This paper describes the theory and the modelling technique of a flexible alternating current transmission systems (FACTS) device, namely, static synchronnous series compensator (SSSC) using an Electromagnetic Transient Program (EMTP) simulation package. The SSSC, a solid-state voltage source inverter coupled with a transformer, is connected in series with a transmission line. An SSSC injects an almost sinusoidal voltage, of variable magnitude, in series with a transmission line. This injected voltage is almost in quadrature with the line current, thereby emulating an inductive or a capacitive reactance in series with the transmission line. The emulated variable reactance, inserted by the injected voltage source, influences the electric power flow in the transmission line.