The power balance is considered as most remarkable issue in power generation system. In this proposed work thermal power systems are connected through tie-line. Thermal power system is designed by considering reheat turbine with single stage, governor, and speed regulator unit. Proportional-Integral-Derivative (PID) controller is implemented to regulate the system operation. In this proposed work, the automatic generation control of three-area interconnected reheat thermal power generating system designed and discussed. The PID controller gain values are tuned with the help of more powerful evolutionary Bat Algorithm (BA) procedure. The proposed Bat algorithm tuned controller response was examined by comparing its performance with other optimization technique, namely Genetic Algorithm (GA) and Particle Swam Optimization (PSO) technique tuned controller response. Additionally, different cost functions-based bat algorithm optimized controller responses are presented and compared. The time domain specification parameters are considered for verifying the better-cost function for designing of controller. The simulated responses evident that proposed bat algorithm tuned controller output yield superior performance over GA & PSO tuned controller. Integral Time Square Error (ITSE) cost function-based BA tuned controller give better controlled response during sudden load demand condition in interconnected power system.