A novel CBPWM strategy for single-phase three-level NPC rectifiers in electric railway traction

In this paper, a new carrier-based pulse width modulation (CBPWM) strategy combined the unipolar and dipolar modulations, is proposed for single-phase three-level rectifiers. The modulation rules are presented in detail. And the proposed CBPWM method can satisfy volt-second balancing principle in the whole modulation index region through compensation. When the normalized modulation signal is in the range from -0.5 to 0.5, it presents the dipolar modulation characteristics, and the main high-order harmonics caused by the proposed CBPWM distribute around quadruple the switching frequency. When the normalized modulation signal is greater than 0.5 or less than -0.5, it presents the unipolar modulation characteristics, the main high-order harmonics caused by the proposed strategy, distribute around double the switching frequency. Therefore, the main high-order harmonics caused by the novel modulation strategy, distribute around double and quadruple the switching frequency in the whole modulation index region. The proposed modulation strategy has the advantages of the fewer high-order harmonics and the requirement of one carrier signal, compared with the traditional unipolar CBPWM. The experimental results verify the feasibility and effectiveness of the proposed novel CBPWM strategy.