低交联度LLDPE用于XLPE绝缘的机械及直流电气性能

Mechanical and DC electrical properties of low crosslinking degree LLDPE for XLPE insulation

为了降低直流电缆绝缘材料的电导率,并提升电导率及直流击穿场强的温度稳定性,同时改善绝缘的高温力学性能,文中引入低交联度线性低密度聚乙烯(linear low density polyethylene,LLDPE)作为直流电缆绝缘材料。文中对比添加不同含量过氧化二异丙苯(dicumyl peroxide,DCP)的LLDPE和低密度聚乙烯(low density polyethylene,LDPE),通过热延伸、动态热机械测试及拉伸试验分析其力学性能,并测试直流电导率和直流击穿场强。结果表明,DCP质量分数为0.7%~2.0%范围内,交联LLDPE具有更低的热延伸率,DCP质量分数为0.7%即可满足高压直流电缆对交联聚乙烯(crosslinked polyethylene,XLPE)热延伸率的要求。由此结果,结合减少副产物的目的,选出DCP质量分数为0.7%的LLDPE作为传统XLPE(DCP质量分数为2.0%的LDPE)的对比及替代绝缘材料。动态热机械测试(dynamic mechanical analysis,DMA)及拉伸试验验证了DCP质量分数为0.7%的LLDPE比传统XLPE拥有更高的杨氏模量及断裂伸长率;电导率及直流击穿实验表明前者在30~90℃拥有更低的电导率及更优的电导率和直流击穿场强的温度稳定性。由于实际电缆中的交联副产物难以脱尽且分布不均,严重影响电缆长期安全运行,可考虑将DCP质量分数为0.7%的低交联度LLDPE用作直流电缆绝缘材料,从而在满足高温机械性能的同时降低直流电导率并增强电导率的温度稳定性。

A way to improve high-temperature mechanical behavior and reduce conductivity of direct-current cable insulation material is introduced by using low-crosslinked linear low density polyethylene(LLDPE).LLDPE and low density polyethylene(LDPE)with different dicumyl peroxide(DCP)content are analyzed.The mechanical properties of LLDPE and LDPE with DCP are assessed by hot-set test,dynamic mechanical analysis(DMA)and tensile test.The DC conductivity and DC breakdown strength of these materials are also tested.The results show that crosslinked LLDPE behaves better than LDPE does in elongation in the range from 0.7 wt.%to 2.0 wt.%DCP addition,and corssslinked LLDPF meets the requirements of HVDC cable insulation for crosslinked polyethylene(XLPE)elongation with 0.7 wt.%DCP addition.In this condition,LLDPE with 0.7 wt.%DCP addition is chosen to be a substitute for traditional XLPE for the purpose of minimizing by-products.DMA and tensile test results verify that LLDPE with 0.7 wt.%DCP addition has higher Young′s modulus and elongation at break than traditional XLPE does.DC conductivity and DC breakdown experiments show that LLDPE with 0.7 wt.%DCP addition has low conductivity,good temperature stability of conductivity and DC breakdown strength at the temperature range from 30℃to 90℃.As the crosslinking by-products in the actual cable are difficult to be removed completely and unevenly distributed,it seriously affects the long-term safe operation of the cable.The low crosslinked LLDPE is recommended as the replacement of the traditional XLPE for the insulation material in HVDC cable insulation system,so as to reduce DC conductivity and enhance the temperature stability of conductivity while meeting high temperature mechanical properties.