The modular multilevel DC-DC converter(MMDC) is a promising solution for the interconnection of MVDC distribution systems with LVDC ports. Due to the symmetric structure and operation scheme, the MMDC is normally only capable of operating with monopole MVDC distribution systems. This paper proposes a bipolar operation scheme of the MMDC. Based on the concept of the flux dc-bias cancellation, a center-tapped high-frequency interface transformer is employed on the MV side of the MMDC. A dedicated operation method is proposed, whereby the power flows of the two MVDC poles can be manipulated independently with a simple control scheme. Compared to the conventional MMDC with monopole operation, no additional component is required and no penalty of increased current rating is imposed on the semiconductor devices or the interface transformer. Moreover, a single-pole operation is also possible with the proposed operation scheme. If the valves of one pole are faulty, it can be isolated from the main converter for maintenance while the MMDC can continue operation without interruption and deliver at least 50% of the rated power capacity. Hence, the reliability and availability of the power delivery can be significantly enhanced. The validity of the proposed bipolar operation scheme of the MMDC has been verified by both simulations and experiments with a down-scale prototype.