In this paper a High Voltage Direct Current (HVDC) transmission system is proposed, based on using three level three phase (3L3P) converters, coupled through a specific transformer connection in order to benefit from natural and selective harmonic cancellation effects. In opposition to classical PWM-VSI- based HVDC systems, the presented converter configuration uses Square Wave Modulation in order to keep the switching loss to a minimum value. Because of the SWM leads to the loose of the control of the input-to-output voltage ratio, special properties of the transmission system are described, regarding the active and reactive power control strategy. For the present contribution, three principal sections are defined; the power flow control; the 3L3P-NPC inverter with its control in SWM, and the dedicated transformer section with its specific connection. Simulation results are shown to highlight the capacity of the proposed system.