## 4 Conclusions  
This article analyzes online monitoring cases of PDs in three large power transformers using both the AE and UHF methods simultaneously. The transformers were selected for testing by the power system operator due to exceeding the typical concentration of hydrogen and acetylene dissolved in oil, indicating the potential presence of high-energy PDs. On the basis of the measurement results and statistical analyses obtained, the following conclusions were formulated:  
* Simultaneous use of AE and UHF methods increases the effectiveness and reliability of PD detection. These methods are based on different physical phenomena (AE method records elastic waves; UHF method records electromagnetic pulses) and are susceptible to varying sources of interference. A high percentage of cases in which both methods simultaneously recorded pulses allowed us to confirm with high probability the presence of real PDs and not acoustic or radio interference. Statistical analyses \begin{table}
\begin{tabular}{c c c} \hline \hline \multicolumn{3}{c}{**The Range of Determined Coordinates of the PD Source**} \\ \hline \(X\) (m) & \(Y\) (m) & \(Z\) (m) \\ \hline
0.85–1.01 * & 0.05–0.95 * & –0.07–0.29 * \\ \hline \multicolumn{3}{c}{**The Modal Value of the PD Source Coordinates**} \\ \hline \(X\) (m) & \(Y\) (m) & \(Y\) (m) \\ \hline
0.96 & 0.76 & 0.11 \\ \hline \end{tabular}  
* The reported data ranges were determined after excluding outliers.  
\end{table} 
Table 11: Ranges and modal values of the \(X\), \(Y\), and \(Z\) coordinates for the estimated location of the PD source in the tested ‘Transformer C’. Figure 19: PD source location in ‘Transformer C’: (**a**) area on the tank showing the calculated range of X and Z coordinates for the PD source; (**b**) a photograph of a sister unit showing the low-voltage winding lead (marked with a red rectangle), the position of which aligns with the calculated coordinates of the PD source. showed that the agreement of the results for 'Transformer A' and 'Transformer B' was 65.6% and 72.8%, respectively, while for 'Transformer C', it was lower (43%). * Monitoring PDs using the UHF method with the function of recording PRPD patterns allows for identifying the type of insulation system defect. For all the tested transformers, characteristic PRPD patterns were recorded, indicating the occurrence of PDs. * Using a multi-channel AE system allows for the localization of defects in individual transformer phases. This allowed one to determine the potential location of discharge sources already at the monitoring stage. For 'Transformer A', AE pulses were recorded near the on-load tap changer; for 'Transformer B', in phase L3 on the high-voltage side; and for 'Transformer C', in phase L2 on the low-voltage side. The data obtained significantly accelerated the procedure for the precise location of the discharge sources using the TDoA technique. The discrepancy in results between the AE and UHF methods may be due to several factors: (i) the presence of interference specific to the given method (e.g., AE may record signals generated by rainfall, which do not affect UHF); (ii) incorrectly selected pulse detection threshold (too low of a threshold causes recording of interference; too high of a threshold prevents the detection of low-energy discharges); (iii) the presence of so-called'silent discharges', which generate negligible elastic waves and may not be detected by the AE method (an example of a silent PD is the formation of carbonization trees on the surface of the pressboard [45]); and (iv) the specific location of the PD source or its large distance from the PD detectors (antenna UHF or AE sensor). Metal structural elements of the transformer (windings, cores, and tank walls) may block the UHF signal, and solid insulation (pressboard barriers, spacers, and strips) may dampen elastic waves generated by PDs.