STEM-EDS of Inception Sites of Vented Trees in a HV XLPE Subsea Cable

High voltage XLPE cable cores have been subjected to wet ageing following the recommendations in CIGRE TB 722. The electrical ageing was performed at 500 Hz and 10 kV/mm. Vented water trees were detected initiating and growing from the semi-conductive screens. The inception sites of the trees were cut by cryo-microtomy and studied using scanning transmission electron microscopy with energy dispersive X-ray spectroscopy (STEM-EDS). Inorganic impurities were detected on both sides of the smooth XLPE-semiconductive screen interface close to the inception site. KEYWORDS High voltage XLPE cables, Vented water trees, STEM-EDS, Inception site