## 3 Results and Discussion
### Comparison of the Frequency Responses through the Statistical Parameters
If this PD source is near the other end of the winding (near tappings t4 or t5), diagnosis is still possible at 37.5 kV, by reversing the test connections. In such a case, the impulse can be applied at '1 V', and the response voltage can be measured the terminals '1U and 1W', by grounding them through the 245 \(\Omega\) resistor. For the PD sources requiring still higher voltages for inception, the diagnosis may not be possible with 37.5 kV level, and the test voltage may need to be increased further, gradually up to the Basic Impulse Insulation Level (BIL). Even for such PD sources, the IFRA based approach is advantageous, as it can additionally cover the diagnosis of a wide range of faults, like mechanical displacements, core magnetization issues, and inter-turn shorts. The pros and cons of \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline \multicolumn{6}{c}{**Statistical Parameter Values Calculated during Different Comparisons**} \\ \hline Frequency & \multirow{2}{*}{Comparisons made for the purpose of the diagnosis of Inter-turn shorts and PD sources} & \multicolumn{2}{c}{PD sources within major insulation} \\  & & & \multicolumn{2}{c}{minor insulation} & \multicolumn{2}{c}{major insulation} \\ \hline  & NC & NC & NC & NC & NC & NC \\  & vs. & vs. & vs. & vs. & vs. & vs. \\  & Sh-1 & Sh-2 & PD-1-Minor & PD-2-Minor & PD-3-Major & PD-4-Major \\ \hline \hline \multicolumn{6}{c}{CSD} \\ \hline
1 kHz-10 kHz & 8.84 & 9.97 & **11.06** & 10.90 & 8.69 & 10.38 \\
100 kHz-1 MHz & 5.26 & 6.29 & 6.57 & 9.97 & 7.41 & 4.67 \\
1 MHz-10 MHz & **12.39** & 8.86 & 9.98 & **15.91** & **13.15** & **13.80** \\ \hline \multicolumn{6}{c}{DABS} \\ \hline
1 kHz-10 kHz & 5.38 & 6.70 & **10.69** & 9.55 & 6.88 & 7.05 \\
100 kHz-1 MHz & 3.87 & 4.36 & 6.46 & 7.75 & 6.20 & 5.24 \\
1 MHz-10 MHz & **8.68** & **7.48** & 8.46 & **13.67** & **9.82** & **9.35** \\ \hline \multicolumn{6}{c}{MM RATIO (absolute)} \\ \hline
1 kHz-10 kHz & 0.65 & **0.52** & **0.45** & **0.41** & **0.55** & **0.53** \\
100 kHz-1 MHz & 0.87 & 0.86 & 0.80 & 0.75 & 0.80 & 0.84 \\
1 MHz-10 MHz & **0.55** & 0.64 & 0.64 & 0.46 & 0.57 & 0.60 \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Comparison between the normal and various faulty cases on statistical parameter basis the proposed approach need to be further explored and compared with other PD diagnostic techniques before generalization. Based on the severity of suspected faults, the following sequence of steps can be suggested for the experimental investigation, so that IFRA can be extended, even for some preliminary diagnosis of sources of PDs. * Step1: Get/Develop (i) the signature pattern FRA, which can represent the transformer in its fault-free condition (Normal (or) Healthy condition) and (ii) Get the BIL value of the winding under investigation \(V_{BIL}\). * Step 2: Start the FRA investigation at a low voltage level (for example, at 5% BIL of the tested winding). * Step 3: Compare FRA results with the signature FRA. * Step4: If there are abnormalities, try to correlate with FRA guidelines/standards, declare the type of the fault (for example \"inter-turn fault\") and,'stop'. * Step 5: If no abnormalities are observed in FRA, increase the test voltage magnitude by (\(\triangle\)V) such that \(V_{New}=V_{Old}+\triangle\)V. (For example, \(\triangle\)V can be set at 5% of BIL)
* Step 6: Compare \(V_{New}\) with \(V_{BIL}\). If \(V_{New}<V_{BIL}\), go to'step 7'. If \(V_{New}\geq V_{BIL}\), go to'step 11'. * Step 7: Repeat FRA investigations. Observe voltage waveforms in the time domain and develop corresponding FRA Plot. * Step 8: Compare the FRA plot at the new test voltage \(V_{New}\), with the Signature FRA plot. While comparing, check whether there are any abnormalities like: (i) additional contributions at high-frequency components in the FRA Plot. (Additionally, traces of'very short duration pulses' on the impulse voltage waveforms can also be observed). * Step 9: If 'Yes', (abnormalities referred in'step 8' are present) declare: \"A potential source of PD may be present!