XLPE Medium voltage underground cable

Underground distribution cables range from 6.6 kV to 33 kV in voltage rating, and XLPE cables that employ crosslinked polyethylene as insulator are generally used.
It may be said that the history of XLPE cable is the history of countermeasures against water tree, a process of insulation deterioration due to water absorption. Water tree is a phenomenon in which water penetrates into insulation under the influence of electric fields forming a dendritic (tree-like) array of voids filled with water, thereby degrading the insulation performance. Countermeasures against this include: minimizing the foreign matters and voids contained in the insulation, reducing the protrusions on the interface of the insulation and relaxing the local concentration of electric fields. These can be realized by dry cross-linking where cables are manufactured without using water vapor pressure, by composing a layered cable insulation where the inner and outer semiconducting layers are structured by semiconductive plastics replacing conductive tapes, and by extruding the resultant semiconducting layers simultaneously with the insulation to carry out triple-layer extrusion. These countermeasures were implemented sequentially in the 1980s, significantly reducing the occurrence of water tree deterioration since that time.

Moreover, water-impervious XLPE cables were developed and applied in the late 1980s centering on the 22- kV and 33-kV XLPE cables, with the aim of improving the reliability further by completely preventing entry of water into the cables. The water-impervious tape consisted of a laminated lead tape which is laminated with a lead foil and plastics to improve the extensibility, making it possible for the tape to follow the thermal expansion and contraction of the cable. This laminated lead layer was bonded on the inside of the cable sheath, constituting a waterimpervious cable. In the 2000s, we eliminated lead from water-impervious cables ahead of others intending to make an environment-conscious cable. To this end, we replaced lead on the water-impervious layer with aluminum to complete an aluminum water-impervious cable, and we are promoting substitution of this cable for lead water-impervious cables.