## 3 Results and Discussion
### Transformer Noise Separation Test in a Substation
#### 3.3.2 Analysis of Test Results  
1. Test accuracy analysis  
The test data are calculated and processed using the phase conjugation method and the equivalent point source method to obtain the noise separation results of three transformers at seven test points. Because the working substation cannot be stopped, the test accuracy cannot be verified by operating one transformer individually. Instead, we verified the test accuracy by analyzing the noise line spectra and A-weighting SPL obtained from the separation sound pressure results of three transformers at different test points, and these noise pressure data have to decrease as the distance increases between the test point and source, and vice versa. \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**No.** & **Equipment** & **Manufacturer** & **Type** & **Quantity** \\ \hline
1 & Microphone & B\&K & 4189 & 11 \\
2 & DAQ system & B\&K & 3050-A-060 & 2 \\
3 & BNC wire & - & - & Certain \\
4 & Computer & - & - & 1 \\
5 & Derrick crane & - & - & 11 \\
6 & Lan cable & - & - & 3 \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Test equipment in a substation. Figure 11: Test scene in substation: (**a**) line array setup for Condition 1, (**b**) line array setup for Condition 2, (**c**) connection between line array and DAQ system, (**d**) connection between DAQ system and computer. The SPL line spectrums and their trend lines from 0 to 3200 Hz for test points 1, 3, 5, and 7 for three transformers in Conditions 1 and 2 are shown in Figures 12 and 13. We choose test point numbers 1, 3, 5, and 7 to show, in Figures 12 and 13, without limiting generality for the sake of clarity of the line spectrum. It can be seen from Figure 12 that the A-weighting SPL line spectrums and the trend lines of the four test points are intertwined, which means the Condition 1 results do not satisfy the law that noise pressure data have to decrease as the distance between test point and source increases, and vice versa. Figure 13 illustrates that the locations of A-weighting SPL line spectrums and their trend lines are highest at test point 1 but lowest at test point 7 for transformer 1, the locations of line spectrums and trend lines are higher at test points 3 and 5 but lower at test points 1 and 7 for transformer 2, and the locations of line spectrums and trend lines are highest at test point 7 but lowest at test point 1 for transformer 3, which means the Condition 2 results satisfy the law that noise pressure data have to decrease as the distance between test point and source increases, and vice versa. The total A-weighting SPLs of the seven test points and the distances \(d_{1}\) between the seven test points and transformers are shown in Figures 14 and 15. As is known to all, the total A-weighting SPL of an acoustic field point should decrease as the distance between the point and noise sources increases, and vice versa. As can be seen from Figures 14 and 15, all test points in Condition 2 strictly meet the above law. Test points 1 and 2 of transformer 1; test points 3 and 4 of transformer 2; and test points 4, 5, 6, and 7 of transformer 3 in Condition 1 do not meet the above law. Figure 12: Noise separation comparison results of 4 test points (Condition 1): (**a**) transformer 1, (**b**) transformer 2, (**c**) transformer 3. Figure 14: Total A-weighting SPL and the test-point-to-source distance of 7 test points (Condition 1): (a) transformer 1, (b) transformer 2, (c) transformer 3. Figure 13: Noise separation comparison results of 4 test points (Condition 2): (a) transformer 1, (b) transformer 2, (c) transformer 3. To sum up, the reason why the separation result of Condition 1 is worse is that the distance between the microphone line array and transformers is too close to regard transformers as point sources. The step of considering the transformer as an equivalent point source in the separation technique makes the separation result have a greater error. The separation results of Condition 2 are more in line with the change law of the sound pressure with distance.