Modular multilevel DC–DC converters(MMDCs) have gained considerable interest due to the increasingly widespread expectations of HVDC grids. Among various topologies, T-type transformerless MMDCs with a wide operation range and fault-blocking capability are a good choice for interconnecting the HVDC links with different voltage levels. This paper provides a comprehensive analysis of this topology, where a simplified equivalent circuit is developed to derive the power flow and balance conditions of T-type transformerless MMDCs. Thereafter, the power conversion and operation principle are characterized in detail. Finally, 320kV/100kV@1GW simulations are conducted to confirm the theoretical expectations.