# Efficient Zero-Sequence Impedance Measurement in Autotransformers Using Low-Voltage Excitation
## 5 Conclusions  
This paper addresses the shortcomings of the zero-sequence impedance measurement method for autotransformers as specified in the ANSI/IEEE standard, which is not easy to implement in the field, and proposes a disconnected test method for measuring the zero-sequence impedance of autotransformers in the field with power applied to the low-voltage side. The conclusions can be drawn as follows:  
1. Using the disconnection test to measure the zero-sequence impedance of the transformer to be tested, the test power supply is placed on the low-voltage side of the transformer, reducing the difficulty and complexity of the test and reducing the cost of the test while enabling the field measurement of the zero-sequence impedance of autotransformers. \begin{table}
\begin{tabular}{c c c c} \hline \hline  & **Standard Test** & \multicolumn{3}{c}{**Disconnection Test**} \\  & **Measured Value** & **Test 4** & **Test 5** & **Test 6** \\ \hline \(V_{0}\) & 1926 V & & - & \\ \(3I_{0}\) & 449.9 A & & - & \\ \(V_{ab}\) & - & 384.3 V & 378.4 V & 378.2 V \\ \(V_{Nc}\) & - & 400 V & 400 V & 398 V \\ \(V_{ca}\) & - & 396.2 V & 394.6 V & 383.5 V \\ \(I_{a}\) & - & 41.7 A & 63.3 A & 68.6 A \\ \(I_{b}\) & - & 0 A & 0 A & 0 A \\ \(I_{c}\) & - & 41.5 A & 63.1 A & 68.6 A \\ \(Z_{\rm Hg-0}\) (\%) & 24.8 & 24.9 & - & - \\ \(Z_{\rm Hg-0}\) (\%) & 16.8 & - & 17.1 & - \\ \(Z_{\rm(H|T)g-0}\) (\%) & 17.2 & - & - & 17.5 \\ Error & - & 0.40\% & 1.78\% & 1.74\% \\ \hline \hline \end{tabular}
\end{table} 
Table 6: Two-phase disconnection test on high-voltage side of three-winding autotransformer. \begin{table}
\begin{tabular}{c c c c c} \hline \hline  & **Standard Test** & \multicolumn{3}{c}{**Disconnection Test**} \\  & **Measured Value** & **Test 1** & **Test 2** & **Test 3** \\ \hline \(V_{0}\) & 1926 V & - & - & - \\ \(3I_{0}\) & 449.9 A & - & - & - \\ \(V_{ab}\) & - & 316.4 V & 319.3 V & 321.7 V \\ \(V_{Nc}\) & - & 315.2 V & 317.1 V & 319.4 V \\ \(V_{ca}\) & - & 396.3 V & 396.4 V & 398.2 V \\ \(I_{a}\) & - & 37.5 A & 39.3 A & 41.4 A \\ \(I_{b}\) & - & 42.7 A & 45.2 A & 48.5 A \\ \(I_{c}\) & - & 38.8 A & 38.6 A & 41.8 A \\ \(Z_{\rm Hg-0}\) (\%) & 24.8 & 25.1 & - & - \\ \(Z_{\rm Hg-0}\) (\%) & 16.8 & - & 17.3 & - \\ \(Z_{\rm(H|T)g-0}\) (\%) & 17.2 & - & - & 17.5 \\ Error & - & 1.19\% & 2.97\% & 1.74\% \\ \hline \hline \end{tabular}
\end{table} 
Table 5: One-phase disconnection test on high-voltage side of three-winding autotransformer. According to the proposed method, the test results were simulated and compared to the standard measurement method. Although there were some errors, the overall error was less than 3%, which proved the practicality of the proposed method, which could be used in engineering practice. This paper derives the measurement and calculation formulas for the zero-sequence impedance of two-winding and three-winding autotransformers, and obtains a suitable testing method for the zero-sequence impedance of autotransformers. Finally, the proposed wire breakage test method was used to simulate and verify the dual-winding and three-winding autotransformers. The simulation results showed that the method has high accuracy, with an impedance percentage error of less than 3%. In order to facilitate calculations in the theoretical analysis of the proposed method, a pure inductance model was used for the transformer. Although the zero-sequence impedance value that can be used in engineering practice can be obtained, ignoring the resistance value in the zero-sequence impedance will still have a certain impact on the experimental calculation results. Further research is needed to improve the accuracy of this method. Conceptualization, M.Z., J.F. and N.Z. ; methodology, M.Z. ; software, J.F. and H.W. ; validation, Q.H. and H.H. ; writing--original draft preparation, M.Z. and J.F. ; writing--review and editing, X.L., H.H., M.Z. and Q.H. All authors have read and agreed to the published version of the manuscript.