## 3 Experimental Setup  
Figure 3 shows schematic diagram of experimental installation, in which four AE sensors were installed on the housing of the transformer. Moreover, the high-frequency current transducer (HFCT) was installed at the ground of the transformer [2,19]. However, all sensors (AE sensor and HFCT) were connected to PowerPD (PD-TP500A). The data storage device was used to view the results on a computer. Moreover, there was the special sensor, an acoustic emission sensor used that was a passive piezoelectric, with a frequency response featured by a peak at 150 kHz where it exhibited a resonance. The frequency range was fixed at 100 kHz to 450 kHz along with transformer function of 4.5-5.0 V/A. Except for disconnection, a split core ferrite was retrospectively allowed to fit in earth straps for HFCT. To provide RF shielding and improve performance in noisy environments, the HFCT was constructed with an aluminum body. When the AE sensor No.1 (Green color) was located on the HV side of the transformer. The AE sensor No.2 (Red color) was located at the next side, which was determined by clockwise direction on the bottom of the transformer. The AE sensor No.3 (Yellow color) was located by the LV side of the transformer, which was opposite to the AE sensor No.1. The AE sensor No.4 (Blue color) was located approximately opposite to the AE sensor No.2 on the bottom position of the transformer. At the beginning, the four AE sensors and HFCT sensor \begin{table}
\begin{tabular}{c c c} \hline \hline
**Gap Time/dT (ms, Milliseconds)** & **Condition Leve** & **Recommended Action** \\ \hline No partial Partial Discharge Signal & Acceptable & Continue normal operation \\  & & Monitoring every 12 months \\ \hline Gap time \(>\) 7 ms & Small PD & Satisfactory (May be non-damaging PD) \\  & & Monitoring every 6 months \\ \hline
3 ms \(<\) Gap time \(<\) 7 ms & Major PD & Engineering review and evaluation need if the trend is increasing consider removal from service in near future \\  & & Monitoring every 3 months \\ \hline Gap time \(<\) 3 ms & Sonous PD & Unsatisfactory, make plans to remove and repair now \\  & & Monitoring every 1 week or continuous monitoring \\  & & should be installed \\ \hline Arcing Signal & Sonous & Engineering review and evaluation need if the arc source is \\  & & important point of equipment, consider removal from \\  & & service in near future \\ \hline Mechanical Signal & Abnormal & Engineering review and evaluation need other \\  & & technologies should be performed to combining to data \\ \hline \hline \end{tabular}
\end{table} 
Table 1: The risk level of PD according to the PowerPD reference. Figure 3: Schematic diagram of experimental installation. connected with PowerPD and the stored data by the computer after measuring. Then, the PD pattern, time domain, and frequency were analyzed by the dedicated software. Detection and coarse location of at least one source can be carried out by moving one or more external sensors to another location on the transformer housing. The combination method is the electrical measurement, which is improved by the confidence in the results of the PD test. When PD signals are detected, the other AE sensors will be moved to the corresponding of the AE sensors, which has detected the PD activity and is located in \(X\)-Axis and \(Y\)-Axis fashion before confirming the exact location of the PD source. The accurate position of the PD source can be determined by the relative time of the acoustic signal through each sensor. This is especially the case when PD bursts were detected from AE sensors and HFCT sensors; it could be concluded that the PD obviously occurred within the transformer. To perform a systematic study, the PD was actually measured by a cast-resin transformer while it's still working through the unusual measurement techniques. In addition, the essential techniques were experimentally used to locate the PD on these transformers. This research, the transformer 24 kV and 1000 kVA in 1991 at a hotel in Bangkok, Thailand was studied.