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The modular multilevel converter (MMC) is a new kind of voltage source converter (VSC) with a high number of voltage levels and considered to be suitable for HVDC transmission. Based on circuit configuration and operation mechanism, this paper presents an mathematical model of MMC. Then the vector control strategy widely used in two-level VSC is applied to the control of MMC. Due to modular structure and cascaded connections, a very high number of voltage levels is obtained by MMC in HVDC applications, for which the nearest level modulation (NLM) become feasible. A combination of NLM and optimized capacitor voltage balancing control is presented to reduce the switching frequency as well as the switching losses of power semiconductors. Time-domain simulation performed in PSCAD/EMTDC shows that the control and modulation methods provide good steady-state and dynamic system performance and significantly reduce the switching frequency of power semiconductors.