In this paper, the transfer function model of heavy duty gas turbine has been developed for doing load frequency control studies. Based on the large signal model of Rowen, small signal model has been developed. This model is much suitable for doing Automatic Generation Control. Proportional integral and derivative secondary controller has been developed for both the small and large signal models to improve the system response. Ziegler Nichols' method, Simulated Annealing and Fuzzy Gain Scheduling have been used for tuning the secondary controller. Ziegler Nichols' method is used as conventional tuning, whereas Simulated Annealing is a search based tuning and Fuzzy Gain Scheduling is adaptive. It is found that Simulated Annealing tuned Proportional Integral Derivative Controller yields better response than other two controllers in both large signal and small signal model of heavy duty gas turbine plant. (C) 2016 Elsevier Ltd. All rights reserved.