## 2 Experiment Setup and Measurement of PD Data
### PD Data Collection from Defect Samples  
After establishing the necessary connections and ensuring proper electrical contact, high voltage was applied to the test object until PRPD patterns looked saturated. Note that these voltages were higher than PDIV and could be regarded as RPDIV. The PD data were recorded for each test sample at the RPDIV. Therefore, two PD measurement conditions were implemented to investigate this issue, as shown in Figure 3.  
Figure 2: (a) Schematic diagram and (b) experimental pictures of epoxy resins with voids at different positions.  
Under the first condition, as the upper brown color section in Figure 3, the PD measurements were conducted for one minute by increasing voltage by 1 kV until the RPDEV was observed.  
Under the second condition, as the lower blue color section in Figure 3, the PD measurements were conducted for one minute from the RPDIV to the PDEV by decreasing voltage by 1 kV. A substantial amount of PRPD patterns were captured during this time. Furthermore, the measurement duration was extended to one hour, allowing for the accumulation of RPRPD patterns near to the PDEV.  
The impact of weak PDs at the PDEV on the accuracy of defect recognition was evaluated by comparing the PD data obtained under these two conditions. This investigation is important for understanding the behavior of weak PDs and their role in defect detection.  
The values of the RPDIV were measured to be 28, 35, and 23 kV for the test samples with void positions at the top, center, and bottom, respectively. The possible reason that the top and bottom voids should have two different PDIVs is that the voids at these positions are closer to the ground electrode. These factors can influence the electrical field distribution and the resulting PD characteristics.  
The PRPD patterns of these test samples provided a visual representation of the PD signals recorded for each void position and are, respectively, shown in Figures 4-6. A stable PD shape was observed in the Void_Top, indicating that the PD phenomenon reached a steady state at the applied voltage. Additionally, a further increase in the voltage did not result in any significant changes in the shape of the PD pattern whose observation duration was longer than 1 min and up to 1 h, which is the limit of the PD instrument.  
Figure 3: PD experimental procedure for preparing PRPD patterns.  
Similar PD pattern shapes for the void positions at the center and bottom can be attributed to various factors, as shown in Figures 5 and 6, respectively. Additionally, other factors may contribute to the similarities in the observed PD pattern shapes, such as the resin properties, void geometry, and electrode configuration.  
PD data were collected during the recording process for a duration of one minute. The PD data obtained at the RPDIV comprised 100 PRPD patterns for each test sample, labeled as Void_Top, Void_Center, and Void_Bottom. Subsequently, the PD data were used as the training dataset for the CNN, as shown in Table 1. Therefore, the CNN could learn the distinctive features associated with each void position and improve its ability to accurately identify and classify defects in future testing scenarios. The PRPD data collection for the testing cases is discussed in more detail in Section 4.  
Figure 4: PRPD pattern with Void_Top in epoxy resin.  
Figure 5: PRPD pattern with Void_Center in epoxy resin.  
Figure 6: PRPD pattern with Void_Bottom in epoxy resin.