| Multilevel converter is more suitable for the high-voltage and high-power applications since it reduced the voltage stress requirement of each single switch compared with two-level converter. Modular Multilevel converter aroused wide concern among scholars since the topology been put forwarded because of the feature that it has no need to use transformer and its high modularity structure so as the high redundancy of breakdown. This article concentrated on the conducted EMI of MMCconverter, starting from the circle current, the voltage ripples of capacitor in sub-module, the characteristics of conducted EMI and the method of restraining common mode voltage were studied.First, the topology of main-circle and the working theory and the relationship between key electric variables was analyzed. The relationship between common mode voltage and the bridge-arm voltage was got so as the relationship between differential mode current and the circle current from the definition. The formation mechanism of common mode interference and differential mode interference was analyzed.Then, the circle current model was built, and the differential equation of circle current was inferred from the model. The characteristic of low frequency harmonic circle current was studied. The calculation of the voltage ripples of capacitor in sub-module was got using the calculation result of low frequency circle current. The influence of bridge-arm conductor on circle current and the voltage ripples of capacitor was studied.At last, the presentation of control PWM in each sub-module was got using double Fourier method. The characteristics of bridge-arm voltage and common mode voltage and differential mode current were studied.The method of restrain the common mode voltage in modular multilevel converter using carrier phase shift modulation was studied. A model was built in MATLAB, the simulation result showd that calculation and simulation results are quite close,and the validity of restraining common mode voltage was proved. |
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