## 3 Case Study Analysis of Online PD Monitoring in Power Transformers
### Transformer B  
#### 3.2.1 Results of Online Monitoring of Partial Discharges  
For 'Transformer B', the transmission system operator implemented PD monitoring following the dissolved gas analysis (DGA) results, which revealed an elevated acetylene concentration of 204 ppm. This finding indicated the presence of high-energy, high-temperature PDs of the sparking type, often described as small, localized arcs. Given the nature of the detected gas and the potential for rapid progression of the defect, a decision was made to conduct a short-term PD monitoring campaign limited to 24 h. The restriction of the monitoring period was primarily motivated by safety considerations and the necessity to mitigate the risks associated with operating a transformer in a pre-failure state. The online PD monitoring systems were activated on 6 November 2023, at precisely 2:02 p.m. An analysis of the measurement results, presented in Figure 10, indicated that the UHF system did not detect any PD pulses until 6:05 a.m. the following day. During this period, the acoustic emission (AE) system recorded two brief intervals of AE pulse activity (from 3:52 p.m. to 4:05 p.m. and from 4:31 p.m. to 4:46 p.m.), both characterized by low intensity (<5%), as well as a single event occurring at 2:22 a.m. the next day. \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) & \(Y\) (m) \\ \hline
0.32–0.70 & 0.28–0.77 & 0.63–1.03 \\ \hline \multicolumn{3}{c}{**The Modal Value of the PD Source Coordinates**} \\ \hline \(X\) (m) & \(Y\) (m) & \(Y\) (m) \\ \hline
0.65 & 0.33 & 0.70 \\ \hline \hline \end{tabular}
\end{table} 
Table 5: Ranges and modal values of the \(X\), \(Y\), and \(Z\) coordinates for the estimated location of the PD source in the tested ‘Transformer A’. Figure 9: Photographs of the tank of the tested ‘Transformer A’ with marked ranges and modal values (indicated by lines and ‘\(\times\)’ symbols, respectively) of the PD source coordinates. Figure 10: Results of online PD monitoring for ‘Transformer B’: amplitude and count of UHF pulses, along with the count of acoustic emission pulses detected by eight AE sensors. The acoustic emission pulses were primarily registered by transducers located on the low-voltage side (CH5, CH6, and CH7) and near the tap changer (CH1). The CH3 transducer continuously recorded sporadic pulses with an intensity below 0.8% throughout the monitoring period, probably due to setting the pulse counting threshold too low. This transducer was located near the radiator and fans, which are potential sources of acoustic interference. Starting at 6:05 a.m. on November 7, both the UHF and AE systems began detecting stable PD pulses, with a sudden and significant increase in their intensity and amplitude observed at 10:35 a.m. The PRPD patterns for UHF pulses, as recorded by the Qualitrol PDM system, were automatically classified as PDs. As illustrated in Figure 11, the concentration of pulses in the first (0-90\({}^{\circ}\)) and third (180-270\({}^{\circ}\)) quadrants of the supply voltage cycle is characteristic of surface and creeping discharge or certain types of void discharge within gas inclusions [44]. Based on these observations, it was determined to discontinue PD monitoring via the acoustic emission method and proceed with the localization of PD sources. To summarize the results obtained by both diagnostic methods, the confusion matrix (Table 6) was calculated. The values obtained show that both methods simultaneously detected PD pulses in only 5.9% of the cases (TP), which means low agreement on the scope of simultaneous registration of PD pulses. In 9.6% of the cases, the UHF method indicated the presence of PD pulses (FP), while the AE method did not detect them. In turn, 17.6% of the cases were situations in which only the AE method registered pulses (FN). It should be noted that up to 66.9% of cases (TN) were situations in which both methods did not detect PD pulses.