## 4 Results and Discussion  
### The Primary Examination of Partial Discharge  
As the first step, the four AE sensors (AE1 to AE4) were installed with housing of transformer as follows from Figure 3. for detected PD signal by PowerPD software. Figure 4 shows a real installation of Acoustic Emission Sensors with 4 positions. The PD signal from PowerPD software showed four signals with different colors (green, red, yellow, and blue) at the position of AE sensor was installed. Figure 5 shows the PD signal of each AE sensor detected with AE1, AE2, AE3, and AE4, in which it was found that the PD signal from PowerPD software was green, red, yellow, and blue color, respectively.  
Figure 4: Installation of Acoustic Emission Sensor (AE1–AE4) with housing of transformer.  
Figure 5 shows the PD signal waveforms obtained from each sensor in four directions of installation. Usually, the location of the PD by sound waves is successful only when the PD waveform has a strong amplitude and repetitive rate with the AE sensor detects at least three channels. It was observed that the PD signal by AE 3 (yellow color) was rather steady; ostensibly that, the amplitude of AE3 was minimum than AE1, AE2 and AE4. The amplitude of AE3 was minimum as the results of PD signal detection were low, which AE3 was installed with low side of this transformer and expected, the PD should not be caused from this side. It was found that the PD signal of AE1, AE2 and AE3 was unsteady, which when compared three signals, it was observed that the amplitude of AE 2 (red color) was lower than AE1 and AE4. The PD should not be caused from the AE2 installation side due to the PD signal was minimum. However, the PD signal of AE1 and AE4 cannot judge which signal was less than the others due to the fact that they have similar signals. Therefore, it is necessary to combine the signals into the same figure for easy analysis, which follows in Figure 6.  
Figure 6 shows comparison of the PD signal waveform with AE1-AE4, it was observed that the PD signal by AE1 (green color) was higher than AE4 (blue color) from peak amplitude. The AE1 sensor was installed on the high side of transformer for detecting the PD signal in this experimental, while the AE4 sensor was installed on the left side of transformer when seen from high side of transformer. When the level of PD signal of AE1 was higher than AE4 with significance, thus, high possibility that the PD was generated from high side transformer which must be detail examined in next step.  
Figure 5: The PD signal waveform obtained from each sensor by four directions installation.  
Figure 7 shows installation of HFCT at ground wire of cast-resin transformer, which the HFCT sensor was sensor number 5 at connect to PowerPD software for shown PD signal waveform. The HFCT sensor was connected to PowerPD due to it was easy to evaluate of condition level of PD with gab time can be easily seen. Figure 8 shows the PD signal waveform from HFCT sensor, it was observed that the gab time/dt was about 8 ms. Where, the signal was the electromagnetic wave signal when characterized by the discharge on the surface of the insulation (Surface Discharge) [11; 23; 24]. From Table 1, this data was evaluated for condition level at small PD due to gab time/dt > 7 ms. whereas this data must be examined in the next step when the AE sensor was moved to investigate the exact position of PD.  
Figure 8: The PD signal waveform from HFCT sensor.  
Figure 6: The comparation PD signal waveform with AE1–AE4.  
Figure 7: Installation of HFCT at ground wire of transformer.