Voltage spectra of three-level inverters

In the area of high voltages and powers, a high quality inverter-fed ac drive is more easily obtained by the use of multi-level inverters. For three-level inverters three different PWM control regions can be introduced where different two-phase (discontinuous) PWM strategies and the different three-phase ones should be applied. This paper deals with the analysis of voltage spectra of different PWM methods for region I of three-level inverters and especially with those two PWM methods that are not used in the case of two-level inverters. The derivation of the analytical equations of motor voltage harmonics for these PWM methods is given. The comparisons of the voltage spectra of the possible PWM methods are presented. It is shown that for the first half of region I (low voltage area) the best solution is the use of dipolar modulation. However, for the second half of this region the harmonic losses associated with dipolar modulation can be sharply decreased by the application of the other three-phase PWM technique. At the same time for two-level inverters this PWM technique produces no acceptable solution.