Remnant Static Mechanical Stresses and Water Tree Ageing of XLPE Power Cables

The main purpose of the work presented in this paper was to experimentally examine possible enhancement of water treeing caused by remnant mechanical stresses, frozen-in during the manufacturing process of extruded XLPE power cables. 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 0,8%. 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 treeing.