Analysis of discontinuous PWM strategies of three-level inverters (I)

In the area of high voltages and powers, the high quality inverter-fed ac drive is more easily achieved by the use of three-level inverters. For three-level inverters three different PWM control regions can be introduced where different two-phase (discontinuous) PWM strategies may be applied. This paper deals with the analysis of voltage spectra of one discontinuous PWM method of three-level inverters when every 60deg of each fundamental period the one motor phase is clamped to dc bar. The derivation of the analytical equations of motor voltage harmonics for this PWM method is given for both realizations: with one and with two carrier waves. The comparison of the voltage spectra of the possible PWM methods is presented. It is shown that in the low voltage area the same value of motor harmonic losses are produced for both one and two carrier wave solutions. However, for the high voltage area the harmonic losses associated with the two carrier modulation are usually considerably lower than in case of one carrier method. The two carrier method can be disadvantageous only for low modulation frequency when the low order harmonics reach the sensitive value