Dielectric behaviour of a new grade of cross-linked polyethylene for HVDC cables

In order to ensure the energy transmission from distant energy generation areas, as renewable energies, to consumption points, it is well known that High Voltage Direct Current (HVDC) presents some advantages compared to High Voltage Alternative Current (HVAC). The constant growth of DC network use in the last decades brings up several questions. Extruded insulation for HVDC cables is being improved and its behaviour needs to be better understood.

In this work, a new grade of insulating material made of cross-linked polyethylene (XLPE), used for cables up to ±550kV DC, is studied. To characterise the cable, the frequency and the transient current responses are measured under various temperatures, humidity and voltages. Comparisons are carried out on three different kinds of samples: a thin plate of XLPE made of pressed melted pellets, a miniaturised model cable with inner and outer semi conductive screens extruded with XLPE, and the same model cable which includes not only the extruded part but also the industrial copper screen and outer sheath.

These geometry differences result in changes in dielectric spectra that are analysed. The impact of the series resistance at high frequency due to the different contact resistance resulting from the different counter-electrodes is in particular highlighted.