An isolated dual MMC DC-DC interfacing converter and its modulation strategy are proposed for the inter-connection of Line Commutated Converter HVDC（LCC-HVDC） and Voltage Source Converter HVDC（VSC-HVDC）. Full bridge MMC is applied in LCC side and half bridge MMC in VSC side for the different characteristics and demands of LCC-HVDC and VSC-HVDC. The two converters are connected via an intermediate-frequency transformer. A quasi two-level modulation method and its submodule voltage balancing strategy are presented to meet the proposed scenario. According to the analysis of the converter working process

stable power transmission and low submodule voltage pressure can be achieved. According to the submodule energy exchanging analysis

submodule capacitor voltages maintain stable by reasonably setting the phase-shifting angle of submodule with proposed voltage balancing strategy. Simulation and experiment results verify the effectiveness of proposed topology and modulation strategy.