Improvements in harmonic mitigation for multilevel AC-Drives for high power applications

Large Current Source PWM AC Drives needs either, a phase controlled rectifier or an Active Front End (AFE). Choosing the AFE implies to use optimized switching patterns in both, rectifiers and inverters. Several authors propose modular current source based AC-Drives (MCSC) as a practical way to handle currents larger than the devices capabilities. Standard SHE was extended to Multilevel Selective Harmonic Elimination (MSHE) for MCSC. MSHE change the harmonic spectra by subtracting the larger harmonics of the total current pattern. It results in cleaner multilevel patterns. This is not the case if symmetrical modulation indexes and balanced currents are not ensured. The inverters shape the output line current meanwhile the rectifiers independently balance the DC currents. Thus, the possibility of having rectifiers operating at asymmetrical conditions exists which generates a threat of harmonic pollution. An alternative approach to generate multilevel patterns in the rectifier stage of MCSC considers the use of Displaced SHE patterns (DSHE). This approach provides total elimination for low order harmonics as well as a reduction of the characteristic harmonics amplitude reducing the total distortion. This work presents the mathematical bases of the DSHE operation. Additionally, it compares the SHE, MSHE, and DSHE for the rectifiers of an MCSC. Simulated results validate the theoretical considerations.