Sammendrag
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 °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 °C. A
cooling system maintained 10 °C at the insulation screen. One
length of the cable was subjected to 2·U0 (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%. ©2013 IEEE
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 °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 °C. A
cooling system maintained 10 °C at the insulation screen. One
length of the cable was subjected to 2·U0 (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%. ©2013 IEEE