Diffusion of Water and Ions in High Voltage XLPE Cable Cores Submerged in Salt Water and Impacts on Testing

Testing of high voltage cables according to the
recommendations by Cigre TB722 is discussed, based on
calculations of water diffusion and measurements of ion
migration in an extruded 52 kV XLPE subsea cable.
Calculations show that, for this cable design, 1000 h is
required to obtain an uniform relative humidity in the
insulation after pre-conditioning, and thus even conditions
for water tree growth. The measured sodium ion
concentration in the insulation screen increased from 2 to
6 ng/mg after two years. This indicates that ions do migrate
into the insulation screen but at a very low diffusion rate.
Water tree analysis of electrically aged samples show that
it is not likely that these ions are causing inception of vented
trees from the insulation screen.
KEYWORDS
High voltage cable, wet design; test regime; pre-
conditioning; extruded insulation; relative humidity,
diffusion of ions; water treeing