## 3 Case Study Analysis of Online PD Monitoring in Power Transformers  
### Transformer A  
#### 3.1.1 Results of Online Monitoring of Partial Discharges  
Figure 4 presents the results of UHF signal and acoustic emission (AE) recordings obtained simultaneously by eight AE transducers mounted on the tank of the tested transformer. Both systems were activated on 17 October 2023, at 3:00 p.m., and from the moment of activation, a high intensity of PD was observed. In the case of PD testing using the AE method, the maximum number of pulses recorded by the CH1 transducer, positioned near the on-load tap changer, was 4660 pulses per minute, representing approximately 60% of the measurement channel capacity. During the first two days of operation of the monitoring system, moderate intensity AE pulses were also recorded by the CH5 transducer, installed near the L1 phase on the low-voltage side. The remaining AE transducers either did not register any pulses or recorded pulses of low intensity. In the case of PD testing using the UHF method, the system recorded numerous pulses with very high amplitudes, reaching 90% of the maximum value of the measurement range. Periods of high and moderate AE and UHF pulse activity predominated in the monitored transformer, while instances of complete signal loss were observed extremely rarely. It is noteworthy that the periods of highest AE and UHF pulse activity largely overlapped, suggesting that their common source was a defect in the insulation system generating PDs. Figure 4: Results of online PD monitoring for ‘Transformer A’ recorded between 17 and 21 October 2023: amplitude and count of UHF pulses, along with the count of acoustic emission pulses detected by eight AE sensors. As mentioned earlier, PD activity depends on various factors, such as voltage fluctuations, including sudden step increases caused by atmospheric and switching overvoltages, the moisture level in the insulation system, and bidirectional moisture migration between the cellulose insulation and oil, induced by temperature changes (water accumulation at the oil-cellulose interface can lead to the initiation of surface discharges). Additionally, the temperature of the insulation system is influenced by factors such as the transformer's variable load, ambient temperature, and the efficiency of the cooling system. As a result of these factors, PDs exhibit characteristics of a stochastic phenomenon, making them difficult to predict. Therefore, in the authors' view, if the transformer testing program includes the localization of discharge sources, this should be carried out immediately upon detection of high and stable AE and UHF pulse activity by the monitoring system. Such a situation occurred in the analyzed case of 'Transformer A'. On the second day of monitoring, from the early morning hours, a stable, high level of UHF and AE pulses was observed. As mentioned previously, the CH1 transducer, installed near the on-load tap changer, recorded the highest acoustic emission activity. The PRPD images (Figure 5) recorded during this period exhibited features indicative of PDs, including a clear correlation with the supply voltage phase and a repetitive pattern of pulse amplitude and count within specific phase intervals (0-90\({}^{\circ}\) and 180-270\({}^{\circ}\)). Importantly, the Qualitrol PDM system software also identified the PRPD pattern as originating from PDs. Based on these data, at 10:30, a decision was made to discontinue monitoring using the acoustic emission method and initiate PD source localization, which continued until 12:30. Throughout the fault localization procedure, the UHF monitoring system continued to record high PD activity. For further statistical analysis to compare the performance of both diagnostic methods in 'Transformer A', the data presented in Figure 4 (number of UHF and AE pulses) were converted to a binary form (pulse detected = 1; no pulses = 0).