Modular multilevel converters（MMCs） belong to a new cascaded multilevel voltage source converter（VSC） suitable for HVDC transmission purpose. The analysis of circuit configuration and operation mechanism of MMC indicates that the potentials of the two AC terminals in one phase are approximately equal， such that these two AC terminals can be short circuited. By doing so， the upper and lower converter reactors in one phase are connected in parallel and hence can be merged into one. Accordingly， the theoretical equivalent circuit model of a MMC can be derived. The derived model has three terminals at the AC side， each one of them is connected to the three-phase power system through a converter reactor. The equivalent circuit model has the same structure as a conventional VSC. In this way， the conventional VSC control strategies can be directly used for MMC. The vector control strategy which is widely used in conventional VSCs is used for a MMC based HVDC transmission system. The time-domain simulation results show that the potentials of the two AC terminals in one phase are nearly equal， and the derived equivalent circuit model of the MMC is valid. Moreover， the vector control strategy is effective for controlling of the MMC based HVDC transmission systems.