Solid-State Transformer Based Fast Charging Station for Various Categories of Electric Vehicles With Batteries of Vastly Different Ratings

This article proposes a solid-state transformer (SST) based power electronic architecture for fast charging three categories of plug-in electric vehicle with batteries of vastly different voltage and power ratings. The SST accomplishes integration to a medium-voltage grid and isolation to the charging ports without the use of an LFT. Delta connected cascaded H-bridge (CHB) is used as the active front-end rectifier and quad active bridges (QABs) are utilized for the isolated dc-dc conversion stage. In the proposed architecture, an additional winding called balancing winding is provided in each of the medium-frequency transformers of the QAB. The balancing winding of the transformers are paralleled such that a circulating current flows among the transformers of the three phases, which eventually reduces the unbalance in the delta-connected clusters. Hence, the magnitude of zero-sequence current required to balance the cluster powers for a certain level of unbalance is also reduced. As a result, the total current in the devices of the CHB decreases and devices with a lesser current rating can be used. The control philosophy and detailed analysis of the system are given in the article. Experimental results with a scaled-down prototype are presented to verify the claims.