## 4 Results and Discussion
### The Position Examination of Partial Discharge Source  
Verify the position of the PD source of cast-resin transformer to confirm the inspection results and analyze from AE sensor and HFCT sensor detection from the primary examination of partial discharge and the insightful examination of Partial Discharge. Figure 12 shows installation of AE sensor and HFCT sensor without housing of transformer. The cast-resin transformer has been removed housing, next, installation of two AE sensor and HFCT sensor. Since there is confirmation of the occurrence of partial discharge on the high-voltage side of cast-resin transformer. Thus, the 1 st AE sensor was installed on the high-voltage side approximately 1.9 m from the center of the transformer. The 2 nd AE sensor was installed in the opposite direction to the 1 st AE sensor approximately 1.2 m from the center of the transformer on the low-voltage side. The two AE sensors have been located at approximately a distance of about 1.9 m. The HFCT sensor was installed at the ground wire of the cast-resin transformer.  
Figure 11: The PD signal waveform from HFCT sensor when all AE sensor installation on housing of high side transformer.  
Figure 12: Installation of AE sensor and HFCT sensor without housing of transformer.  
Figure 13 shows partial discharge when AE1, AE2 and HFCT sensor (UHF1, UHF2 and HFCT4, respectively) detection. The point in the graph was detection of partial discharge, which also shows a sinusoidal waveform for comparison. It was found that all sensors have detected PD, whereas the detection of AE1 sensor and AE2 sensor can have been more than HFCT sensor, which it was observed from point in graph. Furthermore, the distance of partial discharge calculation using the sensor input time difference (TDoA) method [25; 26]. From the PD detection results to be processed to find the PD source, it was found that the distance between the PD source and the AE1 sensor was approximately 0.9 m.  
Figure 14 shows the distance of the PD source obtained from the two AE sensors. When using software analysis of PD distance, it was found that PD source distance was 0.87 to 0.92 m, which obviously that distance value between calculation with TDoA method and software analysis have been similar. It also confirmed the use of the AE sensor to accurately determine distance from the PD source. In addition, the AE sensor for PD detecting was able to accurately determine the location of the PD source even, in case of housing of transformer [27; 28; 29].  
Figure 15 shows evidence of partial discharge on the insulator surface of the high-voltage side. From the primary and insightful examination of partial discharge using AE  
Figure 14: The distance of the PD source obtained from the two AE sensors. **(a)** partial discharge from AE1 **(b)** partial discharge from AE2 **(c)** distance of sensor detection.  
Figure 13: Partial discharge when AE1, AE2 and HFCT sensor detection.  
sensor with HFCT sensor with housing or tank of transformer which it which could not see any vestige of damage from partial discharge. Therefore, in order to confirm that the AE sensor and HFCT sensor have been, an ability to located, the housing of transformer was removed. It was found that there is the PD on the surface of the insulation between the terminal of phase B and the high-voltage tap bar. Furthermore, it has been found that it was detected obviously of partial discharge on the surface and inside in the insulation of transformer; consequently, this is a significant effect on the result. Thus, the equipment would be immediately shut down, correction would be carried out, and the transformer would be put to normal use.