Arm Current Stress Reduction Technique for a Delta-Connected Solid State Transformer Using Zero-Sequence Current Injection

The medium voltage stage of solid state transformer (SST), as a grid-connected converter, is required to generate a negative-sequence (NS) current under certain conditions. Unfortunately, this NS current increases the arm current stress, which limits the operating range of the SST. This article proposes the concept of using a zero-sequence (ZS) current to reduce such a current stress in △-connected SST with a low voltage direct current port (SST+LVdc). An algorithm to determine the optimized ZS current is derived on basis of geometric and logic·. The theoretical analysis and the proposed control algorithm are verified by both simulation and experimental results. The proposed control method gives the △-connected SST+LVdc an advantage over the Y-connected SST+LVdc because it can only work on the △-connected cascaded H-bridge.