Modeling and Analysis of DC Voltage Dynamics in Modular Multilevel Converter

Vector control is the most commonly used control strategy for modular multilevel converter (MMC) based HVDC system. However, due to the complicated structure of MMC, there is no precise mathematical model for the outer DC voltage loop. The conventional model ignores the capacitor voltage dynamics, and the AC voltage boosted operation scenario is not considered. This paper proposes a small signal model for the DC voltage loop of MMC. The capacitor voltage dynamics and different operation modes are taken into consideration. Based on the proposed model, the equivalent capacitor in the DC side is analyzed. It is found that the equivalent capacitor is related to the operation mode, the power factor and the capacitor voltage fluctuation ratio, and the operation mode is the major factor. Furthermore, the simplified model is given for the controller design. Compared with the conventional model, the small signal model is more accurate, and it is applicable to both conventional operation mode (the modulation index within unity) and AC voltage boosted operation mode (the modulation index greater than unity). Finally, the effectiveness of the proposed model is verified by the PSCAD/EMTDC.