Résumé
The present work concerns the study of the cross-linked polyethylene (XLPE) used for high voltage direct current (HVDC) cable insulation. The electrical properties of XLPE have been widely studied under AC stress, however the behavior of these materials under high DC stress is less known and needs thorough investigation. The insulation should be better understood in terms of dielectric behavior and lifetime. A better knowledge of HVDC insulation could allow manufacturers, utilities and TSO’s to propose a relevant qualification processes and to ensure that cable systems will remain safe and operational during their entire lifetime.Firstly, this work introduces HVDC cables and especially the physical and chemical stresses assumed by the cross-linked polyethylene (XLPE) insulation due to service conditions. XLPE insulation is the result of the reticulation of low density polyethylene (LDPE), with a peroxide such as dicumyl peroxide. The cross-linking reaction generates decomposition products in the insulation. The cables, degassed in order to reduce the amount of decomposition products, contain additives such as antioxidant agents, protecting the insulation during the production of HVDC cables and during operation. The stresses can influence the insulation dielectric properties. As example, the presence and the development of electric charges could influence the lifetime of the insulation.The injection and conduction properties, the dielectric loss mechanisms, the breakdown field, the space charge accumulation and some physical and chemical properties have been investigated for the material in its initial state. For this work, XLPE samples (Rogowski cup) with semi-conductive electrodes have been made. Under thermal and electrical stresses, electric charges can be injected in the insulating material and then participate to the conduction, according to different mechanisms. Dominant mechanisms have been identified according to the applied electric field: Schottky-type injection and Space Charge Limited Current (SCLC) conduction. Concerning the dielectric loss mechanisms, the low frequency mechanisms are nearly DC conduction at room temperature and DC conduction for higher temperatures. Furthermore, the dielectric loss factor increases when temperature increases. The breakdown field has been measured at room temperature giving by using the Weibull’s law 375 kV/mm. The space charges have been measured using the Thermal Step Method (TSM). These analyses show that two types of charge are present in the material (homocharge and heterocharge). This effect is influenced by temperature and electric field. The total electric field (sum of the applied electric field and the electric field due to space charge) reaches about 100 kV/mm, when 60 kV/mm is applied. Concerning the physical and chemical properties of XLPE samples, the melting point has been measured at 103°C and the crystallinity ratio is about 39 %. Before ageing, the carbonyl index, due to the slight presence of carbonyl bonds, has been assessed at 0.5.The impact of a combined electric and thermal stress on dielectric properties has been studied at 70, 80 and 90°C under 30 and 60 kV/mm during 857 days of aging tests. Breakdowns have been recorded only for samples aged at 90°C under 60 kV/mm. Increases of electrical capacitance and loss factor, possibly linked to the consumption of the antioxidant, have been observed only at 90°C, after 700 days and 400 days under 30 and 60 kV.mm-1 respectively. Space charge analysis has also shown significant evolutions. Differences have been observed as a function of ageing test temperature, applied electric field stress and ageing time. These results have been used to propose an ageing mechanism taking into account the development of space charges and based on the consumption of the antioxidant, leading to the growth of an XLPE oxidized layer near the semi-conductive electrodes and to a possible reduction in the virtual thickness of the insulation that could explain the sample breakdown.. These results are specific to the testing protocole (Rogowski plaques in air circulation oven) and the proposed theory would need to be confirmed on specimens more representative from an HVDC cable with an aluminium screen (i.e. insulation protected from air).