Multilevel Inverter with Less Number of Isolated dc Bus Voltages

This paper proposes a three-phase 7 levels voltage source inverter employing space vector modulation (SVM) control scheme and with natural dc bus voltages balancing. The proposed inverter is a modular type that can increase its level by adding more cells to each arm. A high frequency transformer (HFT) is proposed in this paper to generate the dc bus voltage of the auxiliary cell from the main cell dc bus voltage. As a result of that the number of isolated dc bus voltages required is reduced by half. In addition, a natural balancing between the main and auxiliary cell dc bus voltages of each arm will be attained due to the HFT turns ratio which result in simplifying the overall control of the whole system. In order to reduce the size of the HFT a judicious modulation index (MI) will be chosen. The proposed system has been simulated using induction motor load. Analyses and simulation have been provided to validate the proposed system. Index Term— MLI, Isolated DC bus sources, HFT, SVM, DC sources balancing.