A Solid-State Transformer with an AC Link for Multi-Port Fast Charging of Electric Vehicles

This paper proposes a solid-state transformer (SST) based fast electric vehicle (EV) charger connected to a medium voltage (MV) AC grid and having multiple DC charging ports. Cascaded H-bridge (CHB) is used as the active front-end rectifier and dual active bridges (DABs) are utilized for DC-DC conversion and isolation. One issue with this topology is the unbalance of the DC-link voltages during unequal loading. Another issue is the large size of DC link capacitors required to decouple the second-harmonic ripple power introduced from the AC grid. To address these two issues, an AC link Solid State Transformer (SST) topology and its control are proposed in this paper. The AC link is constructed by using a three-winding transformer in the DAB converter wherein the third winding of all the converters are connected in parallel. The AC link enables flow of circulating power among the H-bridges in all the three phases and eliminates the issue of voltage unbalance even in 100 % unbalanced loading conditions. The magnetic flux associated with the second harmonic ripple is also cancelled in the AC link due to three phase symmetry and hence the DC link capacitor size is drastically reduced. Simulation results are presented to verify the claims.