# PD Detection and Analysis Triggered by Metal Protrusion in GIS Through Various Methods
## 4 Discussion and Analysis  
Partial discharge occurs when there is a metal protrusion defect in GIS which enhances the electric field at the protrusion. During the experiment, breakdown occurred for 6 mm and 12 mm protrusions on the conductor. According to the PRPD pattern and the maximum discharge amplitude curve detected by the UHF and ultrasonic methods, the process can be divided from the inception of PD to the breakdown of the protrusion on the conductor, into the following sections. 1. At the beginning, PD only occurs near the peak of negative half-cycle of the AC voltage, which is shown in Figure 8a,b, etc. The maximum amplitude slowly increases as the voltage increases, as shown in Figures 11 and 16. 2. PD begins to occur at the positive half-cycle of the voltage as the voltage increases, as is shown in Figure 8c-g, etc. At this point, the maximum amplitude of the positive half-cycle of the voltage increases greatly, as shown in Figures 11 and 16. Figure 24: PDIV detected by different methods. As the voltage further increases, it can be seen that the PD amplitude increases and PD phase is widened, as shown in Figure 8h-j, etc. However, the maximum amplitude increases slightly, as shown in Figures 11 and 16. 4. When the voltage is very high, the discharge becomes very strong and there are some extreme PDs at the peak of the voltage, as shown in Figure 8k,l, etc. The maximum amplitude quickly increases. At this point, breakdown may occur. There is strong ionization around the metal protrusion before PD occurs due to the strong electric field. As the charges drift, positive charges are accumulated around the protrusion, as is shown in Figure 25.  
At this point, the distribution of the electric field changes due to the effect of the space charges. The field for the positive protrusion is weakened, while the field for the negative protrusion is strengthened. This is the reason that PD first occurs at the negative half-cycle of the voltage, and then occurs at the positive half-cycle. As PD starts to occur, the discharges are mainly the streamer discharges, which propagate in a short channel [21]. With the development of the streamer discharge, a large amount of positive charges remain in front of the protrusion, which is shown in Figure 26a. The space positive charges strengthen the electric field in front of the discharge plasma. The streamer discharges of the negative protrusion are dispersive, which make a dispersive plasma sphere around the electrode, as is shown in Figure 26b. This plasma sphere plays a similar effect in increasing the curvature radius of the electrode, which weakens the frontier field. Therefore, the discharge magnitude of the positive protrusion is higher than that of negative protrusion. As the voltage increases, the streamer discharge propagates stepwise. As the voltage further increases, breakdown may occur at the positive half-cycle of the voltage. At this point, the streamer discharge becomes very strong. Although the space  
Figure 26: Space charge distribution graph of protrusion defects under streamer discharge. (**a**) Positive protrusion; (**b**) negative protrusion. Figure 25: Space charge distribution graph of protrusion defects. (**a**) Positive protrusion; (**b**) negative protrusion. charges weaken the frontier electric field, there is leader discharge along the periphery of the positive space charge due to the very high applied voltage, as is shown in Figure 27. The leader discharge propagates in a long channel and its magnitude is high. When the voltage increases to a certain value, breakdown occurs due to the leader discharge through the electrode. Above all, the PD of the protrusion on the conductor occurs at the negative half-cycle of the voltage at the beginning of the process. Then, PD occurs at the positive half-cycle as the voltage increases. Because the PD of the positive protrusion is easier to develop than that of negative protrusion, the maximum amplitude increases greatly. After that, the discharge is mainly streamer discharge.