## 2 Materials and Methods
### Methodology for Online Monitoring of PDs in Tested Power Transformers  
In each of the presented cases of online monitoring of discharges in power transformers, the measurement procedure included the following steps:  
1. [label=Step 0.0]
2. Switching off the power transformer. 3. Installing the antenna (UHF PD detector) in the transformer oil drain valve. 4. Determining the optimal position of the antenna in the oil drain valve. The depth of the antenna was changed in the range of \(\pm 3\) cm relative to the external plane of the tank wall to achieve the minimum, close to unity value of the VSWR (Voltage Standing Wave Ratio). This procedure aimed to adapt the antenna to optimal operating conditions and minimize signal reflection losses. A portable, vector network analyzer, MEASALL KC901S+ (Measall, Chengdu, China), was used to measure the VSWR. 5. Installing the noise-gating antenna near the transformer tank. 6. Installing the eight AE sensors (marked CH1-CH8) on the transformer tank as follows: CH1--on the side wall of the tank, opposite the on-load tap changer; CH2, CH3, and CH4--on the high-voltage side, opposite phases L1, L2, and L3, respectively; CH5, CH6, and CH7--on the low-voltage side, opposite phases L1, L2, and L3, respectively; and CH8--on the side wall of the tank (Figure 3). 7. Verifying the correct operation of individual measurement channels in the acoustic emission (AE) system and proper acoustic and mechanical coupling of AE sensors using the Hsu-Nielsen method (pencil-lead breaks). 8. Determining the level of electromagnetic interference recorded by the noise-gating antenna and the UHF PD detector on the de-energized power transformer. 9. Energizing the power transformer. 10. Determining the average effective value of the acoustic background noise (\(A_{noise\_RMS}\)) recorded by the AE sensors on the energized transformer and the threshold above which the monitoring system will count AE pulses, according to the relationship _threshold_ = \(k\cdot A_{noise\_RMS}\), where \(k\) = 5. \begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Transformer** & **Year of** & **Rated Power** & **Rated Voltage** & **Winding** & **Cooling** & **Test Period** \\
**Designation** & **Manufacture** & **(MVA)** & **(kV)** & **Connections** & **Type** & **Test Period** \\ \hline Transformer A & 1991 & 160 & 230/120/15.75 & YNA0 yN0 & ONAF & 17–24 October 2023 \\ Transformer B & 1978/1996 1 & 160 & 230/120/15.75 & YNA0 yN0 & ONAF & 6–7 November 2023 \\ Transformer C & 1980 & 160 & 230/120 & Y0d11 & ONAF & 23 May–9 June 2024 \\ \hline \hline \end{tabular}  
* [label=Step 0.0]  
\end{table} 
Table 1: Parameters of the tested power transformers. \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Dissolved Gas** & \multicolumn{3}{c}{**Measured Gas Concentration (ppm)**} & \multicolumn{1}{c}{**Typical Gas Concentration**} \\ \cline{2-5}  & **Transformer A** & **Transformer B** & **Transformer C** & **Values (ppm) * \\ \hline Hydrogen (H\({}_{2}\)) & 177 & 291 & 453 & 150 \\ Acetylene (C\({}_{2}\)H\({}_{2}\)) & 79 & 204 & 67 & 55 \\ Carbon dioxide (CO\({}_{2}\)) & – & 3401 & 3216 & 3000 \\ \hline \hline \end{tabular}  
* [label=Step 0.0]  
* Typical gas concentration thresholds set by the power grid operator to indicate proper transformer operation.  
\end{table} 
Table 2: Concentrations of selected gases dissolved in oil in the tested power transformers. Step 10. Determining the level of electromagnetic interference recorded by the noise-gating antenna and UHF PD detector on the energized transformer. * 1. Leaving the online PD monitoring systems in operation for the period specified by the transformer owner (from several days to several weeks), during which parameters of acoustic signals (number of AE pulses per minute) and radio signals (number and amplitude of UHF and PRPD patterns) were recorded. * 1.