Effect of water on the performance of XLPE insulation

Water-tree counting measurements, water and impurity content measurements, and AC breakdown tests were performed on several field-aged medium-voltage extruded cables. The results indicate that the water-tree density increases with the insulation contamination level, particularly for aluminium, sodium, and potassium contaminant. However, neither of these aging factors seems to be correlated with the residual dielectric strength, which, on the contrary, is clearly influenced by the water content. In fact, the results suggest that water is the major aging factor of XLPE (cross-linked polyethylene) cables