## 4 Simulation Results
### Short-Circuit Tests  
The short-circuit test is a representative procedure that is performed for assessing a transformer's characteristics. Note that owing to the limitation of the system configuration for the generation of a rated voltage (154 kV) on the high-voltage (HV) side, a rated voltage (55 kV) was formed on the low-voltage (LV) side, whereas the HV side was configured as a short circuit as shown in Figure 12. We measured the short-circuit current when the \begin{table}
\begin{tabular}{c c} \hline \hline
**Parameter** & **Value** \\ \hline Transformer rating & 45 MVA \\ \hline Rated voltage & 154 kV/55 kV \\ \hline T-side leakage reactance & 12.5\% \\ (Single-phase transformer) & 12.5\% \\ \hline T-side resistance & 0.2\% \\ \hline M-side leakage reactance & 12.5\% \\ (Single-phase 3-winding transformer) & 12.5\% \\ \hline M-side resistance & 0.2\% \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Detailed parameters of Scott transformer (identical to those of the transformer certified by KERI). Figure 11: Result of impedance voltage test for the proposed Scott transformer: (**a**) three-phase current waveforms; (**b**) M- and T-phase current waveforms and vector representation. \begin{table}
\begin{tabular}{c c c} \hline \hline  & **Certified Test Result** & **Results with the Proposed Model** \\ \hline Impedance voltage & 19.25 kV & 19.25 kV \\ \hline M-phase current (RMS) & 0.4090 kA & 0.4089 kA \\ \hline T-phase current (RMS) & 0.4090 kA & 0.4091 kA \\ \hline HV side current (RMS) & 0.1687 kA & 0.1688 kA \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Result of impedance voltage test (a comparison of the result with the certified test report of an actual Scott transformer by KERI). switching operation occurs, while the rated voltage is formed on the LV side shown in Figure 12.  
Figure 12(a) shows the short-circuit current waveform when the T side is configured as an open circuit and voltage is generated on the M side. The actual waveform of the short-circuit current of the transformer certified by KERI is shown in Figure 12(b). As illustrated, the waveform of the short-circuit current for the proposed model is similar to that of the actual short-circuit current. In addition, the peak and RMS current values are almost equal to those shown in Table 4.  
Figure 13: M-phase short-circuit test results of Scott transformer (comparison between short-circuit test results obtained by RTDS and those in the certified test report by KERI): (**a**) waveform of short-circuit test result of the proposed Scott transformer, obtained by RTDS simulation (M-phase); (**b**) waveform in the certified test report (M-phase). The case study system was set in RTDS to evaluate a system condition at a specific time. We measured each feeder's voltage, current, and active power using the CT and PT at 6:14:03 AM in an existing system. Then, we compared the measurement values with evaluated values by the simulations. The comparison results are summarized in Table 7.  
The measurement points (Upward Line and Downward Line in S/S and SP) are marked in blue in Figure 15. It is observed that the results of using the Substep model and the proposed model are similar to the actual measurement data. On the other hand, the results of the Library model differ significantly from the actual measurement data. In addition, Figure 16 compares the accuracy of the results obtained using each model with the actual measurements (Table 7). The accuracy is remarkably low when the Library model was utilized as shown in Figure 16.  
As mentioned in Section 3.2, it should be emphasized that RTS for electric railway systems requires heavy computational resources. The problem is that the Substep library models should completely occupy one of the processor units of real-time simulators (see Figures 9a and 17a). This hinders the flexible calculation of processor units of real-time simulations. In contrast, the proposed model occupied only 5% of a processor unit as shown in Figure 17b.