A long transmission line needs controllable series as well as shunt compensation for power flow control and voltage regulation. This can be achieved by suitable combination of passive elements and active FACTS controllers. In this paper, series passive compensation and shunt active compensation provided by a static synchronous compensator (STATCOM) connected at the electrical center of the transmission line are considered. This paper presents investigation of subsynchronous resonance (SSR) characteristics of the system and proposes a novel method for the extraction of subsynchronous component of line current using filter. The extracted subsynchronous frequency component of line current is injected to the system by the STATCOM which prevents the flow of subsynchronous current through the generator and increases the damping of the system at critical torsional frequencies. This novel technique is termed as Subsynchronous Current Injector (SSCI). The design of SSCI is based on damping torque analysis. The 3-phase model of the STATCOM is based on switching functions. By neglecting harmonics in the switching function, D-Q model is derived which is combined with similar models of the other system components for linear analysis. The results of the linear analysis are validated by carrying out transient simulation based on the detailed nonlinear models. ©2009 IEEE.