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| Effect of Extrusion Crosslinking Degree on the Interfacial Electrical and Mechanical Properties of HVDC Submarine Cable Factory Joints |
| Zhang Yunxiao1,2, Chen Yuxiang1, Lin Wenxin3, Hu Hao1, Liu Yuhao1 |
1. School of Electrical Engineering and Automation Fuzhou University Fuzhou 350108 China;
2. The Innovation Research Institute of Zhejiang University of technology Shengzhou 312400 China;
3. Fujian Shuikou Power Generation Group Co. Ltd Fuzhou 350004 China |
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Abstract High-voltage direct current (HVDC) cross-linked polyethylene (XLPE) submarine cables serve as the core transmission equipment for grid integration of offshore wind power, with the interfacial performance of factory joints directly determining the long-term reliability of the cable system. This study systematically investigates the influence of extrusion crosslinking degree on the interfacial electrical and mechanical properties of factory joints, focusing on the critical weak area of the extruded insulation interface. By adjusting the dicumyl peroxide (DCP) content to achieve graded crosslinking degrees and combining microstructural characterization with macro-performance testing, the relationship between extrusion crosslinking degree and interfacial properties is revealed.
Low-density polyethylene (LDPE) was used as the base material, and extrusion materials with DCP content gradients ranging from 0% to 3.5% were prepared via the impregnation absorption method. XLPE with 2% DCP content served as the bulk material. Single-layer (S-series) and double-layer (D-series) specimens simulating factory joint interfaces were fabricated using a flat-plate hot-pressing method. The crosslinking degree was measured by xylene extraction, interfacial crystalline morphology was observed via scanning electron microscopy (SEM), and interfacial performance was comprehensively evaluated through tensile strength testing, conductivity testing, space charge distribution (PEA method), and DC breakdown testing.
Physicochemical results indicate that an appropriate increase in DCP content enhances XLPE crosslinking degree, while excessive DCP (>2%) reduces it. SEM observations reveal that the interface of double-layer XLPE specimens exhibits tortuous and discontinuous microstructural features, with localized amorphous regions, coherent crystalline regions, and small-sized crystalline regions. The extrusion crosslinking degree significantly influences interfacial microstructure distribution: at relatively low crosslinking degrees (DCP content <1%, crosslinking degree <84.5% of the bulk material), the proportion of amorphous regions at the interface decreases markedly with increasing crosslinking degree, and crystallite size distribution becomes more uniform and continuous. When the extrusion crosslinking degree matches that of the bulk material, the interface develops numerous small-sized crystalline regions, while excessive DCP leads to the formation of microcracks.
Electrical and mechanical tests demonstrate that, except for conductivity, the DC electrical and mechanical properties of double-layer specimens are inferior to those of bulk material without interfaces, confirming that the XLPE interface is a weak point in HVDC submarine cable factory joints. When the extrusion crosslinking degree is below 84.5% of the bulk material, interfacial tensile strength, space charge characteristics, conductivity, and DC breakdown voltage improve significantly with increasing crosslinking degree, with space charge accumulation nearly disappearing at 84.5% crosslinking. However, when the extrusion crosslinking degree matches that of the bulk material, although conductivity decreases and DC breakdown strength slightly improves, interfacial tensile strength and space charge characteristics deteriorate, increasing the risk of deformation, electric field distortion, and even insulation failure under HVDC conditions. Notably, excessive DCP (>2%) causes severe degradation in both electrical and mechanical performance.
In summary, moderately reducing the extrusion crosslinking degree (to approximately 84.5% of the bulk material) enhances interfacial mechanical strength and space charge suppression. This approach not only mitigates scorching risks and improves process stability but also ensures the quality of HVDC submarine cable factory joint interfaces. These findings provide critical theoretical guidance for optimizing the manufacturing process of factory joints.
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Received: 30 April 2025
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2026, Vol. 41 
