Control Algorithm Design and Implementation of Solid State Transformer

In this paper, a design review and control algorithm of a solid state transformer (SST) model intended to replace the existing 60Hz power transformer for the railway application is presented. The proposed smart transformer is a high-frequency power electronic transformer. It is made of multiple cascade AC-DC converter modules, linked by a high voltage DC-link to the multiple parallel dual active bridges (DAB) DC-DC converters. This arrangement of modules is susceptible to the unbalanced voltage of the DC-link capacitors, and the voltages of modules as well. However, if the control system is well designed, the voltage balance can be maintained. The main advantage of the presented SST is that it is capable of controlling the instantaneous voltage and improving the power quality by compensating the harmonics. The proposed control algorithm includes the design analysis of the output DC voltage control, the voltage balancing capability across the DC-link capacitors and the cascaded modules. It guarantees the great performance during the verification process. The validity of the proposed algorithm is verified by PSIM simulation and scale-downed experimental results.