The present study highlights an attempt of integrating the geothermal power plant (GTPP) in automatic generation control of an interconnected system comprising of dish-Stirling solar-thermal system (DSTS) and the conventional thermal system (TS). Generation rate constraints of 3%/min are considered for the TSs. A new fractional-order (FO) cascade controller named as FO proportional (P)-integral (I)-FOP-derivative (D) (FOPI-FOPD) is proposed as secondary controller and performance is compared with commonly used classical controllers. Controller gains and other parameters are optimised using a novel stochastic algorithm called sine-cosine algorithm. The analysis reveals the superiority of FOPI-FOPD over others. The effect of inclusion of GTPP and DSTS is also analysed on the conventional TS, both in a combined manner and separately. Sensitivity analysis reflects the robustness of optimum FOPI-FOPD controller gains and other parameters obtained at nominal and recommend that the optimised parameters do not suffer much deviations and are able to withstand wide fluctuations in system operating conditions, system loading and inertia constant. The dynamic behaviour of the system is studied with 1% step load perturbation in area1. © The Institution of Engineering and Technology 2017.