## 2 Methodology
### Experimental Procedure  
Throughout the investigation (i.e., during the Normal case, inter-turn short cases and the four PD cases), the impulse generator available in the laboratory was used for injecting the switching impulse voltage as a test signal (input) at the '1U' terminal of the high voltage winding (1U-1V). The voltage that was received at the other end (1V) of the winding was observed as the response. The switching impulse voltage generator is of standard design. It can develop a 250 us/2500 us switching impulse with a peak value up to 140 kV, with the tolerances for front, tail & peak values within the permissible tolerances of the standards [29; 30] C1 & C2 values are of 25,000 pF & 1200 pF, respectively. The wave shaping resistors R1 & R2 are 43 k\(\Omega\) & 98 k\(\Omega\), respectively. As the impulse generator is of low capacity (rating: 0.245 kJ, 140 kV, single-stage with a 1200 pF terminal capacitor), the loading effects of the transformer on impulse generator is unavoidable. The wave shape of the input switching impulse voltage and magnitudes of its different frequency components get altered [29; 30]. FRA is on analyzing the frequency response over a wide frequency spectrum and, not on testing the dielectric integrity with standard impulses, such alterations in the  
Figure 4: Photograph of the experimental setup. Figure 3: Circuit for the diagnosis of the source of PD across the minor. shape of the impulse is acceptable. FRA requires the presence of a broad spectrum of frequency components within the test signal. In our case, this distorted switching impulse contained more high-frequency components than the standard switching impulse and enabled the frequency response analysis up to 10 MHz. It can be noted that the effect of the fault can be felt only when it alters the impedance that is offered by the tested winding. Inter-turn shorts can influence on the impedance of the winding at all voltage levels and, therefore, detection of the same is possible, even with low voltage switching impulse. Therefore, in the present work, the experimental work for diagnosis of inter-turn shorts (Sh-1 and Sh-2) were conducted while using the switching impulses of 5 kV peak magnitudes. However, the influence of a potential PD source present in the insulation is found to be voltage-dependent. At low voltages, the frequency response is not getting altered just by the presence of a PD source. The presence of a potential source of PD can influence the effective impedance of the winding only when it offers a low ohmic discharge path across the by-passed portion of the insulation. As long as the switching impulse voltage is below the inception level of PD, the potential sources of PDs offer high ohmic path and, hence, their loading effects are negligible. In our case, the PD was modelled with 2 mm oil gap in series with a 2400 \(\Omega\) resistance, so that its ohmic opposition is very high during its discharge-free condition. For conducting the experimental investigations, the PD model was connected to the transformer by using the tappings t1, t2, and t3. Thus, the presence of the potential sources of the four PD cases (PD-1-minor, PD-2-Minor, PD-3-major, and PD-4-major) was artificially achieved, one-by-one, as referred in Table 1.  
For understanding the loading effects of the PD model at different voltage levels, preliminary tests were conducted for all these four cases by varying the peak voltage of the input (switching impulse) from 5 kV up 40 kV, in steps of 5 kV. The input and response voltage waveforms were captured at all of these voltage levels along with a check for any visual discharge across the oil gap. Rough comparisons were made between these different cases by observing their corresponding input and output waveforms and their frequency components. Discharges across the PD model were found to incept at four different voltages for the four different cases emulated. But, in all the four cases, the inception happened only above 30 kV, and there were differences in waveforms and their frequency components.