The DC/DC converter is one of the key devices of DC grid. The technological requirement analysis of the DC/DC converter is the premise of its topology design and application research. The selection of electrical isolation function and the requirement of fault blocking of DC/DC converters for modular multilevel converter based high-voltage direct current(MMC-HVDC)transmission systems are discussed. The electrical isolation requirements of DC/DC converter under different grounding systems are analyzed in terms of fault characteristics

insulation cost and system reliability. The analysis results show that electrical isolation is needed under symmetric monopole system and non-electrical isolation is better for bipolar system. Besides

DC/DC converter fault blocking demand is carefully investigated in terms of extermal and internal faults of the DC/DC converter. Finally

the analysis of electrical isolation and fault blocking demand of the DC/DC converter are verified through simulations.