Variable frequency operation control of modular multilevel converter based on carrier phase-shift modulation

Applying the modular multilevel converter (MMC) into medium-voltage/high-voltage high-power drive systems is an intrinsically challenging problem because the output voltage distortion is serious when the converter works at low frequency operation, as a result of the sub-module capacitor voltages fluctuations' aggravation. For this matter, a method of injecting the high frequency common mode voltages and the circulating currents was designed from the perspective of balancing the upper and lower arm powers dynamically based on the analysis of the mathematical model of a MMC. The carrier phase-shift technology was adopted in the drive system to reduce the switching frequency and losses. The implementations to make the phase voltage as N+1and 2N+1levels were discussed. By combining the carrier phase-shift modulation and low frequency operation sub-module capacitor voltage restraining method, a voltage control strategy along with its system architecture and implementation were presented for variable frequency operation. The proposed control strategy achieves the goal of balancing voltages in variable frequency operation and makes the MMC and motor work stably. At the same time, a satisfied output harmonic characteristic is acquired at low switching frequency. Finally, the constructed Matlab/Simulink simulation and experimental prototype verify the availability of the proposed control strategy at variable frequency operation. ©2015 Chin. Soc. for Elec. Eng.