A double-T DC-DC modular multilevel converter (DC-DC-MMC) topology for high-voltage dc (HVdc) applications is presented in this paper. The topology is intended to interconnect large power dc grids with different voltage levels or to control the power flow in meshed dc grids. A two level control hierarchy is used to regulate the dc voltage of each cell. At the top level, dc and circulating ac currents are used to control the total energy in every branch. At the bottom level, the balance of the cell capacitor voltages within a branch is carried out by balancing (N-1) capacitor voltage deviations with respect to the average capacitor voltage. Finally, the operation of the converter is validated by means of PSCAD simulations. Results for the operation of the DC-DC-MMC controlling the power flow between two HVdc grids with different voltage levels are presented. The robustness of the converter has also been proven by simulating the disconnection of one pole of the bipolar dc grid.