An Improved Modulation Strategy for a Hybrid Multilevel Inverter

Multilevel cascaded and hybrid inverter systems conventionally use three level bipolar PWM for each individual inverter, with their carriers phase shifted to achieve maximum harmonic cancellation within each phase leg. This has been shown to give a harmonic performance equivalent to APOD modulation of a NPC multilevel inverter, when the switching frequencies are scaled to achieve the same number of overall switch transitions. In contrast, PD modulation of a NPC inverter is harmonically superior, because it places harmonic energy directly into the carrier harmonic for each phase leg, and relies on cancellation of this harmonic across phase legs as the line-to-line voltage is developed. Using this understanding, a new discontinuous modulation strategy is proposed for a hybrid inverter which achieves a similar level of improvement in lineto-line harmonics compared to conventional three level modulation of this type of inverter.