Thermal aging and morphology of high voltage crosslinked polyethylene cables

High Voltage polyethylene (PE) and crosslinked polyethylene (XLPE) cables extruded and cured under different conditions were obtained from major cable producing companies and studied to define their physical, chemical and morphological structures. All XLPE cables were found to comprise of crosslinked network (gel) and uncrosslinked extractable fractions (sol). The amount of extractables, crosslink density, melting characteristics as well as morphology were found to differ according to the method of heat transfer used during crosslinking. The effect of extractables on the melting of the original cables and on annealing process, especially at high annealing temperatures, was found to be different from one cable to another. The morphologies throughout the cables are sheaf-like and the appearance of those morphological entities upon annealing remained unchanged. An understanding was obtained of the annealing mechanisms operating at different temperatures which resulted in multiple melting behavior. One process was shown to be caused by a separation of sol from gel in the melt. On the other hand, morphological structure was found to be altered on a lamellar scale upon thermal and thermoelectrical aging in terms of the lamellar thickness and void content. Understanding of the complicated multiple melting behavior and molecular changes taking place in field aged cables was deduced from knowledge of isothermal annealings carried out on the original cables. While a deterioration in the dielectric breakdown strength was observed for the laboratory cured XLPE samples annealed at temperatures up to 95 °C, the breakdown strength was found to increase for samples annealed between 100 °C and 107 °C.