## 3 Case Study Analysis of Online PD Monitoring in Power Transformers
### Transformer C  
#### 3.3.1 Results of Online Monitoring of Partial Discharges  
Figure 13 shows the results of two weeks of monitoring conducted on 'Transformer C' from 23 May 2024 to 6 June 2024. Three hours after the transformer was switched into operation in measurement channel CH7 (with the transducer positioned opposite the L2 phase on the lower voltage side), the acoustic emission system suddenly began recording stable pulses of moderate intensity, registering 1079 pulses per minute--approximately 14% of the channel's capacity. From that moment, that is, from 23 May, 6:23 p.m., to 24 May, 2:15 p.m., the system practically continuously recorded AE pulses of variable intensity (from 0.2 to 21%). The periods in which the AE system and the UHF system recorded pulses simultaneously overlapped by up to 85.7%, which allows us to assume that their source was PDs. The maximum relative amplitude of the UHF pulses was 45.3% during this period. The next periods in which the CH7 transducer recorded pulses occurred in the intervals from 25 May, 2:55 a.m. to 12:17 p.m., and from 26 May, 2:42 a.m., to 27 May, 4:01 a.m. Activity was also recorded in the CH4 measuring channel (transducer placed opposite the L3 phase on the high-voltage side) in the time interval from 25 May, 3:06 p.m., to 27 May, 2:32 a.m. On 27 May from 1:20 to 1:48 pm, a very high (up to 93%) intensity of AE pulses was recorded in the eight measuring channels. The UHF monitoring system also recorded PD pulses at that time with a maximum amplitude of 79%. One of the possible causes of the sudden ignition of PD could be a switching overvoltage in the transformer field or an atmospheric overvoltage, as numerous lightning discharges occurred during this period, which is confirmed by the map shown in Figure 14.  
Figure 12: (**a**) Photograph of a fragment of the ‘Transformer B’ tank, showing a Cartesian coordinate system applied for PD source localization using the TDoA method, with 18 measurement points marked where AE transducers were installed, as well as the solution area (red rectangle); (**b**) representative AE waveforms recorded at measurement points P1, P2, P3, and P4, with the onset times of the pulses marked by red lines; and (**c**) a graphical representation of the PD source localization results. Figure 13: Results of online PD monitoring for ‘Transformer C’ recorded between 23 May and 6 June 2023: amplitude and count of UHF pulses, along with the count of acoustic emission pulses detected by eight AE sensors. The PRPD pattern for UHF pulses (Figure 15), recorded at that time, was characteristic of a defect: carbonized tracks at the leads of the winding [38]. After 13:48, acoustic emission pulses were still recorded by the CH4 transducer only, which monitored the L3 phase from the HV side, with the pulse intensity decreasing and not exceeding the level of 39%. The pulses were continuous, undying, and disappeared completely after only 5 days, i.e., on 1 June at approximately 22:00. In the remaining phases on the high-voltage side (L1 and L2), no AE pulses from PD were recorded apart from single events. On the low-voltage side, a high intensity of AE pulses (up to 30%) was detected in phase L2. During the 2 weeks of monitoring system operation, discharges were recorded daily, with the highest activity occurring on 26, 30, and 31 May and on the day of system dismantling (6 June). Therefore, immediately after completing the monitoring, a procedure was carried out to locate the source of partial discharges in phase L2 on the low-voltage side. Figure 14: Map of lightning discharges recorded on 27 May between 12:00 and 18:00 (total: 7333 discharges), with the location of the tested power transformer highlighted. Data source: [[https://burzowo.info/archiwum](https://burzowo.info/archiwum)]([https://burzowo.info/archiwum](https://burzowo.info/archiwum)) (accessed on 18 January 2025). Figure 15: PRPD pattern of UHF PD pulses recorded on 27 May 2024, at 1:46 p.m., in the tested Transformer C’.