A New Arm Voltage Control Scheme for a Single Phase Modular Multilevel Converter

Modular Multilevel Converters (MMCs) are widely used in medium to high voltage/power applications such as in energy transformation and grid connected systems. MMCs benefit from being redundant and allowing transformer-less operations. Unfortunately, they suffer from having a high cost due to the high number of sub-modules (SMs). Moreover, they are hard to control. In order to operate the MMCs properly, the load current, the circulating current and the capacitor voltages should be controlled. The usual control methods are synthesized in order to control the average lower and upper arm capacitor voltage as well as the load current. However, in some special cases, the capacitor values could change due to the aging of the capacitors, or the SM may stop working completely. Thus, creating an imbalance in the capacitor voltages of each arm in the system. This paper presents a new control scheme that allows to control the voltage of the upper and lower arms separately while controlling the load current at the same time. The usual and the new proposed controllers are developed and compared through software simulation. Finally, the conclusion is drawn.