Modular multilevel converters

A modular multilevel converter (MMC) is one of the promising topologies for medium- to high-voltage, high-power applications. This chapter deals with the operation, control, and applications of an MMC. This converter can be realized with a wide range of submodules to meet the application requirements. Some of the popular submodules are half-bridge, full-bridge, flying capacitor, and cascaded half-bridge. The submodule operation, switching states, and their features are presented in this chapter. To achieve the MMC control objectives, a simple classical control method with pulse width modulation is presented. The past and present developments of pulse width modulation schemes for an MMC are briefly discussed in this chapter. The classical control method has independent controls to achieve each control objective of an MMC. The submodule capacitor voltage control is designed to achieve the balancing between the submodules in each arm while maintaining the average leg voltage at its nominal value. On the other hand, the current control is designed in the synchronous reference frame to ensure perfect tracking of output currents while reducing the circulating currents in an MMC. These independent controls are analyzed through case studies. Finally, the commercial applications of an MMC are summarized in this chapter.