Analysis of an Improved Voltage-balancing Control Method of Modular Multilevel Converter Based on Amplitude-adjustable Carrier

Realize the control of the balance of DC capacitor in power-unit is paramount for the normal operation of the modular multi-level converters. The traditional control method achieves the DC capacitor voltage balance by adjusting the trigger pulse of each power-unit. At the same time, the DC capacitor is constantly absorbing and releasing energy, so that the voltage of the DC capacitor cannot be maintained at a fixed value which means that the capacitor voltage fluctuation problem still exists while the traditional control method has been used. Moreover, the quality of the output voltage will also decrease. In this paper, an improved voltage-balancing control method based on amplitude-adjustable carrier is proposed to suppress the adverse effects of the DC capacitor voltage fluctuation. The improved DC capacitor voltage control method is based on volt-second equivalent, and the carrier of each power-unit is rebuilt according to the real-time voltage of the DC capacitor. Finally, we will get a reconstructed trigger pulse to achieve the control of the output voltage of each power-unit which effectively improve the quality of the output voltage of the modular multi-level converters. The simulation experiments and results built on MTALAB/SIMULINK have proved the effectiveness of the improved control method.