Operation of Three-phase Modular Multilevel Converter (MMC) with Reduced Number of Arms

Conventional three-phase MMC consists of six-arms, each arm is controlled to generate unipolar voltage ranging from zero to V-dc where V-de, is the input DC voltage. Each arm voltage has two components (DC and AC components). In this work, the operation of MMC with reduced number of arms (four arms only), correspondingly reduced cost is presented. This mode can be also used with the conventional three-phase MMC in case of faulty leg. In the presented approach, only two legs (i.e. four arms) are considered, where two out of the three phases are connected to these legs, while the third phase is tapped from the mid-point of the DC supply. To assure generating three-phase output voltages, the AC components in the presented approach will depend on the line-voltages. As the voltage per arm is limited to V-dc, the peak of line-voltage in the suggested mode is limited to 0.5V(de), i.e. the generated phase voltages will have a peak of 0.5V(de)/root 3 (i.e. 57.7% compared to conventional three-phase MMC with same input voltage). A detailed illustration of the operational concept of the proposed architecture is presented in this work. The simulation results of the suggested approach is presented to validate the proposed concept.