During specific low-demand periods, such as Eid Fitr 2023, the Cawang transmission subsystem in Indonesia experienced significant over-voltage issues due to excess reactive power, compromising grid stability and efficiency. This study proposes a solution using a Static VAR Compensator (SVC) optimized by the Firefly Algorithm to enhance voltage stability. The Firefly Algorithm, a meta-heuristic optimization technique, was employed to determine the optimal placement and capacity of the SVC within the grid. Simulations conducted using DIgSILENT PowerFactory demonstrated that the optimal configuration, with a 784.8829 MVAR SVC placed at bus 1PGSAN/I-5, effectively reduced voltage deviation by 0.6485%, leaving only six buses over-voltage. This approach highlights the potential of advanced optimization techniques in addressing over-voltage challenges and improving power system reliability. Further validation of the results was achieved by comparing various scenarios, underscoring the robustness of the proposed method. © 2024 IEEE.