Control Strategy of Delta-Connected Solid State Transformer Under Unbalanced Grid Voltage

Abstract

The Solid State Transformer (SST) or Power Electronic Transformer (PET) is a key technique in the future distribution grid. The mainstream selection of SST MV stage is cascaded H-bridge, but till now, researchers have not reached a conclusion on whether star-connected CHB or delta-connected CHB is better. This paper gives a comparison on input power capability of the two configurations under unbalanced grid voltage. The input current stress and operating range under unbalanced grid voltage of the two CHBs have been analyzed. A zero-sequence current injection algorithm is proposed to reduce the phase current stress of delta-connected PET which can increase its operation range. The analysis and control schemes are supported by both simulation and experimental.