The use of thyristor converters in existing tripole high voltage direct current (HVDC) transmission system suffers some inherent defects similar to those of the traditional HVDC transmission system. For this reason

this paper proposes an improved scheme which uses the modular multilevel converters (MMCs) based on full bridge sub-modules (FBSMs) to replace the thyristor converters in pole 3

and the converters in pole 1 and pole 2 are also replaced by the MMCs based on clamp double sub-modules (CDSMs). Meanwhile

the coordination control between the three poles is proposed. In order to suppress the transient overvoltage and minimize the grounding current and power fluctuation in the transition process

the voltage inverse control

power coordinated control and the control strategy of keeping current zero in pole 3 are also proposed. The performance of the proposed tripole HVDC transmission system is validated through time-domain simulation study in PSCAD/EMTDC. The results show that the tripole HVDC transmission system based on the proposed control strategy can achieve a satisfactory response characteristic

while maintaining the balance of capacitor voltage and minimizing the grounding current and power fluctuation.