Water treeing of XLPE cables during dynamic mechanical tension

The main purpose of the work presented in this paper was to experimentally examine possible water tree enhancement caused by dynamic mechanical tension. The experiments were performed using samples of 12 kV XLPE cables exposed to static and dynamic tension. A test rig was designed allowing application of static and dynamic strain at mechanic oscillation frequencies of 0, 0.1 and 0.01 Hz at a maximum amplitude of 6% elongation. During ageing the cable samples were soaked in tap water at 30°C and an effective 50 Hz AC voltage of 14 kV (E_Max= 5.2 kV/mm) was applied across the cable insulation. The degree of water tree degradation was characterized using optical microscopy investigation of 0.4 mm thick methylene blue stained slices. Both the density and growth rate of bow-ties and vented water trees were found to increase significantly when applying mechanical tension. Compared to the non-strained cable sections the number and length of water trees were found to increase by approximately 100 and 50%, respectively. No significant difference in water tree lengths were observed between samples aged at 6% static and dynamic mechanical strain. - The results are in good agreement with the mechanical damage theory of water treeing.