热循环处理下电缆绝缘空间电荷特性研究

Study on Space Charge Characteristics of Cable Insulation Under Thermal Cycling Treatment

电缆绝缘特性的变化与实际运行工况有关,绝缘层的温度会随负荷波动发生变化,恒温热处理不能对其进行模拟。为了研究电缆投运时绝缘层的温度变化对其交联聚乙烯(cross-linked polyethylene,XLPE)绝缘空间电荷特性的影响,对3个不同电缆厂家生产的110 kV电缆绝缘分别进行了最高温度为80℃、90℃和100℃的热循环处理,并结合傅里叶红外光谱(Fourier transform infrared spectroscopy,FTIR)、差示扫描量热法(differential scanning calorimetry,DSC)和空间电荷实验结果,分析了热循环20次前后XLPE绝缘的空间电荷特性变化。结果表明:热循环处理前,XLPE残留的交联副产物等杂质成为试样电极处异极性电荷的主要来源;热循环处理后,试样内部出现次级结晶,同时XLPE支链和极性端基增多,使得其陷阱数目增大,且以浅陷阱为主;热循环处理温度越高,不饱和基指数越小,极化阶段平均体电荷密度越小,试样中生成的陷阱数目越少;在热循环刺激作用下,XLPE的熔融温度升高,结晶度增大,试样内部的深陷阱数目增多;热循环处理温度越高,次级结晶形成的片晶厚度越厚,试样中生成的深陷阱数目越多。短时适当提高电缆投运时的绝缘层温度,有助于减少电缆绝缘内部杂质以及生成的陷阱数目。

The change of cable insulation characteristics is related to the operating conditions,and the heat treatment of constant temperature can not simulate the temperature change of insulation layer caused by load fluctuation.To study the effect of temperature change of the insulation layer on space charge characteristics of XLPE during cable operation,110 kV cables produced by three different cable manufacturers are chosen for thermal cycling treatment with highest temperature of 80℃,90℃and 100℃respectively.Combined with Fourier transform infrared spectroscopy(FTIR),differential scanning calorimetry(DSC)and space charge measurements results,the space charge characteristics of XLPE insulation before and after 20 heat cycles are analyzed.The results show that the impurities like cross-linking by-products in XLPE become the main sources of heteropolar charges near the electrodes before thermal cycling treatments.After thermal cycling treatments,there are secondary crystals in the samples and the branching chains and polar end groups of XLPE increase,which cause the numbers of traps increase,based primarily on shallow traps.The higher the temperature of heat treatment is,the smaller the unsaturated band index is,and the smaller the charge accumulation per unit volume in the polarization stage,the fewer the traps in the samples.The melting temperature and crystallinity of XLPE increase under the effect of thermal cycle stimulation,causing the numbers of deep traps increase.The higher the temperature of heat treatment,the thicker the lamellae,and the more deep traps in the samples.It is beneficial to reduce the impurities and the numbers of traps in XLPE by elevating the temperature of insulation layer in a short-time appropriately at the beginning of cable operation.