Semi conducting for cable

The main use for semi-conducting compounds are as electrical screening or shielding in medium voltage cables which are designed to operate from 6 to 30kV.  These screening layers are put next to the conductor and on the outside of the insulation; their purpose being to contain the electrical field within the insulation and to eliminate any electrical discharges adjacent to the insulation.

A typical construction for a medium voltage cable consists of an aluminium conductor covered by a screening layer, then by a polyethylene or ethylene propylene rubber insulation followed by a further screening layer.  The coefficient of expansion of the insulation layer is typically ten times greater than that of the aluminium and when the cable is at its maximum operating temperture of 90oC, a large enough gap can formed to allow electrical discharges to occur.  The semi-conducting layer then serves to even out the stresses associated with these discharges, which would otherwise attack the insulation at specific points.

Semi-conducting compounds also have the effect of filling in the interstices of the conductor giving a smooth surface for the insulation.  This reduces the electrical flux lines around each individual wire that make up the conductor, which can reduce the stress by 10-15%. 

Generally screens are required for polyethylene insulations above 3.6/6 kV and for 6/10 kV for EPR insulations.  The inner conductor screen is bonded to the insulation but the outer screen can either been bonded or strippable for ease of installation.

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