An Investigation on Thermal Ageing of 500 kV EHVDC XLPE Cables Based on Molecular Structure Changes

This paper investigates the thermal ageing mechanism of 500 kV extra high voltage direct current (EHVDC) cross-linked polyethylene (XLPE) cables, focusing on molecular structural changes. In this study, a self-constructed multi-field ageing experimental setup was utilized to conduct combined multi-physical field ageing experiments on ring-cut XLPE cable samples. Electrical properties such as DC conductivity, resistivity, and DC breakdown strength were analyzed. Simulation using molecular dynamics illuminated the thermal decomposition process of polyethylene, revealing a three-stage ageing mechanism. Experimental results indicated that conductivity initially decreased then increased with ageing time, while the breakdown strength decreased after 144 hours of thermal stress. Molecular chain scission and crosslinking reactions during ageing contributed to the alteration of electrical properties, which was supported by free volume theory analysis. The findings provide essential insights into improving the transmission efficiency and durability of high performance submarine cables.