**3.4 PD endurance under impulse voltages**

PD endurance time tests were carried out under a square wave with a peakto-peak voltage of 7 kV. After the application of this voltage, a corona will be found around the rod electrode, destroying the PI tape and leading to breakdown. This voltage parameter was selected for the purpose of achieving breakdown within less than 2 h.

Eight samples were tested at each aging time. The Weibull plots were employed, and B10 of endurance time was calculated. B10 of all the stages are shown in **Figure 12**. B10 in stage 1 varies from 70 to 95 min; this fluctuation is maybe caused by the limitation of sample quantity. In stage 2, the trend is obvious. B10 decreases from 83 to 68 min. While in stage 3, B10 declines linearly with aging time, from 68 to 40 min. It can be found that thermal aging in stage 1 will not change the endurance of PI. The reason of decreasing endurance in stage 2 is assumed to be the reduction of PDIV. However, endurance in stage 3 also decreases, while the PDIV is recovered to the initial level. This change is more likely related to the material changes inside the sample caused by thermal aging, which will be discussed in the next section.
