An effort of incorporating geothermal plant (GTP) in automatic generation control and analysing its impact along with other renewables on conventional thermal system is focused in this work. Suitable generation rate constraint is considered for thermal units. Fractional-order (FO) Proportional Integral—FO Proportional Derivative (FOPI-FOPD) is used as a secondary controller implementing the innovative concept of FO cascade controller. Dynamic performance of FOPI-FOPD is compared with the normally used traditional controllers and its parameters are optimized using the Sine Cosine Algorithm. The superiority of FOPI-FOPD is clearly revealed in the analysis over others. Effect of integrating GTP and other renewables is also highlighted on the thermal system. The system is studied considering 1% step load perturbation in area1. © Springer Nature Singapore Pte Ltd. 2019.