Space Vector Modulation for Cascaded Asymmetrical Multilevel Converters Under Fault Conditions

This paper proposes a new space vector modulation (SVM) strategy for asymmetrical cascaded multilevel converters (ACMCs) operating under fault conditions. When the converter loses one power cell, the remaining ones can overmodulate, if the modulation strategy is not properly adapted to the fault condition. As in ACMCs the series-connected power cells operate with distinct voltages and frequencies, the overmodulation problem is more difficult to address, when compared to other multilevel converters. In this context, the proposed SVM entirely avoids overmodulation of the converter under any fault condition. In an ACMC, the faults in the low-voltage-high-frequency (LVHF) cells are more problematic, as the converter loses the fine voltage adjustment capability provided by the PWM. The proposed SVM is carried out in the converter output line-to-line voltages coordinate system, in which all the voltage vectors have only integer entries. As a result, it is possible to simplify the implementation of the proposed algorithm. Besides assuring that the LVHF cells do not overmodulate as much as possible, the SVM flexibilizes the implementation of different switching sequences for the remainder operational LVHF cells, making it possible to optimize the quality of the output line-to-line voltages, without penalizing the switching losses. Simulation and experimental results are given to validate and to demonstrate the performance of the proposed SVM.