Abstract
Solid-State Transformer (SST) has been investigated to replace or work in conjunction with the conventional Low Frequency Transformer (LFT). The LFT even though efficient and reliable, it does not provide room for additional demands in the recent grid also known as Smart Grid (SG). These demands are but not limited to: Supply to DC system; means of online communication; Renewable Energy Sources (RESs) integration; Renewable Energy Storage Systems (RESSs) integration to the grid; correction of voltage sag and power factor; active and reactive power control; bidirectional power flow. On the other hand, SST is capable to provide all functionalities and flexibilities needed by the SG that the LFT cannot provide. However, the SST is not as efficient and reliable as LFT. This work improved and simulated a designed SST. Also, the work analyzed performance of the designed improved SST. To achieve the proposed SST, the Inductor-Capacitor (LC) filter implemented at the output of the designed SST was replaced by an (Inductor-Capacitor-Inductor) LCL filter and also an additional LCL filter was implemented at the input of the designed improved SST to further mitigate harmonics. Furthermore, instead of using three single phase Dual Active Bridge (DAB) converters at the DC-DC unit of the designed improved SST, a single three phase DAB was used. This action enabled use of a less number of power electronic switch that is, Insulated Gate Bipolar Transistor (IGBT) in the design of the improved SST (24 instead of 36). Furthermore, the High Frequency Transformer (HFT) was redesigned with the view to reducing the length of the conductor used for the two sides of the HFT. This in turn helped reduced core and copper losses which improved efficiency of the improved SST. The designed three phase SST was simulated on MATLAB R2018a environment using Simulink blocks. The results of the simulation of the designed SST in terms of input and output current and voltage, Total Harmonic Distortion (THD) as well as efficiency were obtained and analyzed as presented in the paper. Consequently, THD of the designed improved SST with supply from the grid and Renewable Energy Sources (RES) were 1.42% and 1.39% as against those obtained with the earlier designed SST: 2.16% and 2.15% respectively. A reduction of 34.25% and 35.34% were respectively achieved. Furthermore, efficiency of the SST at 25%, 50%, 85% and 100% loading conditions were obtained as 90.03%, 95.01%, 97.06%, and 97.05% as against those obtained in the base paper: 86.46%, 93.20%, 96.01% and 96.61% respectively. Improvements in the efficiency of: 3.96%, 1.8%, 1.08% and 1.07% respectively were obtained. The results clearly indicated superiority over those obtained with the earlier designed SST and therefore, the designed improved SST was found to be more efficient than that was improved.

Key words: Solid-State Transformer, smart grid, converters, efficiency, total harmonic distortion, and low frequency transformer.