Résumé
During the operation of the cables, their insulation is subjected to simultaneous or consecutive electrical, thermal, mechanical and environmental stresses. These stresses contribute to the initiation and development of some processes of XLPE degradation; processes that lead, in time, to the insulation breakdown and, therefore, to the removal and failure of the cables. This chapter presents the mechanisms of XLPE insulations degradation generated by the electric field, namely the electric discharges, the electric and water trees. Initially, the distribution of the electric field inside the insulations (i.e., in the absence and in the presence of the space charge) is analyzed and shown to be non-uniform. The existence of macroscopic defects (e.g., protuberances, non-uniformities of the semiconductor layers, cavities, clusters of impurities, etc.) determines large local values of the electric field, which can exceed the maximum permissible limits imposed on each type of cable (starting from which the mechanisms of insulations degradation are initiated). It is outlined then the phenomena of aging, degradation and failure of the XLPE insulation due to the electric field and its values are presented for each characteristic phenomenon and degradation mechanism. Further, the physical processes of initiation and development of partial discharges and electric and water trees, their characteristic parameters, the factors that contribute to the intensification of these processes, as well as the methods of increasing the initiation times and reducing the development speed of the mechanisms are analyzed in detail. The resistance of the XLPE insulations to each of the three degradation mechanisms is studied in detail, and some results from literature regarding methods of increasing their values are presented, respectively, the improvement of the XLPE characteristics and of the insulations manufacturing technologies. In addition, the mechanisms of space charge accumulation and its influence on the degradation mechanisms of XLPE insulations are investigated.