**3.3 PDIV results**

PDIV tests were carried out with AC of 50 Hz. Eight specimens were tested at each aging time. The peak-to-peak value of applied voltage is recorded for the PDIV. All the data of each aging time are Weibull plotted and shown in **Figure 10**. The B10 parameter is employed here, which means the 10% probability of PD occurrence. Besides, the Weibull scale parameter (α) and shape parameter (β) are also employed and shown in **Table 2**.

From the estimation of Weibull plot, B10 and α can be calculated at each aging time. In stage 1 shown in **Figure 10(a)**, B10 changes from 1590 (at 0 h) to 1715 V (at 192 h), while α varies from 1750 to 1810 V. It indicates that B10 is more scattered after the thermal aging at 270°C. In stage 2 shown in **Figure 10(b)**, both B10 and α show similar behavior, while in stage 3 (**Figure 10(c)**), both B10 and α are scattered, compared with the other two stages.

B10 can be employed to show the reliability of PI in severe working conditions. As shown in **Figure 11**, this "reliability" can be divided into three stages corresponding to the three aging stages. B10 increases sharply in the first 200 h, and then it becomes stable till the end of stage 1. With the beginning of stage 2, the high humidity makes B10 drop linearly with aging time, which indicates the risk of PI application in wet environment. As mentioned above, the high temperature in stage 3 will recombine the PI chains and remove the moisture in the bulk, resulting in the recovery of B10. However, a decrease of B10 occurs after its recovery, from 1680 to 1640 V.


### **Table 2.**

*PDIV statistics during aging (n = 8).*

*Electrical Endurance of Corona-Resistant Polyimide for Electrical Traction DOI: http://dx.doi.org/10.5772/intechopen.93253*

**Figure 11.** *Trend of PDIV B10 of at 50 Hz during aging.*

**Figure 12.** *B10 of PD endurance in all the stages.*

By checking **Figures 5, 7** and **11**, one can find that B10 is closely related to the change of ε'. A capacitor model can be used here to explain this relation. In this model, the air gap between rod electrode and sample surface is equivalent to a capacitor, the PI tape to another one. With the increasing ε' of PI, the electric field in air gap will increase simultaneously. For instance, the ε' at 50 Hz increases 8% (from ~3.8 to ~4.0) in stage 2; B10 in **Figure 11** decreases from 1660 to 1520 V (~9%) correspondingly.
