to experimentally examine possible enhancement of water
treeing caused by remnant mechanical stresses, frozen-in
during the manufacturing process of extruded XLPE
The experiments were performed using 3m long samples
of 12 kV XLPE cable cores, exposed to water and
different static tensions during ageing at an effective 50
Hz AC voltage of 14 kV (Emax = 5.2 kV/mm). Reference
cable samples as well as mechanically stressed cable
samples were examined at 20, 40 and 60 °C. At each
temperature the following 3 types of samples were
examined: i) Reference cable sample, with conductor, ii)
non-strained cable sample, without conductor, iii) mechanically
2 % strained cable sample, without conductor.
After removal of the cable conductor at 20 °C the
insulation was found to longitudinally shrink by a about
The degree of water tree degradation was characterized
using optical microscopy investigation of 0.5 mm thick
methylene blue stained slices.
The resulting density of bow-tie trees were found to
slightly increase when applying mechanical tension.
Highest numbers of water trees were found in the
reference and the mechanically stressed cable samples.
The results support previous findings and are in good
agreement with the mechanical damage theory of water