# Interpretation of Frequency Response Analysis for Fault Detection in Power Transformers
## 4 Proposed Fault Identification Guidelines  
The most common transformer faults and their impacts on various frequency bands based on extensive literature studies are listed in Table 3. Only end-to-end open and short circuit tests are considered in the study, as these are the most common essential test configurations as per the IEEE and IEC standards. Each of the faults listed in Table 3 has a distinct effect on the transformer frequency response. For example, core defects show a large variation at the low-frequency region, small variation at the mid-frequency region, and no variation at the high-frequency region. \begin{table}
\begin{tabular}{l l l c c c c} \hline \hline \multicolumn{1}{c}{**NO**} & \multicolumn{1}{c}{**Sensitivity to Fault**} & \multicolumn{1}{c}{**Test Type**} & \multicolumn{1}{c}{**LF**} & \multicolumn{1}{c}{**MF**} & \multicolumn{1}{c}{**HF**} & \multicolumn{1}{c}{**Reference**} \\ \hline
1 & Radial winding deformation & Short circuit test & No & Small & Large & [1,3] \\
2 & Axial winding deformation & Short circuit test & No & Small & Small & [1,3,38,39] \\
3 & Bulk winding deformation & Open circuit test & No & Large & No & [1,36,3] \\
4 & Core defects & Open circuit test & Large & Small & No & [1,3,36] \\
5 & Higher contact resistance & Open circuit test & No & Small & Small & [1,36] \\
6 & Short circuit fault & Short circuit test & Small & Large & No & [1,2,36] \\
7 & Inter-disc fault & Short circuit test & No & Small & Large & [22] \\
8 & Open circuit winding & Open circuit test & No & Large & Small & [1,39] \\
9 & Winding looseness due to transportation & Open circuit test & No & Small & Large & [1] \\
10 & Floating shield with local insulation & Open circuit test & Small & Small & Large & [1,2] \\
11 & Presence of oil & Open circuit test & No & Large & Large & [2,40,41] \\
12 & Measurement direction & Open circuit test & No & No & Large & [3] \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Effect of various transformer conditions on three frequency regions of the FRA signature. Figure 8: Example of variation between the measured and reference frequency responses: (**a**) small variation and (**b**) considerable or large variation. The proposed flowchart shown in Figure 9 is constructed based on Table 3. This flowchart can be adopted to determine the possible type of fault when comparing the FRA measurements at different frequency regions. The flowchart may indicate an \"undefined\" condition for certain frequency response variations that have not been recorded previously. Table 4 provides additional information for FRA interpretation. It shows the change in the frequency response due to various faults. The proposed table describes how each  
Figure 9: The proposed FRA interpretation flowchart. \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline
**NO** & **Sensitivity to Fault** & **Test Type** & **LF** & **MF** & **HF** & **Reference** \\ \hline
13 & Effect of temperature & Open circuit test & Small & Small & Small & [3,41] \\
14 & Loss of clamping pressure & Short circuit test & No & No & Small & [13] \\
15 & Tap changer cooking & Open circuit test & Large & No & No & [42] \\
16 & Tap changer pitting & Open circuit test & Small & Small & Small & [42] \\
17 & Winding insulation degradation & Open circuit test & No & No & Large & [43,44] \\
18 & Clamping structure damage & Open circuit test & No & Small & No & [36] \\
19 & High voltage bushing fault & Short circuit test & No & Small & Small & [5,39,40,44] \\
20 & Increasing moisture content & Short circuit test & Small & Small & Small & [41,43,45] \\
21 & Bad measurement practice & Open and short test & Large & Large & Large & [3] \\
22 & No faults & Open and short test & No & No & No & Obvious \\ \hline \hline \end{tabular}
\end{table} 
Table 3: \(Cont.\)fault type impacts the frequency and magnitude by using a simple arrow, the direction of which indicates the deviation of the measured response compared to the reference FRA signature.