In order to further investigate the diagnostic method of the local aging type of cables by polarization and depolarization current method (PDC)

this paper analyzes the characteristics and the reasons of the PDC third branch parameters of cables with different local aging types. Eight groups of cable samples were prepared

and four of them were subjected to a thermal aging at 140 ℃ for 108 h

216 h

324 h and 432 h

respectively. The other four groups of samples were subjected to an accelerated water tree aging for 20 days

40 days

60 days

and 80 days at 7.5 kV and 400 Hz

respectively. PDC tests and scanning electron microscope observation were performed on the aged samples. PDC branch analysis results show that the time constant of the third branch of the thermal aged samples increases with time

while the time constant of the third branch of the water tree samples decreases with time. In addition

the asymmetry coefficient τas of the third branch time constant of the aged samples is higher than 1 and increases with time. The results of scanning electron microscopy show that there are a large number of isolated micropores in the thermal aged samples

and there are microcracks between the micropores of the water tree samples. The analysis shows that the thermal aging mainly generates isolated micropores

resulting in an increase in the local capacitance of the sample and a small decrease in the resistance

so the time constant of the third branch increases with time. The water tree sample can generate water tree channels

resulting in a significant decrease in the local resistance of the sample

so the time constant of the third branch decreases with time. According to the time-varying characteristics of the time constant of the third branch of PDC

different local aging types can be identified

and the local aging condition can be further judged combined with the τas value. If there is only a single aging form of thermal aging or water tree aging in the cable

it is preliminarily considered that τas=1.3 can be used as a standard to distinguish thermal aging and water tree aging.