A multilevel current source converter (CSC) for controlling electrical power transmission in a high voltage direct current (HVDC) transmission system includes an alternating current (AC) side for input/output of AC to/from the CSC; a direct current (DC) side for input/output of DC to/from the CSC; and a plurality of modular CSC cells connected in parallel with each other between the AC side and the DC side of the CSC, each modular cell comprising a plurality of switches. The CSC is configured for being connected in series with an HVDC transmission line and for injecting a voltage in series with the HVDC transmission line at the DC side of the CSC.