This paper presents an isolated DC/AC/DC converter using a middle frequency transformer coupling two modular multilevel converters (MMC), suitable for interconnecting DC transmission lines of different voltage levels in high voltage direct current (HVDC) system. The basic operational principle of the isolated module multilevel DC/DC converter (IMMDCC) is analyzed. The dynamic model of IMMDCC is studied in detail and the transient relationship between DC side and AC side of IMMDCC is revealed, which is physically straightforward for understanding the power transfer in IMMDCC. The control strategy in D-Q coordinate system is put forward, and the fault characteristic and corresponding protection method is analyzed. Finally, computer simulation using Matlab/Simulink is performed to verify the dynamic model and the proposed control strategy. The simulation results show good performances and the quick response ability of the proposed control strategy.