## 3 Tests of the Developed PD Monitoring System on Power Transformers  
### Test of the Monitoring System During Induced Voltage Test of 330 MVA Power Transformer  
After successfully finished laboratory tests, it was decided to check the effectiveness of the system operation on a real power transformer. First such measurement was carried out during the induced voltage test with partial discharge measurement (IVPD) on a power transformer of 330 MVA. The tank of the investigated transformer, already at the production stage, was equipped in two ceramic dielectric windows placed in side-walls of the tank, i.e., near the L1 phase (Figure 26) and the L3 phase. This unfortunately prevented the active dielectric window with the built-in acoustic emission sensor (ISO-002-AWD/UHF) from installation and testing. The induced voltage test was carried out strictly according to the scheme described in the IEC 60076-3 standard [106] and taking into account the strict internal safety regulations in the high voltage laboratory. Due to these restrictions, the installation of the  
Figure 24: Measurement results obtained with conventional electric method: (**a**) instantaneous value of apparent charge \(q\), (**b**) PRPD pattern. Figure 25: PRPD patterns obtained in a monitoring system for partial discharges pulses registered in a measurement channel: (**a**) of high frequency, (**b**) of ultra-high frequency. high-frequency current transformer (ISO-002-HF sensor) on the grounding of the neutral point was not allowed. Therefore, the registration of the PD pulses was performed using two UHF antennas, whereas the employees of the HV test station conducted the measurement of apparent charge (according to IEC 60270 standard) using multichannel digital PD detector type DDX 9121b (Haefely, Basel, Switzerland). The time sequence for the application of test voltage for induced voltage test with partial discharge measurement (IVPD) is shown in Figure 27. In the first stage, the voltage was increased to the level of \((0.4\times L_{r})\surd 3\), which for a transformer with rated voltage of a winding \(U_{r}=410\) kV corresponded to the value \(U_{m}=94.7\) kV. At that voltage level, the background noise (\(q=3.5\) pC) was registered. Then, the voltage was increased for 1 min to the level of \((1.2\times U_{r})\surd 3=284\) kV, at which partial discharge pulses of the apparent charge from a several dozen to 150 pC were registered occasionally. According to the scheme of the IVPD test, the voltage was increased to the 1-h PD measurement voltage level (\(U_{m}=374\) kV). This voltage level was held for 5 min. During that time stable PD pulses of apparent charge \(q=600\)-700 pC were registered. Then the voltage was raised to the enhancement voltage level (\(U_{m}=473.4\) kV) and held there for 1 min. At this test stage, the apparent charge grew to approx. 1500-1800 pC. For the IVPD test, the monitoring system was configured to the operation in a diagnostic mode, which enabled the observation and registration of UHF time waveforms in real-time. In Figure 28 the exemplary time waveforms of duration 20 ms were presented, which were registered for three main IVPD test voltage levels, i.e., 284, 374, and 473 kV. PD pulses were detected by the UHF probe installed near the phase L3. The probe installed on the opposite side of the tank, near phase L1, independently from the level of the test voltage, registered only background noise (about 20-30 mV). This means that the PD source was probably in phase L3 or its neighborhood. For the partial discharges of low intensity and low apparent charge (max. 150 pC), the UHF probe registered pulses, however, their amplitude did not exceed 45 mV (approx. 1.5-2 times higher than the background noise). In turn, by \(U_{m}=374\) kV, for partial discharges of apparent charge equal to 600-700 pC, the UHF pulses of definitely higher amplitude (120-135 mV) were registered.