## 3 Tests of the Developed PD Monitoring System on Power Transformers
### Test of the System on 31.5 MVA Power Transformer  
The next object, on which the PD monitoring system was tested was a transformer with a rated power of 31.5 MVA and voltage of 115/33 kV. Before the prototype system installation, the localization of PD sources was carried out by the trilateration technique. For this purpose, eight piezoelectric AE sensors type PAC R15\(\alpha\) and 8-channel AE system type DiSP (Physical Acoustic Corporation, Princeton, NJ, USA) were used. The coordinates of the AE sensors mounted on the transformer tank of dimensions \(4\times 1.3\times 2.6\) m are listed in Table 5. \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Sensor Index** & **X (m)** & **Y (m)** & **Z (m)** \\ \hline
1 & 1.20 & 1.40 & 1.30 \\
2 & 2.05 & 0.45 & 1.30 \\
3 & 3.05 & 1.40 & 1.30 \\
4 & 4.00 & 1.10 & 0.65 \\
5 & 3.00 & 0.75 & 0.00 \\
6 & 2.00 & 0.45 & 0.00 \\
7 & 1.06 & 1.05 & 0.00 \\
8 & 0.00 & 0.95 & 0.65 \\ \hline \hline \end{tabular}
\end{table} 
Table 5: Coordinates of AE sensors. Figure 28: Exemplary UHF PD pulses registered for different voltage levels during the IVPD test: (a) 284 kV, (b) 374 kV, (c) 473 kV. The amplitude of the registered signals was relatively low and equal to 30-45 dB ref 100 \(\mu\)V (peak at around 66 dB). The results of the localization obtained with the trilateration technique showed that in the investigated power transformer there are two sources of partial discharges. The first source was detected on the high voltage side (115 kV), between phases L1 and L2 (coordinates: x = 1.9 m, y = 1.0 m, z = 1.25 m). In turn, the second PD source was detected on the low voltage side (33 kV), at the point of LV connections (coordinates: x = 1.3, y = 0.1 m, z = 1.5 m). The graphic results of the PD source localization are presented in Figure 29.  
After performing the measurements which showed the existence of partial discharge sources, the works preceding the prototype system installation were carried out. First, the power transformer was turned off and disconnected from the HV transmission lines. Then, the oil excess was drained to  
Figure 29: Graphic presentation of the localization results of two partial discharges sources by the trilateration technique and the distribution of the eight AE sensors (blue dots). the level enabling opening the revision hole in the upper tank cover. After opening the revision hole, the active dielectric windows and the remaining elements of the ISO-002-ADW/UHF sensors were immediately installed, in order to minimize the time of contact of the oil with humid air (Figure 30). The investigated power transformer was equipped in two sensors ISO-002-ADW/UHF and one sensor ISO-002-HF installed on the grounding wire of the neutral point of the HV winding (Figure 31). The optic fiber and power supply cables were protected by putting them into the corrugated PCV pipes with a diameter of 25 mm. A waterproof enclosure of the monitoring system with data concentrator module MPD-001, server, and power supply was placed opposite to the power transformer. The last stage of the installation works was configuring the data transmission between the PD monitoring system and SCADA substation system type SYNDIS ES (Mikronika, Poznan, Poland), which registered basic operation parameters, such as voltage, load, and oil temperature. After filling the power transformer tank with oil and checking the tightness of revision holes, the power transformer was turned on and the partial discharges monitoring system was started. Figure 32 presents the characteristics of PD intensity (pulse rate per 15 s), active power, voltage, and top-oil temperature obtained within 8-days-operation of the PD monitoring system. Figure 31: Photograph of the PD on-line monitoring system installed on 31.5 MVA substation power transformer. Figure 30: The installation of the active dielectric windows with UHF antennas (ISO-002-ADW/UHF-1 and ISO-002-ADW/UHF-2) in the revision holes of the upper cover of the 31.5 MVA power transformer tank.