# PD Detection and Analysis Triggered by Metal Protrusion in GIS Through Various Methods
## 3 Results
### Results of Protrusion on the Conductor
#### 3.1.1 Detection Results for Ultrasonic Method  
The PRPD (phase-resolved partial discharge) patterns for 3.5 mm protrusion on the conductor detected using the ultrasonic method are shown in Figure 7. Figure 7a shows the pattern of the inception discharge. At this point, PD only occurs at the negative half-cycle of the AC voltage. Then, PD occurs at the positive half-cycle when the voltage increases to 185.28 kV, as is shown in Figure 7b. The maximum discharge magnitude at the positive half-cycle of the voltage is bigger than that of the negative half-cycle at a higher voltage, as shown in Figure 7c-i. Figure 8 shows the PRPD pattern for the 6 mm protrusion on the conductor. As shown in Figure 8a,b, PD only occurs at the negative half-cycle of the AC voltage at the inception discharge, which is similar to that for the 3.5 mm detect. Figure 8c shows that PD begins  
Figure 7: PRPD pattern for 3.5 mm protrusion on the conductor detected by ultrasonic method (**a**) at 170 kV; (**b**) at 185 kV; (**c**) at 190 kV; (**d**) at 205 kV; (**e**) at 225 kV; (**f**) at 236 kV; (**g**) at 246 kV; (**h**) at 256 kV; (**i**) at 266 kV. to occur at the positive half-cycle of the voltage. From Figure 8d-g, it can also clearly be seen that the discharge amplitude at the positive half-cycle of voltage is higher than that of the negative half-cycle with increasing voltage. This is different from the results for 3.5 mm protrusion, which are shown in Figure 8h-l. At this point, the discharge becomes very strong. It should be noted that an extremely high discharge occasionally occurs at the peak of the AC voltage. Breakdown occurs when a voltage of 270.87 kV is kept constant for 1 min. The discharge pattern for a 12 mm metal protrusion on the conductor is shown in Figure 9. Figure 9a,b show that PD firstly occurs at the negative half-cycle of the AC voltage and then occurs at the positive half, as can be seen in Figure 9c. Figure 9d-f show that the PD magnitude at the positive half-cycle clearly increases as the voltage increases. Figure 9g-i show that an extremely high discharge occurs at the voltage peak as the voltage further increases. Breakdown occurs when a voltage of 265.25 kV is kept constant for about 20 s.  
Figure 8: PRPD pattern for 6 mm protrusion on the conductor detected by the ultrasonic method (**a**) at 130 kV; (**b**) at 140 kV; (**c**) at 150 kV; (**d**) at 155 kV; (**e**) at 160 kV; (**f**) at 170 kV; (**g**) at 180 kV; (**h**) at 200 kV; (**i**) at 222 kV; (**j**) at 231 kV; (**k**) at 250 kV; (**l**) at 271 kV. It is shown in Figure 10 that the PDIV (PD inception voltage) under different length defects clearly decreases as the length increases. Figure 11 shows the curve of the maximum discharge amplitudes under different voltages. It can be seen that the discharge amplitude increases as the voltage increases. For a 12 mm protrusion, the maximum discharge amplitude increases slowly when the voltage is below 105 kV. Then, the discharge amplitude greatly increases at a voltage of 105 kV. When the voltage increases from 105 kV to 175 kV, the amplitude increases slightly. Finally, the maximum amplitude increases quickly until breakdown. The result of a 6 mm protrusion is similar to that of a 12 mm protrusion. At first, the amplitude increases slowly as the voltage increases. When the voltage is 175 kV, the maximum discharge amplitude greatly increases. Then, the amplitude increases slightly, and finally, it increases quickly. It can be seen from Figure 11 that the maximum discharge amplitude of 3.5 mm firstly increases slowly, then increases greatly as the voltage increases. It is expected that the  
Figure 10: Inception voltage for different length detects. Figure 9: PRPD pattern for 12 mm protrusion at conductor detected by the ultrasonic method (**a**) at 77 kV; (**b**) at 91 kV; (**c**) at 106 kV; (**d**) at 116 kV; (**e**) at 136 kV; (**f**) at 160 kV; (**g**) at 189 kV; (**h**) at 216 kV; (i) at 256 kV. amplitude will slightly increase and then increase quickly as the voltage further increases.