New modulation strategy for asymmetrical cascaded multilevel converters under fault conditions

This paper proposes a new and simple hybrid modulation strategy for asymmetrical cascaded multilevel converters (ACMCs) operating under fault conditions in the low-voltage-high-frequency (LVHF) cells. The theoretical analysis is explained through an example, an ACMC with one 1pu and one 2pu cell per phase. The modulation strategy integrates the benefits of both geometrical and space vector approaches. The last one is used to define the voltages for the high-voltage-low-frequency (HVLF) cells, such as to avoid the overmodulation of the LVHF cells. The carrier-based geometrical approach is employed for the LVHF cells to simplify the implementation of the algorithm, since the PWM signals are obtained via the comparison of modulating voltages to triangular carriers, with no need to define switching sequences for the low voltage vectors. As a result, the proposed algorithm is considerably simplified when compared to the space vector. Simulation and experimental results are given to validate the proposed method and to demonstrate its good performance.