Soft Start-Up Technique for Solid State Transformer

Power electronics improvements make Solid State Transformer (SST) technology an attractive choice for direct integration of MVAC grid with LVDC/LVAC. However, during the startup of SST, the starting current (inrush current) and over-voltage can damage the semiconductor devices if the starting control strategy is not properly executed for all the stages of SST. The design of the startup procedure is dependent on the power conversion stages and associated control systems. For this reason, it is essential to carefully design the startup process, particularly in the case of modular structures with distributed DC links. Therefore, this article proposes an easy and reliable soft starting technique of SST for two types of applications: MVAC/LVAC (MV/HF DC-DC/ LV) and MVAC/LVDC (MV/HF DC-DC). The proposed control keeps the starting grid current within limits to avoid overcurrent protection failure by using active switching in a controlled manner. Furthermore, the natural cell voltage/power balancing at H-bridge cells in the MV stage of SST simplifies the implementation of the proposed control. It is experimentally tested using a 600V(MVAC)/200V(LVDC)/100V(LVAC), down-scal CHR-based SST prototype setup.