In this article Fuzzy-Proportional Integral Derivative controller (Fuzzy-PID) is proposed for Automatic Generation Control (AGC) problem of multi area thermal hydropower system. At the outset, a two area hydro thermal power system is developed in the MATLAB/SIMULINK environment. Non-linearities like backlash, transport delay (TD) and generation rate constraint (GRC) are incorporated at the suitable positions in the system and fuzzy-PID controllers are kept as a secondary controller to diminish AGC problem. The superiority of fuzzy-PID controller is explored by comparing with integral, proportional integral (PI) and PID controllers. The controller parameters have been tuned with Glow-worm Swarm Optimization (GSO) algorithm and the Integral Time Absolute Error (ITAE) is used as an objective function. The results are verified through simulations and experiments. The optimized fuzzy-PID controller shows good closed-loop responses in control multi area thermal hydropower system.