In most modern MV cables, especially underground, cables are shielded or screened with an earthing conductor. The screen consists of lapped copper tape or metallic foil usually less than 1mm in thickness, which is the interface between the conductor and the insulation (PVC, XLPE). The main purpose of this conductive screen is to maintain a uniformly divergent electric field and contain the electric field within the cable core. The conductor screen is made from semi-conductive material, which is designed to hold back voltage. This smooths out the surface irregularities of the conductor by making the voltage on the inside of the insulation the same. This semi-conductive screen material is based on carbon black, which is dispersed within a polymer matrix. The concentration of carbon black needs to be sufficient to ensure adequate and consistent conductivity. It must also be optimized to provide a smooth interface between the conducting and insulating components of the cable, which is important as it decreases the occurrence of high stress regions on the cable.
The semi-conductive screen also reduces voltage stresses where the conductive components interface with the insulating components. It accomplishes this when the expansion of the insulating layer is typically 10 times greater than that of the conductor, so that when the cable is at its maximum operating temperature of 90oC a large enough gap is formed to allow electrical discharges to occur. This then serves to even out the stresses associated with these discharges which would otherwise attack the insulation at specific points along the cable.
The screen wires are connected at each extremity of the circuit in solidly grounded systems, dispersing the circulating currents of the MV cable. When connecting the screen wires, it is important to note whether the installation is using current transformers (CT’s) for the circuit protection. Any currents on the screen wires will be phasor summed in the conductor by the CT. In the case of fault currents, this is often directly out of phase with the conductor current. As the screen passes through the CT as part of the cable it is then necessary to bring the made-up screen wires back through the CT in the opposite direction prior to grounding. This ensures the screen current is effectively cancelled out to ensure the correct operation of circuit protection devices.