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This paper presents automatic generation control (AGC) of interconnected two equal area and three unequal area thermal systems provided with single reheat turbine and generation rate constraints of 3% per minute. A maiden attempt is made to apply integral plus double derivative (IDD) controller in AGC. Controller gains in the two area system are optimized using classical approach whereas in the three area system controller gains and governor speed regulation parameters (Ri) are simultaneously optimized by using a more recent and powerful evolutionary computational technique called bacterial foraging (BF) technique. Investigations reveal on comparison that Integral (I), proportional-integral (PI), integral-derivative (ID), or proportional-integral-derivative (PID) all provide more or less same response whereas integral-double derivative (IDD) controller provides much better response. Sensitivity analysis reveals the robustness of optimum gains of IDD controllers and optimum values of speed regulation parameters at nominal loading condition to wide changes in system parameters such as loading condition, inertia constant (H), reheat time constant (Tr) and reheat coefficient (Kr) from their nominal values. The system performance is examined considering 1% step load perturbation in area1.