This paper addresses the limitation of current technologies in the interconnection between medium-voltage (MV) DC and high-voltage (HV) DC grids. One transformerless approach is proposed to achieve bidirectional high power transfer and high step up/down ratio, which can be configured to interface with different grids connection. The proposed technology integrates modular multilevel converter (MMC) modules into resonant dc/dc converters. Power is transferred through both DC and AC paths. Equivalent circuit is developed and power flow is analyzed to clarify the system operation. Simulation results are presented to verify the validity of the proposed approach.