In this paper a transformerless Static Synchronous Series Compensator (SSSC) to increase the power capacity of a 230 kV transmission line is proposed. In order to eliminate the transformer, a 15-level cascade H Bridge (CHB) inverter is employed within the proposed compensator structure to inject reactive power to the transmission line. The theory of instantaneous p-q power using in the compensator is in such a way that allows for the system to be controlled in one phase. However, this method is used with three-phase systems. The proposed control method is also able to balance the voltage of dc-link capacitors at a desirable level. Modulation technique used by the inverter is Phase-Shifted PWM. In order to validate the proposed compensator’s application, a three-phase transmission line is tested by MATLAB software. The transmitted active power of this transmission line is equal to 160 MW. The results obtained from a dynamic simulation are presented. Simulation results indicated 39% increase in the transmitted active power of the line.