The DC-link voltage control has a significant impact on the characteristics and stability of voltage source converter based high voltage DC(VSC-HVDC) transmission system. Using the AC-side current, DC-side current and circulating current as the state variables, this paper established a new state space model of MMC. The concept of inner electric potentials of MMC was proposed based on this model. The decoupled control of the AC side current, DC side current and circulating current can be archived by using th...