of the proposed controller are simultaneously optimized by considering integral time absolute error (ITAE) as an objective function with a step load perturbation (SLP) of 0.01 p.u. in area 1. The dynamic behavior of the system is compared with a recently published results in terms of settling time, peak overshoots and undershoots which proves the superior behavior of the proposed controller. Further in order to reflect the sensitivity and robustness of the proposed control method the system behavior is also shown by increasing the system loading.