A power electronic transformer (PET) fed nine-level H-bridge inverter for large induction motor drives

This paper is devoted to the investigation of a 500 HP induction machine drive based on a nine-level 4.16 kV H-bridge inverter. Previous work in the area of controlling H-bridge inverters at such power levels report enhancement in the performance of a conventional staircase modulation technique by employing optimization of switching angles to minimize voltage distortion. However, the DC link voltages of each level are typically held constant. Such a control scheme enables elimination of a maximum of three dominant harmonics (5^th, 7^th and 11^th) to synthesize a given fundamental voltage with a nine-level waveform. Moreover, at lower modulation depths, the nine-level operation degenerates into seven or even fewer levels. This restricts the number of harmonics that can be eliminated to two (5^th and 7^th) or less. An alternative strategy to produce a required fundamental voltage with a nine-level waveform by controlling the DC bus voltages is presented in this paper. This control scheme enables elimination of four dominant harmonics (5^th, 7 ^th, 11^th and 13^th) over the entire range of operation. The required DC link voltage control is achieved by employing active power electronic transformers for isolation, thereby supplying varying AC voltage to the front-end rectifiers. Operating principles, spectral structure and design consideration are discussed. Computer simulations backed up by experimental results are presented in the paper