# PD Detection and Analysis Triggered by Metal Protrusion in GIS Through Various Methods
## 3 Results
### Comparision of Results for Different Methods
Above all, the PD detection capability of the pulse current method is the weakest among the three methods. The results for the UHF and ultrasonic methods show similarities, including PRPD patterns and the variation trend of maximum amplitude with increasing voltage. This can be attributed to the fact that although the two methods detect different physical quantities (ultrasonic signals correlate with discharge energy, while UHF signals relate to the rising gradient of the pulse current), both signals originate from the pulse current generated by PD. Thus, under the same voltage, their results--whether PRPD patterns or maximum amplitude trends--exhibit consistency. The key distinction lies in the more pronounced amplitude variation in ultrasonic signals compared to UHF (Figure 23), as the discharge energy increases significantly with voltage, whereas the rise gradient of pulse current changes only marginally. In addition, the main difference between the UHF and ultrasonic methods is their sensitivity. As the results show, the sensitivity of the ultrasonic method is higher than UHF. One reason is that the ultrasonic signal is related to the discharge energy, which can easily be detected due to it changing more obviously with the development of the PD. Another  
Figure 23: The normalization results for maximum amplitude as voltage increases: (**a**) 3.5 mm HV protrusion; (**b**) 6 mm HV protrusion; (**c**) 12 mm HV protrusion; (**d**) 12 mm LV protrusion. reason is that the ultrasonic sensor is closer to the PD source than the UHF sensor. This leads to the ultrasonic signal decaying relatively less than UHF. The different sensitivities lead to their different minimum detection discharge magnitude. The order of minimum detection discharge magnitude by different methods is as follows: pulse current > UHF > ultrasonic. This leads the PDIV detected by different methods to be different, which can be explained by Figure 24.  
In Figure 24, the solid line is the curve of maximum discharge as the voltage increases, and the horizontal dotted lines are the minimum discharge magnitude detected by different methods. The intersection of the vertical and horizontal dotted lines is the PDIV. Because the minimum detection discharge magnitude is pulse current > UHF > ultrasonic, the PDIV is pulse current > UHF > ultrasonic. The differences in sensitivity among the detection methods stem from their respective signal sources, propagation characteristics and anti-interference capabilities. The pulse current method, which is directly affected by electromagnetic interference in the circuit and relies on higher discharge levels to be triggered, exhibits the lowest sensitivity. The UHF method, which detects electromagnetic wave signals, offers moderate sensitivity but faces challenges such as electromagnetic interference, signal attenuation over long propagation distances and the fact that significant signals are only generated during the streamer discharge stage. In contrast, the ultrasonic method utilizes acoustic signals directly excited by discharge energy, producing detectable responses even in the initial discharge stage. Additionally, acoustic signals are less susceptible to electromagnetic interference and have shorter propagation paths, resulting in them having highest sensitivity. These fundamental differences lead to a hierarchical relationship in the minimum detectable discharge levels, which is as follows: pulse current method > UHF method > ultrasonic method.