Condensation of Water Vapour in XLPE Insulation at Different Cooling Rates and Pressures

Abstract

Temperature cycling and temperature gradients can cause super saturation of water vapor in the insulation. Condensation of water can then occur and high hydrostatic pressures can be established within water droplets embedded in the polymer. Water treeing is closely related to the condensation of water, and the main purpose of this paper has therefore been to examine the effect of different cooling rates and pressures on water condensation in XLPE insulation during temperature cycling. 1 mm thick samples cut from a XLPE cable with a super clean grade XLPE (Borealis LE 4201 S) were subjected to temperature cycles from 90 to 65, or 90 to 20/spl deg/C at cooling rates ranging from 0.12 to 120/spl deg/C/min. The total pressure during cycling was atmospheric or 100 bars. The insulation was characterized by water content measurements and microscopy SEM examinations. The solubility of water was about 244 g/m/sup 3/ at 90/spl deg/C independent of the applied pressure. The results show that a relatively small but significant increase of about 65 g/m/sup 3/ of water were measured for the fastest cooling rates at atmospheric conditions (1 bar) after 50 cycles. No significant change was measured for the slow cooling at the same conditions. An even smaller, but not significant, increase was measured at 100 bars, indicating that the high external pressure to some extent suppresses the formation of micro-voids. It is likely that the relatively low water content after the temperature cycles was due to a very low density of condensation nuclei within the insulation avoiding the formation of micro-voids. This was confirmed by the SEM analysis.