A multiport medium voltage (MV) isolated DC-DC converter with integrated energy storage, suited for connection of low voltage (LV) and medium voltage (MV) DC grids is presented. A multi-stage structure, adapted for MV ratings, is composed of multiple identical submodules connected in an Input-Series- Output-Parallel (ISOP) configuration. Each submodule has three fully bidirectional ports, that are galvanically isolated. This is achieved by means of a three-winding medium frequency transformer, where each leakage inductance combined with a resonant capacitor creates a resonant tank, benefiting from soft switching operation. Each submodule has two additional buck/boost stages, to actively control the power exchange with external DC circuits. The inner resonant stage is operated in open-loop and adapts its mode of operation based on the actual power flow. The remaining ports, dedicated to the storage, can be operated independently from the power flow between the MV and LV side, in order to charge or discharge the storage elements. Switched mode simulations are used to verify and demonstrate various operational modes and performance of the converter.