Phd Priscillia DANIEL

“Electrical properties study of cross-linked polyethylene insulated cables in presence of gaseous cavities aged under DC or DC with harmonic voltages”

In order to accommodate large scale renewable energy, electrical networks integrates more and more high-voltage direct current (HVDC) systems. Interface between AC and DC networks is ensured by power converters which also generate harmonics. It is known that during crosslinking process of polyethylene (XLPE), microcavities appear inside polymer matrix. This study aims to examine possible impact of these harmonics in presence of microcavities. Depending on their size and on the electrical field, partial discharges (PD) can occur in these microcavities. Two types of cables have been aged: a medium voltage AC (MVAC) cable with millimetric cavities induced artificially and a model cable with an HVDC XLPE grade. MVAC cable sections have been submitted to DC voltage under temperatures between 90 to 120°C. Model cables have been subjected to superimposed DC plus 1800 Hz AC voltages under temperatures from 70 to 90°C. Ageing have been characterized by PD, current and dielectric spectroscopy measurements. In presence of millimetric cavities, ageing have caused a decrease in PD inception voltage (PDIV) and an increase in PD magnitude. For model cable, PDIV values have been assigned, according to a proposed model, to cavities of about 10 µm in diameter. Conduction currents were increased depending on ageing. Finally, dielectric spectroscopy characterisations have shown differences between thermal and electrothermal ageing. Electrical field enhancement factors under DC voltage have been estimated in various cavity geometries thanks to numerical simulations. These factors reveal to be higher than the ones used under AC voltage. Space charges involvement in PD phenomena inside cavities is discussed based on measurements and simulations.