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Turns out a high voltage cable can have semiconductive layers in its design. For example here's a rather old patent covering some specific way of producing such cable.

This doesn't make much sense. Yes, insulating layers are obvious and also a cable can contain a conductive screen to reduce dielectric losses.

What's the need for a semiconductive layer?

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The material described in the patent referred to is not semiconductor material in the sense that we understand it in the electronics field: The material is described as a polymer base "mixed with a conductivity imparting agent such as carbon black".

In other words, it forms a weakly conducting, yet not perfectly insulating, shield layer around the conductor element. Modern parlance would refer to this as a partially conductive material, I suggest.

The application seems to be similar to the use of a very high value resistor between an earthing strap, and the building earth such straps are often connected to.

Referring to the default source, WikiPedia:

At the inner (2) and outer (4) sides of this insulation, semi-conducting layers are fused to the insulation.[4] The function of these layers is to prevent air-filled cavities between the metal conductors and the dielectric so that little electric discharges can arise and endanger the insulation material.

On the same page, more information is forthcoming from the section on cable joints, though applicable to the cable itself, as well:

The black parts in this picture are semi-conducting rubber parts. The outer one is at earth potential and spreads the electric field in a similar way as in a cable terminal. The inner one is at high-voltage and shields the connector of the conductors from the electric field.

While this is not definitive first-hand knowledge, such use of semiconducting, or to be more precise, partially conductive i.e. high resistance shield layers seems to be common knowledge in the literature.

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The patent describes another connotation of semiconductivity, something we nowadays call either static dissipative or static shielding properties. Hints are found throughout the patent, e.g. talking about adding carbon black to the PE or PV base material. Carbon black is an old fashioned way of reducing the capability of insulators to build up a static charge.

It is very helpful for data cables to not build up a static charge, as such a charge can very easily be transferred to a device that the cable is being plugged into and may damage inside circuitry.

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Without semiconducting layer E and H fields are not uniformly concentrated in insulation material. In other words where E and H fields concentrated more at places where cable contact with earth or contact with neighbouring phases and least concentrated at other places. This causes more Electromagnetic energy flows at some places causes heating and damaging of cable insulation. Semiconducting layer makes entire outer circumference of insulation at earth potential in turn uniform distribution of Poynting vector P=EXH i.e., Electromagnetic energy.

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  • \$\begingroup\$ This is actually the reason for the semiconductive layers: to equalise the electric field around the entire cable. \$\endgroup\$ – Li-aung Yip Jul 26 '15 at 19:13
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It is used for uniform flow of stresses that may develop on outer surface of conductor and outer semiconductor for, to draw the leakage current due to damage insulation

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