—Substation Expansion Planning (SEP) aims to estimate the expansion capacities of the existing and new substations (SS) along with their locations and time of construction. This study dints to solve real-world SEP problem for the power sector of Tamil Nadu, a state in India. Cost minimization has been considered as the main objective along with the constraints. Several discrete and continuous design variables make the SEP problem as complex. Hence the application of optimization technique is required to solve the problem. In this study, Firefly Algorithm (FA) has been implemented to solve SEP problem of Tamil Nadu. The simulation results imply that FA provides the optimal results with better convergence rate.