## 4 Measurement and Analysis of PD
### PDs of a Power Frequency Cycle  
In order to assess the detection performance of the built-in sensor and verify its superiority, the PDs with a power-frequency cycle (20 ms) are analyzed under different voltages, with a sampling frequency of 100 MHz. Figures 14 and 15 show the waveforms of PDs detected by the built-in antenna and the external antenna for different voltages, respectively.  
The built-in sensor is able to detect a small discharge when the voltage is only 5 kV, whereas the initial discharge is only detected by external sensor when the voltage reaches 6 kV. When the voltage is set to 5.5 kV, signals detected by the built-in sensor indicate there is a continuous and evident discharge in the high-voltage switchgear. However, the external sensor only detects continuous and evident PDs when the voltage is increased to 6.5 kV. Those experimental results show that the designed  
Figure 12: Experimental circuit diagram.  
Figure 13: The high-voltage switchgear setup. (**a**) side and (**b**) front view.  
built-in sensor possesses good detection sensitivity, and it detect insulation defects earlier than the external sensor.  
Figure 14: PDs detected by the built-in antenna under different voltages.  
Figure 15: PDs detected by the external antenna under different voltages.  
The waveforms of PDs shown in Figures 14 and 15 indicate that the PDs detected by the external sensor possess more burrs, from which we can conclude that the external sensor is seriously affected by the external electromagnetic interference. Compared with the external sensor, the built-in sensor shows a better anti-interference capability because of the electromagnetic shielding effect of the all-metal casing.  
By analyzing the PDs detected by the built-in sensor under different voltages as shown in Figure 14, it can be concluded that the discharge times and the amplitude of PDs will increase as the applied voltage increases. Those discharges occur intensively in the negative half-period of the power-frequency cycle, because a positive voltage is applied to the needle electrode. The corona onset voltage of the negative half-period is less than that of the positive half-period because of the polarity effect. Partial and continuous discharges in the negative half-period are firstly observed when the applied voltage is less than the breakdown voltage. In order to verify the correctness of the above analysis, the applied voltage is sampled as the power-frequency reference for PDs, but gathering the integral PDs with low sampling frequency is challenging because the PD in the positive half-period is less and not continuous. Therefore, sampling frequency is set as 250 MHz, and the experimental results are consistent with the above analysis as shown in Figure 16. Figure 16b,c are an enlarged view of PDs occurring in the positive half-period and negative half-period, respectively. The information indicates that PDs occurring in the positive half-period possesses a larger amplitude, and the maximum amplitude is positive, while the maximum value of PDs occurring in the negative half-period is negative. Moreover, there are evident differences in the waveform of PDs occurring in the different half-period. Subsequently, the single PDs occurring in the positive half-period are acquired and verified based on the previously presented characteristics.