Space Charge Profile of TR-XLPE Cables Aged Under Multiple Stresses

In this paper the space charge distribution profile along the thickness of tree-retardant crosslinked polyethylene (TR-XLPE) cables, commonly used in medium voltage system and degraded under multiple factors as temperature, AC and impulse voltages are presented. A comparison of the space charge distribution along the insulation thickness (radial direction) of samples extracted from 15 kV TR-XLPE cables, 2 AWG Cu conductor, manufactured under dry curing process was done. Two 7 m long cable samples were submitted to accelerated ageing; one sample was under thermal stress and the other was under electrothermal factors combining thermal cycles with 25 kV AC voltage applied continuously and impulse voltage transients. Looking at the results it can be concluded that after the first thermal cycle the space charge is reduced in comparison with the profile of the new sample. This confirms that crosslink by-products are expelled under thermal treatments. The space charge density is higher in the central region of the insulator thickness of the new sample and aged samples as well. It is also evident that the electrothermal ageing cycle is more severe because it releases more quantity of space charge along the insulation thickness than the thermal one.