Capacitance of Cables

Capacitance is the ability of cable to store electrical charge. It is defined as ration of electrical charge (Q) to electrical potential (V).

  Q=C/V Col/Volt

In LV, MV and HV Cables may be single core with earthed metallic layer around the core or 3 cores with earth metallic screened around each core. In both cases electrostatic field is contained within the earth screened and is substantially radial.

The capacitance of single core cable is

C= ???/18 log (D/d) μF/Km

 D= Diameter over Insulation (mm)

 d= Diameter of Conductor (mm)

 ?o= Relative permittivity of dielectical material

?r =4.8 for PVC

?r= 2.3 for XLPE

?r= 3.2 for Butyl rubber

?r= 4 for Silicone rubber

?r= 4.5 for XLPE-Filled

The relative permittivity depends on temperature and frequency. It describes the characteristics of insulation material. This is also used for calculating low voltage cables capacitance without any metallic layer.

Hence capacitance depends upon the spacing between conductors. Power losses in cables can be achieve by compacting the conductor. Moreover mutual capacitance is also reduced by compacting the conductor.