The XLPE insulation possesses very good electrical, mechanical and thermal characteristics in medium voltage dc power cables. This type of insulation is excellent chemical resistant and also resistant to cold. Due to various advantages, the XLPE insulated type has vastly displaced the traditional classical paper-insulated types in medium voltage dc power cable.
In order to prevent the penetration of moisture and also to extend the duration of life, the XLPE insulated medium voltage dc power cables are designed with longitudinally water-proof screen including an additional swell tape and a PE outer sheath. The manufacture of this sheath is based on high density polyethylene (HDPE), in which an additive organic peroxide is mixed. Due to the heating and pressure the molecule chains are joined each other, assuring the transion from thermoplastic to elastic condition.
In comparison to PVC and paper-insulated medium voltage dc power cables, the advantages of XLPE insulated medium voltage dc power cables are possessing a low dielectric factor, such as it is 100 times smaller than of PVC-insulated medium voltage dc power cables. Moreover, a better dielectric constant value has an effect on the low mutual capacitance, the short circuit to ground and the charging current of XLPE insulated medium voltage dc power cables. The good properties of XLPE insulated medium voltage dc cables remain constant at a long temperature range.
In order to avoid any damage, the XLPE insulated medium voltage dc power cables should carefully laid and installed. It must be ensured that the cables should not be pulled over the hard or sharp edges. The cable ends must be water-tight-sealed. After cutting the length both ends must be sealed immediately. A laying depth of 60 to 80 cm is recommended. Single conductor medium voltage dc power cables are normally arranged in a trefoil touching or tringular shape. For laying in conduits, specially the influence of thermal insulation of air space between the cable and the inner wall of conduit should be considered.