TCSC as a dynamic system, besides its capability in increasing power transfer in transmission lines, can be used to enhance different power system issues. TCSC's different advantages can be classified as steady-state and transient ones. During a fault, TCSC can enhance power quality by limiting the current and help to keep the voltage as high as possible. In this paper, the application of TCSC to enhance one of the important power quality issues, i.e., voltage sag is investigated. Different operating modes of TCSC have different impacts on the voltage of the bus that the line equipped with TCSC is connected to. Transferring to bypass mode upon occurrence of a fault is a salient feature of TCSC to improve voltage sag. The simulations on a sample network reveal these points.