DC field measurements around a cable end

Even under corona-free conditions, insulating structures (materials) may be charged in DC fields. This is due to the fact that air has a finite ionic conductivity. The finite conductivity of air results from charges generated by background radio activity and cosmic radiation. Typical ion concentrations on ground are 107 to 1010 ions/m3 which leads to conductivities ranging from 10-16 to 10-13 S/m. An experiment has been conducted where the end of a terminated high voltage XLPE cable was used as an insulating structure. The field distribution around the stripped cable end was measured with an applied DC voltage of -140 kV using a rotating field probe. The measurements are compared to simulations, which do not specifically take into account space charge effects. A big discrepancy is observed, showing the relevance of taking into account space charge effects when modeling HVDC insulation components subjected to air.