Thesis Justine BILLORE

“Thermal aging of XLPE for HVDC cables”

To connect the different points of the network, the transmission and distribution of electric power used cables. Since few years, cables and especially their insulation part are evolving, going from paper insulation to a synthetic insulation. Nowadays, the insulation part of HVDC cables is in crosslinked polyethylene or XLPE.

Their insulation part is the weak point of this cables. In order to ensure their lifetime, understanding and mastering their ageing is essential. Ageing studies of cable insulation have already been carried out, focusing on electrical markers such as space charge or partial discharges. The link between the electrical markers and the physical and chemical properties was not identified.

This manuscript is interested in the study of the thermal ageing of crosslinked polyethylene. The studied material is a commercial grade used in the HVDC cable production. The study consist of the achievement of different campaign of thermal ageing at 135 °C then the characterisations of the aged samples. The tracked properties are both physical and chemical like the crystallinity rate, the characteristic temperatures (e.g. Tm), and the mechanical properties. The dielectric and electrical properties were also tracked to identify the impact of the thermal ageing on the permittivity, on the conductivity, and on the conduction and injection mechanisms under electric field.

From this study, we are proposing a scenario of crosslinked polyethylene thermal ageing associated to several electrical, chemical and physical markers.

This Ph.D. thesis work was performed with the group “Materials for Electrical Engineering” Ampère laboratory, the IMP laboratory and funded by the institute for the energy transition SuperGrid Institute.