## 7 Field Test
### Field-Measured PD Signal  
A 6 kV MYJV cross-linked polyethylene cable with a length of about 250 m from a 110 kV substation to a coal-washing plant in a coal mine in Shanxi Province was detected as having partial discharge. The field-measured PD signal can be seen in Figure 17a. With the aim of fully verifying the noise reduction effect of different methods, multiple narrowband interference signals with amplitudes of 0.5 mV, 1 mV and 1.5 mV and frequencies of 1 MHz, 1.5 MHz and 2.2 MHz are added, as shown in Figure 17b.  
S_VMD, IIE-MVMD, SVD-EWT and ISVD-MVMD are applied to denoise the field-measured signal, and the noise reduction results can be seen in Figure 18.  
Figure 16: Field monitoring equipment. **(a)** HFCT; **(b)** industrial control computer.  
Figure 17: Field data. **(a)** Field-measured PD signal; **(b)** field-measured PD signal with narrowband interference.  
Figure 15: Online monitoring system diagram.  
According to Figure 18, there is still residual noise after noise reduction by S_VMD, which will affect the defect diagnosis and location of subsequent cables. The waveform residual noise after noise reduction by IIE-MVMD and SVD-EWT is small, but the PD signal waveform distortion is more serious. After noise reduction by ISVD-MVMD, the WGN and narrowband interference signals can be suppressed to the greatest extent, and the PD waveform characteristics after noise reduction are obvious.  
The calculation results of the NRR and the ARR of the field-measured PD signal can be seen in Table 11.  
According to Table 11, the NRR value of the PD signal denoised by ISVD-MVMD is 4.5008, which is the largest among different noise reduction methods. This shows that the PD signal obtained by ISVD-MVMD has the least residual noise content. The ARR value is 0.2017, which is only slightly larger than the SVD-EWT. However, compared with SVD-EWT, ISVD-MVMD retains the waveform characteristics of the PD signal completely, and the PD signal is easy to identify. The high NRR value and the low ARR value indicate that the noise reduction effect is good. Therefore, this method is also superior to the noise reduction effect of the actual collected PD signal in the field measurement. The time-consuming and operating efficiency indicators of the algorithm are shown in Table 12.  
It can be seen from Table 12 that the algorithm time of this method is close to that of the IIE-MVMD method and the SVD-EWT method, which is much smaller than that of the S_VMD method. Through the construction of the algorithm operation efficiency index, the algorithm operation efficiency of this method is the highest, which has certain advantages for the noise reduction situation with a large amount of data.  
\begin{table}
\begin{tabular}{c c c c c} \hline
**Evaluation Index Values** & **S\_VMD** & **IIE-MVMD** & **SVD-EWT** & **ISVD-MVMD** \\ \hline NRR & 3.0746 & 3.7609 & 3.0407 & 4.5008 \\ ARR (\%) & 26.61 & 52.06 & 18.30 & 20.17 \\ \hline \end{tabular}
\end{table}
Table 11: Noise reduction performance evaluation of field-measured data.  
Figure 18: Noise reduction results of different methods in field-measured PD signals. (**a**) S_VMD; (**b**) IIE-MVMD; (**c**) SVD-EWT; (**d**) ISVD-MVMD.