# Interpretation of Frequency Response Analysis for Fault Detection in Power Transformers
## 5 Case Study: Short Circuit Fault  
For examining the feasibility of the proposed flowchart in Figure 9, laboratory testing was conducted on an 11/0.433 kV, 500 kVA transformer, as shown in Figure 10. The transformer windings were lifted from the main tank to access the tap changer. A short circuit fault in the winding was then created by shorting tap terminals 2-7 in-phase (R), as shown in Figure 11. A turns ratio test was performed on the transformer before and after applying the fault. The results of the measured V-ratio are presented in Table 5.  
The FRA measurement was performed using a commercial frequency response analyzer on the tested winding open-circuit configuration before and after applying the short circuit fault, as shown in Figure 12. The fault causes shifting of the response at the low and mid-frequency regions toward higher frequencies, which is attributed to the reduction in the inductive components due to the shorted turns. Similarly, within the low and mid-frequency regions, the magnitude increases due to reduced inductance from the shorted turns. These changes align well with the variations listed in Table 4. \begin{table}
\begin{tabular}{l c c c c c c c} \hline \hline \multirow{2}{*}{**Faults**} & \multirow{2}{*}{**Test Type**} & \multicolumn{4}{c}{**Magnitude**} & \multicolumn{4}{c}{**Shifting Frequency**} \\ \cline{3-8}  & & **LF** & **MF** & **HF** & **LF** & **MF** & **HF** \\ \hline Radial winding deformation & Short circuit test & - & - & - & - & - & - \\ Axial winding deformation & Short circuit test & - & - & - & - & - & - \\ Bulk winding deformation & Open circuit test & - & - & - & - & - & - \\ Core defects & Open circuit test & \(\downarrow\) & \(\downarrow\) & - & - & - & - \\ Higher contact resistance & Open circuit test & - & \(\uparrow\) & \(\uparrow\) & - & - & - \\ Short circuit fault & Short circuit test & \(\uparrow\) & \(\uparrow\) & - & \(\rightarrow\) & - & - \\ Inter-disc fault & Short circuit test & - & \(\uparrow\) & \(\uparrow\) & \(\uparrow\) & - & - \\ Open circuit winding & Open circuit test & - & \(\downarrow\) & \(\downarrow\) & - & - & - \\ Winding looseness & Open circuit test & - & - & \(\uparrow\) & - & - & - \\ Floating shield with local insulation & Open circuit test & - & - & \(\downarrow\) & - & - & - \\ Presence of oil & Open circuit test & - & - & - & - & - & - \\ Measurement direction & Open circuit test & - & - & \(\downarrow\) & - & - & - \\ Effect of high temperature & Open circuit test & - & - & - & - & - & - \\ Loss of clamping pressure & Short circuit test & - & - & \(\downarrow\) & - & - & - \\ Tap changer coking & Open circuit test & \(\downarrow\) & - & - & - & - & - \\ Tap changer pitting & Open circuit test & - & - & - & - & - & - \\ Winding insulation degradation & Open circuit test & - & - & - & - & - & - \\ Clamping structure damage & Open circuit test & \(\downarrow\) & - & - & - & - & - \\ High voltage bushing fault & Short circuit test & - & - & \(\uparrow\) & - & - & - \\ Increasing moisture content & Short circuit test & - & - & - & - & - & - \\ Bad measurement practice & Open and short test & \(\uparrow\) & \(\uparrow\) & \(\uparrow\) & \(\uparrow\) & \(\uparrow\) & - \\ \hline \hline \end{tabular}  
* No changes; \(\downarrow\): Decrease; \(\uparrow\): Increase; \(\leftarrow\): Shifting towards lower frequencies; \(\rightarrow\): Shifting towards higher frequencies.
* No variation.  
\end{table} 
Table 4: Characteristics of FRA signature due to transformer faults. The CC between the FRA signatures for normal and short circuit faults was calculated to quantify the fault severity, and the result is shown in Table 6. Based on the proposed CC margins given in Figure 8 and Table 2, the results show a small variation at the low-frequency region, large variation in the mid-frequency region, and no variation in the high-frequency region. This is consistent with the short circuit fault identification using the proposed flowchart in Figure 9.  
Figure 11: Measurement setup at normal and short circuit fault conditions.