## 3 Case Study Analysis of Online PD Monitoring in Power Transformers
### Transformer A
#### 3.1.2 Results of Partial Discharge Source Localization  
Based on information obtained from the AE monitoring, it was determined that the PD source was most likely located near the on-load tap changer. Therefore, the localization procedure was confined to this area (the side wall of the transformer tank). To pinpoint the PD source using the TDoA technique, pulses were recorded using four AE transducers in three different configurations of their positions, referred to as Test 1, Test 2, and Test 3. A common Cartesian coordinate system, shown in Figure 6, was adopted for all tests.  
The coordinates describing the positions of the acoustic emission AE transducers for each measurement test are provided in Table 4. Figure 7 shows exemplary time waveforms of acoustic emission pulses, with the signal onsets marked by a red line, which were automatically estimated using the AIC method. Based on the recorded time series, the coordinates of the discharge source locations were calculated for each test. The graphical representation of the obtained results is shown in Figure 8, while Table 5 presents the ranges and modal values (most frequently occurring values) of the calculated PD source coordinates.  
\begin{table}
\begin{tabular}{c c c} \hline  & **AE: Pulse (1)** & **AE: No Pulse (0)** \\ \hline
**UHF: Pulse (1)** & TP = 59.2\% & FP = 16.4\% \\
**UHF: No Pulse (0)** & FN = 17.9\% & TN = 6.4\% \\ \hline \end{tabular}
\end{table}
Table 3: Confusion matrix for pulses recorded by UHF and AE methods in ‘Transformer A’.  
Figure 6: Photographs of the tank of ‘Transformer A’ with installed AE sensors and the Cartesian coordinate system adopted for PD source localization using the TDoA technique: (**a**) Test 1, (**b**) Test 2, and (**c**) Test 3.  
\begin{table}
\begin{tabular}{c c c c c} \hline \hline \multirow{2}{*}{**Test**} & \multirow{2}{*}{**AE Sensor**} & \multicolumn{3}{c}{**AE Sensor Coordinates**} \\ \cline{3-5}  & & **X (m)** & **Y (m)** & **Z (m)** \\ \hline \multirow{4}{*}{Test 1} & CH1 & 1.130 & 0 & 0.985 \\  & CH2 & 1.580 & 0 & 0.700 \\  & CH3 & 1.140 & 0 & 0.710 \\  & CH4 & 1.630 & 0 & 0.980 \\ \hline \multirow{4}{*}{Test 2} & CH1 & 1.130 & 0 & 0.985 \\  & CH2 & 1.580 & 0 & 0.700 \\  & CH3 & 1.250 & 0 & 0.360 \\  & CH4 & 0.560 & 0 & 0.720 \\ \hline \multirow{4}{*}{Test 3} & CH1 & 1.130 & 0 & 0.985 \\  & CH2 & 0 & 0.590 & 0.895 \\ \cline{1-1}  & CH3 & 1.140 & 0 & 0.700 \\ \cline{1-1}  & CH4 & 0 & 0.790 & 0.750 \\ \hline \hline \end{tabular}
\end{table}
Table 4: The coordinates and designations of the AE transducers used in the individual measurement tests (i.e., Test 1, Test 2, and Test 3).  
Figure 8: Graphical representation of PD source localization results obtained from individual measurement tests on Transformer A\({}^{\wedge}\): (**a**) Test 1, (**b**) Test 2, and (**c**) Test 3.  
Figure 7: AE time waveforms with the pulse onset marked (red line) and estimated using the AIC method and recorded during measurement tests on ‘Transformer A’: (**a**) Test 1, (**b**) Test 2, and (**c**) Test 3.  
The tests carried out using the TDoA technique confirmed that the most probable source of PD pulses is a defect occurring inside the on-load tap changer or its immediate vicinity, e.g., in the tap lead insulation. Upon completion of the monitoring and localization of PD, the area corresponding to the range of defect coordinates was marked on the tank of the tested transformer for the operational services of the power grid operator, while the modal value, i.e., the most frequently obtained solution, was indicated with an '\(\times\)' (Figure 9).