10.1.1 Structure

The name cables is given to long current-carrying devices that carry their own insulation and present an earthed outer surface. In this context, overhead lines for example, are not considered as cables. Power cables have a coaxial structure: essentially, they comprise a central current-carrying conductor at line voltage, an insulation surrounding the conductor and an outer conductor at earth potential. AC cables are generally installed as a three-phase system and hence the outer conductor should only carry fault and loss currents. In practice, a more sophisticated construction is adopted. The interfaces between the metal conductors and the polymeric insulation would tend to include protrusions and voids; features that would lead to electrical stress enhancement and premature failure [1]. To overcome this, a polymer semicon, a conductive polymeric composite, is placed at both interfaces. The inner semicon, the insulation and the outer semicon are co-extruded to ensure the interfaces are smooth and contaminant free. Surrounding this cable are layers to protect the cable during installation/operation and carry the loss/fault currents. These layers also serve to keep out water, which may lead to water treeing (section 10.4.2). A schematic diagram of a power cable is shown in Figure 10.1.

Figure 10.1: Cut-away section of a power cable

10.1.2 Voltage Ratings