## 4 Simulation of Transformer Inrush Current Using 154 kV Transformer Test Report
### Calculation of Leakage Impedance Using the Results in Test Report  
To select the appropriate transformer model, the leakage impedance of the transformer must be calculated. Since this transformer is a three-winding transformer, the leakage impedance can be calculated using Equations (1)-(3).  
\begin{table}
\begin{tabular}{|p{142.3 pt}|p{142.3 pt}|} \hline
1. Structure and Exterior test & 12. Insulation strength test of operation and control circuit \\
1.1. Structure test & 13. Noise level \\
1.2. Exterior test & 14. Short circuit test of transformer \\
2. Turns ratio measurement, polarity test, and angle displacement test & 14.1. Short circuit test (Primary -Secondary winding) \\
2.1. Turns ratio measuremnt & 14.2. Short circuit test (Primary -Tertiary winding) \\
2.1.1. Primary - Secondary winding & 14.3. Winding temperature during short circuit test \\
2.1.2. Primary -Tertiary winding & 14.4. Confirmation test after short circuit test \\
2.1.3. Secondary - Tertiary winding & 14.4.1. Turns ration measurement, polarity test and angle displacement test \\
2.2. Polarity test and angle displacement test & 14.4.2. Impedance voltage and full load loss test \\
3. Impedance voltage and full load loss test & 14.4.3. No load loss and excitation current test \\
4. No load loss and excitation current test & 14.4.4. Winding resistance measurement \\
5. Winding resistance measurement & 14.4.5. Power frequency withstand voltage test \\
6. Power frequency withstand voltage test & 14.4.6. Induced withstand voltage test and partial discharge test \\
7. Induced withstand voltage test and partial discharge test & 14.4.7. Insulation power factor test \\
8. Insulation power factor test & 14.4.8. Motor test \\
8.1. Phase A & 14.4.9. Insulation strength test of operation and control circuit \\
8.2. Phase B & 14.4.10. Variation ratio of reactance \\
8.3. Phase C & 14.4.11. Visual inspection after short circuit test \\
9. Temperature rise test & 15. Poly Chlorinated Biphenyls (PCB) analysis test in insulation oil \\
9.1. Temperature rise test & 16. Winding insulation resistance measurement \\
9.1.1. Primary and Secondary winding & 16.1. Before short circuit test \\
9.1.2. Tertiary winding & 16.2. After short circuit test \\
9.2. Dissolved gas test & 17. Frame and core insulation test \\
10. Lightning impulse withstand voltage test & 17.1. Before short circuit test \\
11. Motor test & 17.2. After short circuit test \\
11.1. Fan power consumption measurement and rotating direction test & 18. Conclusion \\
11.2. On Load Tap Changer (OLTC) operation test & \\ \hline \end{tabular}
\end{table}
Table 1: Test list in transformer test report.  
\[Z_{1} =\frac{1}{2}(Z_{12}+Z_{13}-Z_{23}), \tag{1}\] \[Z_{2} =\frac{1}{2}(Z_{12}+Z_{23}-Z_{13}),\] (2) \[Z_{3} =\frac{1}{2}(Z_{13}+Z_{23}-Z_{12}), \tag{3}\]  
where  
\(Z_{1}\): Leakage impedance of primary side;  
\(Z_{2}\): Leakage impedance of secondary side;  
\(Z_{3}\): Leakage impedance of tertiary side;  
\(Z_{12}\): Per-unit leakage impedance measured from winding 1, with wind 2 shorted and winding 3 open;  
\(Z_{13}\): Per-unit leakage impedance measured from winding 1, with wind 3 shorted and winding 2 open; and  
\(Z_{23}\): Per-unit leakage impedance measured from winding 2, with wind 3 shorted and winding 1 open.  
Based on the calculation method, the value is obtained using Excel as shown in Figure 3. If the values in the red boxes are inputted, the base impedance, equivalent per-unit (pu) impedance, and actual value are calculated. Finally, the values required for the EMTP model are the actual values shown in the green box. The unit is calculated in ohms.  
In Figure 3, the equivalent pu impedance of the tertiary side has a negative value; and, hence, the actual value is calculated as the negative value. This means that the equivalent impedance calculated using the measured results has a negative value. However, the actual leakage impedance value is positive. Since the user cannot input negative impedance in the saturable transformer model in EMTP, the BCRRAN model should be used.