•The present study highlights the attempt of incorporating geothermal power plant (GTPP), dish-Stirling solar thermal system (DSTS) and high voltage direct current transmission (HVDC) link, with the conventional thermal system, in automatic generation control of an interconnected power system under deregulated environment. Appropriate generation rate constraints are provided in thermal systems.•A new fractional order (FO) cascade controller named as FO proportional-integral-FO proportional-integral-derivative (FOPI-FOPID) is proposed as secondary controller and its performance is compared with the commonly used classical controllers.•A stochastic algorithm, Sine Cosine Algorithm (SCA) is used to optimize the controller gains and other parameters.•Analyses of the dynamic responses reveal the superiority of FOPI-FOPID over the others in terms of settling time, peak deviation and magnitude of oscillation.•Performance index comparison is carried out and integral squared error is selected.•The effect of GTPP, DSTS and HVDC link have been examined separately and the responses disclose that integration of HVDC link to the combined system having both GTPP and DSTS along with thermal leads to better dynamics.•Sensitivity analysis of SCA optimized FOPI-FOPID controller parameters obtained at nominal Disco Participation Matrix (DPM) validate that they are healthy enough and need not be optimized for change in DPMs. The present study highlights the attempt of incorporating geothermal power plant (GTPP), dish-Stirling solar thermal system (DSTS) and high voltage direct current transmission (HVDC) link, with the conventional thermal system, in automatic generation control of an interconnected power system under deregulated environment. Appropriate generation rate constraints are provided in thermal systems. A new fractional order (FO) cascade controller named as FO proportional-integral-FO proportional-integral-derivative (FOPI-FOPID) is proposed as secondary controller and its performance is compared with the commonly used classical controllers. A stochastic algorithm, Sine Cosine Algorithm (SCA) is used to optimize the controller gains and other parameters. Analyses of the dynamic responses reveal the superiority of FOPI-FOPID over the others in terms of settling time, peak deviation and magnitude of oscillation. Performance index comparison is carried out and integral squared error is selected. The effect of GTPP, DSTS and HVDC link have been examined separately and the responses disclose that integration of HVDC link to the combined system having both GTPP and DSTS along with thermal leads to better dynamics. Sensitivity analysis of SCA optimized FOPI-FOPID controller parameters obtained at nominal Disco Participation Matrix (DPM) validate that they are healthy enough and need not be optimized for change in DPMs.