TCSC (Thyristor-Controlled Series Compensator) has been used in modern power systems to increase power transfer and system stability. Besides, due to its controllable feature, TCSC has been used to work as a solution to sub-synchronous resonance (SSR) problems. However, it is not clear so far whether TCSC’s inherent characteristic can avoid SSR and its effectiveness for SSR damping. In this paper, an equivalent circuit of TCSC is used to better understand how TCSC performs at sub-synchronous frequency ranges. Therefore, interactions of TCSC behaviors and SSR components can be viewed individually. The effects of TCSC operation interfered by sub-synchronous components are discussed. Results show that TCSC has the capability to damp SSR, but this capability can be deteriorated if TCSC firing logic cannot be effectively maintained.