Research on Unbalanced-Load Correction Capability of Two Power Electronic Transformer Topologies

Compared with conventional power transformer, power electronic transformer (PET) has many merits and an increasing possibility to replace it in the future. In this paper, the unbalanced-load correction capability of two H-bridge based three-phase three-stage modular PET topologies, the separate-phase-connection (SPC) one and the cross-phase-connection (CPC) one, are analyzed and compared. Both of the SPC and CPC consist of three stages: a modular multilevel ac-dc input stage, an isolation stage with several independent modular dual active bridge (DAB) dc-dc converters, and a dc-ac output stage with single-phase-inverter parallel-connected structure. Based on the control strategy, it is found that the SPC is suitable for dealing with full-range unbalanced load under the condition of increasing the input-stage current stress. For the SPC feeding, one load or three loads with identical power polarity, the current stress shall be theoretically increased up to 115% of the rated one, while the percentage is 153% when the SPC feeds three loads with different power polarity. On the contrary, the CPC is only suitable for dealing with partial unbalanced-load conditions. Extendedly, the third so-called full-range-power auto-balance PET is proposed to deal with unbalanced load, the performance of which is verified by simulation results. A downscaled prototype for SPC and CPC is built and tested. Experimental results verify the unbalanced-load correction performance of SPC and CPC.