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This paper presents some new technologies of voltage source converter (VSC), i.e., modular multilevel VSC, for high voltage direct current transmission system based on VSC (VSC-HVDC). The origin of modular multilevel VSC and its essential working mechanism are clarified in detail. Then, the effective protection measures of submodule and the technology of controlled submodule capacitor voltages balancing are discussed. And the capacitance installed in each submodule is calculated. Concerning HVDC transmission based on modular multilevel VSC, an improved modulation control technology is presented. And the precharge technology of individual submodule capacitors is described. The technologies and conclusions are validated by simulations in PSCAD/EMTDC linked to MATLAB/SIMULINK. The theory analysis and simulation results indicate that these new technologies have great potentiality in the application of VSC-HVDC projects in future.