This paper is an endeavor to enhance the performance of the Automatic Generation Control (AGC) by adopting cascade PD-FOPID (Proportional Derivative-Fractional Order PID) controller in a two-area mutually connected thermal power plant with Generation Rate Constraint (GRC). The performance of the cascade PD-FOPID controller is validated by contrasting PID and FOPID controllers implemented in each area as AGC. The basic goal of the design of these controllers is to lessen the area control error (ACE) of corresponding area by conceding the frequency and tie-line power deviation. Group Hunting Search (GHS) algorithm is adopted to explore the gain parameters of the controllers to lessen the objective function (ITAE). A small step load transition of 0.01 pu is enforced in area-1 to investigate the controller performance. Cascade PD-FOPID controller optimized by GHS algorithm performs precisely better than PID and FOPID controller in the proposed system. Â