With the exception of low-voltage cables, the electric field in power cables is limited to the volume of the dielectric by an inner and an outer conductive layer. The field distribution or voltage gradient is equivalent to a homogenous cylinder and is therefore represented by a homogenious radial field.
The value of the voltage gradient at point x within the insulation can be calculated using a logarithmic relationship. The electrical field strength is highest above the conductor screen, below the insulation and lowest below the insulation screen, above the insulation.
In high-voltage DC cables, a temperature gradient can change the conductivity of the insulator which leads to significant distortion of the electric field.
The values for dielectric strength of insulating materials is taken from chemistry.mdma.ch .
| $\frac{U_e}{1000} \frac{1}{r_x \ln\left(\frac{r_{osc}}{r_{isc}}\right)}$ | homogenous radial field |
| $\frac{\delta_i \frac{U_e}{1000} \left(\frac{r_x}{r_{osc}}\right)^{\delta-1}}{r_{osc} \left(1-\left(\frac{r_{isc}}{r_{osc}}\right)^{\delta_i}\right)}$ | function of the radial position $r_x$ |
| Material | Dielectric strength | Reference |
|---|---|---|
| PE | 21.7 | |
| HDPE | 19.7 | |
| XLPE | 19.7 | same as HDPE |
| XLPEf | 19.7 | same as HDPE |
| PVC | 11.8 | range between 11.8 and 15.7 |
| EPR | 23.6 | like Polypropylene/polyethlyene copolymer |
| IIR | 23.6 | |
| PPLP | 28.7 | like Aramid papers, calendered |
| Mass | 12.2 | like Aramid papers, uncalendered |
| OilP | 12.2 | like Aramid papers, uncalendered |
| PP | 23.6 | |
| SiR | 26.0 | range between 26 and 36 |
| EVA | 19.3 | like Ethylene-chlortrifluoroethylene copolymer |
