A Staircase Modulation for Asymmetric Inverter Operating with Equals Fundamental Voltage and Minimum THD

Nowadays, static power converters have a transcendental role at an industrial level. The two-level converter is still the most used, such as in the case of use in AC electrical machine drives or the inclusion of renewable energies in electrical systems. However, due to its admirable advantages, the multilevel converter is a valid alternative for higher quality and efficiency benefits. In particular, one of the most studied topologies is the cascaded H-Bridge, which can be classified according to its DC voltage ratios as symmetrical and asymmetrical. The latter makes it possible to obtain a high-quality output voltage waveform using fewer inverters per phase. However, this has drawbacks such as: (i) regeneration due to the modulation technique, (ii) asymmetric power distribution among the inverters, and (iii) loss of modularity. In this work, it is proposed to design a modulation technique for the generation of a stepped phase voltage based on the sum of modified square waves generated by H-bridge inverters in cascade with asymmetric DC voltage ratio in order to obtain a distribution of symmetrical active power between the cells and a minimum THD of the output phase voltage. For the above, the minimization of the THD of the output phase voltage is performed. As a result, the DC voltages and firing angles are obtained for each modified square waveform. Thus, for six cells per phase with m = 1.0, 6.976% of THD is obtained, and an efficiency of 95.32%.