Purpose/Aim
XLPE is widely used in cable insulation due to its excellent electrical, mechanical and physicochemical properties. However, thermal stress can greatly accelerate the degradation of XLPE during its long-term operation. This paper reports a critical ageing-time phenomena of XLPE cable insulation degradation on the basis of mechanical, dielectric and chemical properties assessment under thermal aging condition.
Experimental/Modeling methods
XLPE specimens were cut from 110 kV commercial cables with different service years into square shapes and thermal ageing was carried out at 135℃, 155℃ and 180℃. AC breakdown test, tensile test, Fourier transform infrared spectroscopy (FTIR) and thermogravimetric analysis (TG) were used to analyze the effects of thermal aging and life prediction.
Results/discussion
Taking the cable with a service life of 20 years as an example, results demonstrated that the elongation at break of XLPE initially decrease slowly from 611% to 551.22% with thermal ageing time, followed by an abrupt decline to 138.44% after 216 h at 155℃. The AC breakdown strength, pyrolysis activation energy and carbonyl index displayed a sudden change similar to the elongation at break after 216 h. The dramatic change of properties could also be seen for results at 135℃ and 180℃. From this aspect, XLPE insulation after the critical ageing time can be regarded as failure state and end of its life. 
Conclusions
During thermal ageing, macroscopic properties including AC breakdown strength, elongation at break, carbonyl index, and pyrolysis activation energy of XLPE specimens all display an abrupt change after early stable stage. It could be concluded that there is a critical time related to the temperature during thermal aging process, which could be considered as the end of life for XLPE specimens. Due to the critical ageing-time phenomenon, the assessment of XLPE residual life needs to be reconsidered.
 

XLPE,thermal ageing,critical ageing-time phenomena,ageing assessment,life prediction