Cables semiconducting layer

Figure I 2.3 Construction of a) low voltage and b) extra high voltage cable. In the latter, the semiconducting layers equalise the electric field and prevent corona discharges.

Figure 3.10 shows the physical and electrical data of a 400-kV cable used for comparison. An existence of semiconducting layers introduces an error in the charging capacity of the cable. The relative permittivity of the insulation (XLPE) is converted from Equations 2.4 to 2.7, according to Equation 3.72, in order to correct the error and achieve a reasonable cable model [10] ... 

It is well-known that a semiconducting layer exists on the surface of a cable conductor, which occasionally produces a significant effect on a cable transient. The impedance of the semiconducting layer was derived in Reference 22 and may be implemented into a cable-impedance calculation. It should be noted that the admittance of the semiconducting layer is far more important than its impedance, from a transient analysis viewpoint. 

Ametani, A., Y. Miyamoto, and N. Nagaoka. 2004. Semiconducting layer impedance and its effect on cable wave-propagation and transient characteristics. IEEE Trans. Power Deliv. 19(4) 523-531. 

The carbon black in semiconductive shields is composed of complex aggregates (clusters) that are grape-like stmctures of very small primary particles in the 10 to 70 nanometer size range (see Carbon, carbon black). The optimum concentration of carbon black is a compromise between conductivity and processibiUty and can vary from about 30 to 60 parts per hundred of polymer (phr) depending on the black. If the black concentration is higher than 60 phr for most blacks, the compound is no longer easily extmded into a thin continuous layer on the cable and its physical properties are sacrificed. Ionic contaminants in carbon black may produce tree channels in the insulation close to the conductor shield. 

Power Cables. The materials mosdy used to produce power cables are ethylene copolymers loaded with conductive carbon black for semiconductive shielding layers, polyethylene or ethylene—propylene mbber-based compounds as insulations, and either thermoplastic materials (eg, polyethylene, PVC) or thermosetting (based on chlorinated polyethylene (CPE), chlorosulfonated polyethylene (CSPE), chloroprene, etc) for jackets. 

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