## 3 Electric Field Detection Method of Degraded Insulators Based on Sensitive Insulators
### Detection and Test Platform of Degraded Insulators  
In order to explore the electric field characteristics of sensitive insulators and improve the contactless detection method of degraded insulators, a 220 kV line insulator detection platform was built. The insulator string was composed of 15 insulators. An optical electric field sensor based on the Pockels effect was installed at the front end of the insulating rod to detect the spatial electric field strength of the insulator string [21]. The schematic diagram of the detection site and test are as follows Figure 12.  
The insulators were numbered from the conductor side to the cross arm side by #1-#15. During the test, the #1, #3, #5, #7, #9, #11, #13, and #15 insulators were replaced with zero-value insulators with a resistance of 2 M\(\Omega\). The horizontal distance between the electric field probe and the insulator string was 30 mm, and the space electric field of the insulator string was measured. Measurement results are shown in the table below. According to Table 3, in the case of degradation in the insulator string, it will affect the space electric field of degraded insulators and other insulators. Moreover, in the case of degraded insulators at the high-voltage end, due to the large voltage borne by the high-voltage end, the voltage on the insulator string will be redistributed after degradation, resulting in obvious changes in the space electric field at all insulators in the insulator string. In the case of degraded insulators at the middle-voltage end and the low-voltage end, the space electric field at other insulator positions had few changes due to small voltage borne by degraded insulators. Figure 12: Test diagrams and schematic diagrams of sensitive insulators. (**a**) Test diagram of sensitive insulators. (**b**) Schematic diagram of detection device. To intuitively reflect the change of an insulator electric field, the influence of degraded insulator position on space electric field distribution of insulators was analyzed through the change rate of space electric field strength \(\sigma_{ij}\):  
\[\sigma_{ij}=\left|\Delta E_{i,j}^{f}-\Delta E_{i}^{s}\right|/\Delta E_{i}^{s} \tag{9}\]  
where \(i\) represents the position of insulators, \(j\) represents the position of degraded insulators (\(i,j=1\)-\(15\)), \(f\) represents the insulator string with degraded insulators, and \(S\) represents the insulator string with good insulation performance. After many tests, the change rate of space electric field of several insulators near the low-voltage side was more stable and closer to meeting the conditions of sensitive insulators. The test results are shown in the Table 4 below. \begin{table}
\begin{tabular}{c c c c} \hline \hline  & \multicolumn{3}{c}{**Rate of Change of the Space Electric Field of Insulators at a Low-Voltage End**} \\ \cline{2-4}
**Zero-Value Position \(j\)** & \multicolumn{3}{c}{**Low-Voltage End**} \\ \cline{2-4}  & \(\sigma_{13j}\) & \(\sigma_{14j}\) & \(\sigma_{15j}\) \\ \hline
1 & 0.051 & 0.110 & 0.101 \\
2 & 0.083 & 0.078 & 0.088 \\
3 & 0.092 & 0.054 & 0.055 \\
4 & 0.078 & 0.054 & 0.064 \\
5 & 0.057 & 0.051 & 0.052 \\
6 & 0.054 & 0.054 & 0.058 \\
7 & 0.060 & 0.059 & 0.059 \\
8 & 0.057 & 0.041 & 0.061 \\
9 & 0.050 & 0.073 & 0.057 \\
10 & 0.037 & 0.053 & 0.051 \\
11 & 0.054 & 0.028 & 0.052 \\
12 & 0.057 & 0.016 & 0.053 \\
13 & 0.076 & 0.056 & 0.064 \\
14 & 0.044 & 0.071 & 0.062 \\
15 & 0.067 & 0.082 & 0.057 \\ \hline \hline \end{tabular}
\end{table} 
Table 4: The rate of change in the field strength of the low-voltage end space when 1 zero-value insulator was included.