Water Treeing in Subsea XLPE Cables with Thermal Gradient

Water treeing is a well-known ageing mechanism in polymeric cable insulations. In case of wet designed medium voltage subsea cables, very few service failures have been reported even when the designs allows seawater to be in direct contact with the insulation screen. During service there is normally a temperature gradient across the cable insulation system due to the resistive heating of the conductor. It has previously been shown for model cable insulation systems that both the initiation and growth rate of water trees are strongly dependent on the relative humidity at room temperature. This paper shows that a temperature gradient influences both the water ingress into the cable core and the radial distribution of water trees in the insulation. This provides an explanation for the low occurrence of failures due to water tree growth in wet designed subsea cables. Two lengths of a 36 kV XLPE cables have been subjected to a thermal gradient of 40 degrees C over the cable core. The cables were initially dry, and were then submerged in water to allow water to diffuse into the insulation system. The current loading resulted in a conductor temperature of 50 degrees C. A cooling system maintained 10 degrees C at the insulation screen. One length of the cable was subjected to 2.U-0 (36 kV) prior to water tree analyses. The other length of cable had no voltage applied and was used for water content measurements. One sampling has been performed after 8 months of ageing. These measurements were compared to the results of numerical simulations of the radial water distribution in the cable insulation. The numerical calculations show that the relative humidity (RH) decreases towards the conductor. The measurements show that the water distribution in the cable insulation is in good agreement with the calculations. In the water tree analysis only bow-tie trees were observed. Most of the water trees were found in the outer parts of the insulation where the numerical calculations indicated that the RH was over 70%. In the inner parts of the insulation, where the RH was below 70%, no or few trees were found. The results show that water trees only occurs if the RH is above 70%.