Research about the model and steady-state performance for modular multilevel converter

Modular multilevel converter (MMC) is a potential topology for the medium or high voltage and high power applications due to the modular structure, high efficiency, low distortion and high reliability. In this paper, a four order large signal model is built with the state variables of the summation of all the capacitor voltages, the difference between the total voltages of the upper arm and lower arm, the circulating current and the output current. Based on the model, the steady-state performance can be evaluated and the accurate steady-state expressions of the circulating current, the arm current and the average capacitor voltage can be obtained. It can be inferred that the circulating current is mainly composed of the DC component and the second harmonic. For the upper capacitor voltages and the lower capacitor voltages, the even harmonics are with the same phase and the odd harmonics are with the inverse phase. Furthermore, the DC bias of the capacitor voltage is a little different from the rated value. At last, a three-phase 48 sub-modules platform is built to verify the accuracy of the four order model and the steady-state expressions. The simulation and experimental results are highly matched with the theoretical expressions. ©, 2015, Chinese Machine Press. All right reserved.