In this paper, based on the Voltage Source Convertor(VSC), the topology and mathematical model of the MMC is studied, based on the topology of the submodule and submodule group, the block, half block and deblock states are studied, and the equivalent circuit and mathematical model of the submodule and submodule group are put forward. On this basis, the block, half block and deblock state can be simulated using the equivalent circuit and mathematical model of the submodule and submodule group. Furthermore, the five type simulation model with different simplification degree are set up. In the study of the MMC, the simulation model with different simplification degree can be chose according to the different simulation purpose.Based on the simulation model with different simplification degree, the following issues are studied:1. Studied the charging process of the MMC, the half block charging process is put forward, with which the voltage of the capacitor in the MMC can be charged to the voltage when the MMC operated in steady state, in result, the AC current of the MMC have no inrush and distortion after the deblock of the MMC; 2. Studied the grouping control of the submodels, the floor modify number shift algorithm are put forward, with which, the similar effect of the Carriage Phase Shift PWM(CPSPWM) can be achieved, and the voltage balance control between the sub module group can be realized.3. Studied the reason of the circulation current, based on the equivalent circuit of the double frequency component, the mathematical model of the steady state, instantaneous mathematical model in the static coordinate and instantaneous mathematical model in the double frequency coordinate are studied, and the Circulation Current Suppression Control(CCSC) algorithm is put forward based on the three mathematical model.4. Studied the design method of the submodule capacitor, based on the power characteristic of the MMC in the steady state, the mathematical equation of the capacitor voltage is deduced out, by the curve fitting method, the mathematical equation of the magnitude of the capacitor voltage is deduced out, furthermore, the design method of the submodule capacitor is acquired.5. Based on the simplified simulation model, with the parameters of the Zhou Shan five terminal HVDC system, the simulation model of the five terminal HVDC system is built up, the power balance and voltage control are studied, such as primaryslave control, voltage drop control, voltage margin control and dropmargin hybrid control.When studied different problems, different simulation model with different simplification degree are used, ensured the simulation precision, at the same time, the simulation speed is also improved.
