## 3 T-Head Cable Partial Discharge Signal Acquisition
### Experimental Acquisition Method of Partial Discharge Signal  
The pressurization methods of the partial discharge test include the constant pressure method and the step-by-step pressurization method. The constant voltage method is to apply a constant initial discharge voltage to the insulation in the sample until the insulation is broken down. Although this method is most in line with the actual insulation working voltage situation, it takes a long time. The step-by-step voltage method can quickly realize the whole process of simulated discharge, obtain a large number of experimental data, and greatly improve the experimental speed. The constant pressure method and the constant voltage method have the same insulation aging effect. Therefore, the step-by-step voltage method with faster speed is adopted in this paper.  
The data samples under different defects were collected in the experiment with 0.02 s as the cycle. According to the experimental platform shown in Figure 4, the voltage is quickly raised to the rated voltage of the joint U\({}_{0}\) = 8.7 kV. If there is a discharge phenomenon, the voltage applied at this time is recorded, and if there is no discharge phenomenon, the voltage is continuously raised. Under the same conditions as the above conditions, the five data samples were measured, and the mean value of the five data samples was taken as the initial discharge voltage. The initial discharge data of different defects are obtained by working for 0.5 h at the initial discharge voltage. The initial discharge voltage of the defects obtained by the experiment is shown in Table 3. According to the table, the initial discharge voltage of the insulation scratch defect is close to the rated voltage of the joint of 8.7 kV, while the other two defects are greater than the rated voltage of the joint of 8.7 kV.  
To accelerate the deterioration of insulation, a staged step-up method is adopted. Starting from the initial discharge voltage, each voltage gradient is about 5 kV, each voltage gradient is maintained for 30 min, and data samples are collected until the discharge becomes serious (such as the number of discharge pulses becomes dense and the experimental site hears a significant discharge current sound). The pressurization is  
Figure 5: Defect model diagram.  
The distribution of partial discharge amplitude-pulse width-discharge times of each defect is shown in Figures 9-11. The discharge times of different defects are different. The distribution range of partial discharge amplitude and pulse width of insulation scratch is mainly concentrated in the range of [\(-\)1,1] A and [0.08,0.12] us, and the maximum amplitude and maximum pulse width are 1.75 A and 0.47 us, respectively. The length of the discharge amplitude distribution interval of the casing fouling is narrow, but the discharge distribution is more uniform, and the approximate \"pyramid\" type concentrated area can be observed. The pulse width amplitude of the loose joint is distributed in a \"one-character\" shape along the x-axis, and the maximum amplitude and the length of the amplitude distribution interval are larger than the other two types of defects.  
Figure 8: Waveform of joint loosening defect.  
Figure 10: Casing fouling defect amplitude–pulse width–discharge times diagram.  
Figure 9: Insulation scratch defect amplitude–pulse width–discharge times diagram.