This study presents the automatic generation control of an unequal four-area thermal system with appropriate generation rate constraint and governor dead band (GDB). Performances of several classical controllers, such as single degree-of-freedom proportional-integral-derivative (PID), two degree-of-freedom PID (2DOF-PID) and three degree-of-freedom PID (3DOF-PID) as secondary controllers are evaluated separately in the system. An attempt is made to apply the successful evolutionary optimisation technique named as biogeography-based optimisation technique for simultaneous optimisation of several variables, such as controller gains, setpoint weights and so on. Comparison of dynamic responses corresponding to PID, 2DOF-PID and 3DOF-PID reveals that 3DOF-PID controller outperforms the others. Sensitivity analysis reveals that the optimum gains and other parameters of the optimal controller obtained at nominal conditions are robust and need not be reset for wide changes in system condition like system loading, system parameters such as inertia constant (H), synchronising coefficient (T-ij) and GDB. The performance of 3DOF-PID controller is also studied with different step-load perturbations (SLPs) and random-load perturbations (RLPs). Analysis proves that 3DOF-PID controller performs better than 2DOF-PID at different SLPs and RLPs.