Synergistic effects of mechanical, electrical, and thermal stresses on XLPE insulation breakdown

Abstract

High-voltage direct current (HVDC) cables are essential for the centralized transmission of power from offshore wind farms and large-scale onshore new energy bases. The DC breakdown strength of cross-linked polyethylene (XLPE) insulation, when subjected to thermal-mechanical-electric coupled fields, is a critical factor in determining the operational safety and reliability of these transmission systems. The classical breakdown models are fail to quantitatively characterize the breakdown performance of XLPE under multi-field coupling conditions. This study experimentally investigates the DC breakdown properties of XLPE under the influence of combined electrical, thermal, and tensile stresses. Experimental results indicate that increasing tensile stress reduces the breakdown strength of XLPE. Elevated temperature further aggravates such degradation through synergistic interactions with mechanical loads. In conjunction with low-field nuclear magnetic resonance (LF-NMR), dynamic mechanical analysis (DMA), broadband dielectric spectroscopy (BDS) and thermally stimulated depolarization current testing (TSDC), the molecular network structure, chain segment dynamics, and trap distribution in XLPE are systematically analyzed in relation to tensile stress and temperature. According to the characterization results, the coupling effect of tensile stress and high temperature lessens intermolecular entanglement and interactions, leading to improved segmental mobility. This is accompanied by a reduction in trap energy levels and an acceleration of carrier transport. Based on the critical threshold for electric field-induced oriented molecular chain motion that triggers insulation failure, a breakdown model under coupled thermal-mechanical-electric stresses for XLPE is established. The combined effects of tensile stress and high temperature decrease the activation barrier for molecular motion, making the chains more susceptible to electric field-induced displacement and reducing the breakdown field. This model quantitatively elucidates the relationship between molecular relaxation and breakdown strength, offering theoretical insights for optimizing the performance of high-voltage cable insulation in complex service environments.