SCR-Front-End Regenerative CHB Drive with Improved Harmonic Profile

Cascaded H-bridge (CHB) motor drive is an attractive solution for high-power industrial motor drives due to its modularity and scalability. The existing phase-shifting transformer in the CHB drive can eliminate the low-order harmonics at the grid, which makes CHB drives outperform other MV topologies due to their “perfect” harmonic profile. However, in the applications where frequent braking or regenerative capability are required, the conventional CHB drives cannot be employed due to the diode-front-end (DFE) structure. To obtain the regeneration capability, several solutions based on IGBT active front-end rectifier (AFE) have been proposed to convert the regenerative power to grid in each CHB power cell. However, these IGBT-based methods in regenerative CHB drives introduce the extra switching sideband harmonics that cannot be effectively eliminated by the existing phase-shifting transformers. The advantage of the CHB drives is thus sacrificed. In this paper, a new solution is discussed by replacing the DFE rectifier with an SCR (Silicon-Controlled Rectifier)-Front-End rectifier/inverter in each power cell. Unlike the existing IGBT-based solutions, the harmonics generated by the SCR rectifier/inverter can be eliminated by the existing phase-shifting transformer. Thus, to comply with the grid harmonic standard IEEE std 519–2014, a small $L$1 filter between the grid and SCR rectifier/inverter is required in each power cell. To further remove the additional filters while complying with IEEE standards, in this paper, a new 54 pulse phase-shifting transformer is introduced for a seven-level CHB drive and studied with an improved harmonic profile. Finally, the proposed SCR-Front-End solution is validated on a seven-level CHB inverter in simulation. This method can be generalized for any n-level CHB motor drive.