Decoupling control for the capacitor voltage and the circulating current harmonics of the modular multilevel inverter

This paper focus on the control of the module capacitor voltages and the circulating current harmonics of the modular multilevel inverter (MMI). The harmonic elements of the circulating current and the capacitor voltages are analyzed with a single phase state equations of the system deduced in this paper. And then the three phase state equations are transformed into the double line-frequency negative-sequence dq0 coordinate system. The equations in the d and q frame of axes indicate the characteristics of the harmonic circulating currents and the zero sequence equations indicate the characteristics of the dc components of the modular capacitor voltages and the circulating currents. So, a decoupled control strategy of the harmonic circulating currents and the dc circulating currents is proposed. In the dq coordinate system, the traditional strategy is modified to suppressing the harmonic circulating currents perfectly, and in the zero sequence coordinate system, the capacitor voltage and the dc circulating currents are controlled. In different coordinate system, the controllers do not affect each other. The final simulation and experimental results indicate the mentioned conclusions.