Power Electronic Transformer with H-Bridge Cells Switched in Stair-case Mode and A Voltage Source Inverter in High Frequency Mode in MV Stage

This paper presents a three-stage (Medium Voltage (MV), DCDC, and Low Voltage (LV)), either side grid-connected Power Electronic Transformer (PET) system. The MV stage of the PET system is realized using multiple IGBT based HB cells. Each phase's last HB cell is terminated to a SiC MOSFET based 2L Voltage Source Inverter (VSI). The H-Bridge (HB) Cells are switched in stair-case mode to reduce switching losses in the MV stage. The VSI is switched at a high frequency, resulting in a net higher switching frequency, ensuring a sinusoidal profile in the MV stage. When compared to Si IGBT-based solutions, the proposed PET systems achieve the same effective switching frequency with reduced semiconductor loss in the MV stage. The proposed technique ensures that HB cells manage active power while the VSI unit handles reactive power. This allows the VSI DC link voltage to be realized via a floating DC link. A voltage balancing mechanism is proposed that maintains the VSI's DC link voltage at a desirable level. The DC link voltages of all the MV cells are naturally regulated by operating Series Resonance Converters (SRCs) at resonance in the DCDC stage. The proposed control is verified experimentally in a 5 kV A, 600 V(MV Grid)/100 $V$ (LV grid) PET system.