Harmonic reduction in multilevel inverters

The use of multilevel inverters has increased markedly during the last decade. These new types of inverters are suitable for high voltage and high power application due to their ability to synthesize waveforms with better harmonic spectra. Numerous topologies have been introduced and widely studied for utility and drive applications.;With the advent of power electronics and the proliferation of non-linear loads in industrial power applications, power harmonics and their effects on power quality have become a topic of concern. The goal is to use the equipment that generates harmonics as low as possible. There are many practical harmonic elimination methods for multilevel inverters; the performance of these methods is related to the number of switching angles and the number of selected harmonics. Directly applying these methods would not guarantee finding the best switching angles for all modulation index and voltage step combinations.;Increasing the number of levels reduces harmonics, so it is logical to investigate higher-order multilevel inverters. Maximizing the levels for different multilevel inverters' topologies is accomplished by setting their input voltages to certain values. The selective harmonic elimination method for asymmetrical multilevel inverters can eliminate more specific harmonic than symmetrical multilevel inverters, for the reason that we can solve not only for switching angles but for the input voltages. However, that does not completely solve the problem since the total harmonic distortion is not significantly improved and the solution range is restricted.;Minimizing the total harmonic distortion by solving for the switching angles is demonstrated for single-phase and three-phase systems for symmetrical and asymmetrical multilevel inverters. The results improve significantly on previous methods. Some suggestions are made to overcome the main limitation of the optimal switching method, which is the unchanging output voltage.