The present work deals with an unequal two-area multi-source thermal-split shaft gas turbine (SSGT)-based automatic generation control (AGC) system. Each of the thermal units is associated with both reheat turbine and generation rate constraint (GRC). The whale optimization algorithm (WOA) is used for simultaneous optimization of controller gains and other parameters. Through investigations it is observed that combination of integer order proportional–integral–derivative with filter (PIDN) controller and fractional-order integral-derivative (FOID) controller (PIDN-FOID) provides upperhand over classical controllers with respect to lesser peak deviations, settling time, and smaller values of cost function. Different analyses also show that involvement of interline power flow controller (IPFC) and ultra-capacitor (UC) one at a time or both together improves the system dynamics with much-reduced oscillations, lesser peak deviations, and settling time. Sensitivity analysis at higher value of step load perturbation (SLP) is performed for checking the robustness of controller. © Springer Nature Singapore Pte Ltd. 2019.