Conductors

Date: 2015-10-07; view: 489.

CABLE MATERIALS

Cables

Cables have been used in transmission and distribution networks since the early days of the electrical power industry. Generally, long distance power transmission is carried out through overhead lines. However, transmission and distribution in densely populated urban areas mostly uses underground cables. Although significantly more expensive than the overhead lines, the cables are preferred in urban areas due to safety, reliability and aesthetical considerations. As a result of developments in insulating materials and manufacturing techniques, high voltage cable technology has improved significantly over the years. With a continuous increase in the overall length of cable networks, questions regarding reliability, failure modes and diagnostics of such cables have assumed greater significance.

Cables are constructed using a variety of materials for conductors, insulation, screening and armoring. The most common conductor materials are copper or aluminum of high purity (99,5% purity), since the resistivity of such materials significantly increases with impurity content. Sometimes, sodium is also used as the conductor material. In recent years, the possibility of using high temperature superconducting materials for power applications is also being examined. The choice between aluminum and copper is normally based on resistivity (ρ), cost as well as mechanical and manufacturing considerations. Table 1 compares resistivity values for a few materials used in cables. For a given current rating, aluminum requires a larger conductor cross-sectional area than copper. Both solid and stranded conductors are used; the choice depends upon total cross-sectional area, flexibility and manufacturing considerations. For larger cross sections, stranded construction with alternate layers spiraled in opposite directions is generally preferred.

Table 1 Resistivity of Some Conductors at 20°C