# PD Detection and Analysis Triggered by Metal Protrusion in GIS Through Various Methods
## 3 Results
### Comparision of Results for Different Methods  
There are differences between the detection results of different methods for the same defect, which are mainly reflected in the PD detection ability, PDIV, PRPD pattern and discharge magnitude, etc. The pulse current method only detected signal for a 12 mm protrusion on the conductor during the experiment. Although UHF and the ultrasonic method both detect signal under all defects, the PDIV and PDEV (partial discharge extinction voltage) are different, as Figure 22 shows. It can be seen from Figure 22 that PDIV and PDEV both decrease as the protrusion length increases. The PDIV and PDEV of the protrusion on the enclosure are lower than those of the conductor, although the defect length is the same. In addition, PDIV and PDEV detected by the ultrasonic method are lower than those detected by the UHF method when the defect is short. For example, the PDIV and PDEV for a 3.5 mm protrusion on the conductor detected by the UHF method are 1.32 and 1.22 times higher than those detected by the ultrasonic method, respectively. For all 12 mm defects (including both conductor and enclosure protrusions), the PDIV and PDEV measured by the UHF and ultrasonic methods are completely consistent. This indicates that for such large-sized defects, both methods possess sufficient sensitivity to accurately capture the actual PD inception activity. However, for smaller defects (3.5 mm and 6 mm conductor protrusions), the PDIV and PDEV measured by the UHF method are higher than those obtained by the ultrasonic method; therefore, the ultrasonic method is more sensitive for detecting short protrusion defects. Regarding a 12 mm protrusion on the enclosure, although the PD amplitude is significantly lower than that for conductor protrusions of the same size, and the PDIV and PDEV measured by both methods remain consistent. This demonstrates that for large-sized defects, even when the PD amplitude is relatively low, both methods can still simultaneously detect PD inception. When PD occurs at the positive half-cycle of voltage, the PD signal is only detected by UHF. Then, the PRPD pattern will be like the results of the 3.5 mm and 6 mm protrusion on the conductor, as shown in Figures 12a and 13a. This is because the initial genuine PD signals may be missed due to their amplitudes falling below the UHF detection threshold, while the larger discharge magnitudes during the positive half-cycle are captured first. Similarly, for the 12 mm protrusion on the enclosure where PD magnitudes are relatively lower, both the UHF and ultrasonic methods might only detect discharges during the positive half-cycle when the discharge magnitudes are sufficiently large, while still missing the initial negative half-cycle discharges. Comparing Figures 7-9 to Figures 12 and 13, and Figure 19 to Figure 20, it can be seen that the PRPD patterns for the UHF and ultrasonic methods are similar to each other  
Figure 22: PDIV and PDEV of different defects. (**a**) PDIV; (**b**) PDEV. under the same voltage. The normalization results for maximum amplitude are shown in Figure 23. The figure shows that the results for the 12 mm protrusion on the conductor and enclosure detected by UHF are similar to the results detected by the ultrasonic method, as shown in Figure 12d,e. The maximum amplitude first slowly increases, then greatly increases. After that, the amplitude increases slightly, and finally increases quickly. The PDIVs of 3.5 mm and 6 mm protrusion at conductor detected by UHF are higher than those by the ultrasonic method, so the ranges of their normalization curves are different. However, the trends of the curves are similar in the same range, as shown in Figure 12a,b. In addition, Figure 12 shows that the results detected by ultrasonic change rapidly, while the results detected by UHF change gently. This difference further confirms that the ultrasonic method exhibits greater sensitivity to minute discharges, while the UHF method demonstrates a weaker response to initial weak discharges.