基于超低频介损和U-I滞回曲线的XLPE电缆水树老化状态评估

Condition Assessment of Water-tree Aged XLPE Cables via Very-low Frequency DielectricLoss and U-I Hysteresis Curve

为准确地评估交联聚乙烯(cross linked polyethylene,XLPE)电缆的水树老化程度,该文提出一种基于超低频介损和U-I滞回曲线的评估方法。对3组XLPE样本分别进行0、96和168h的加速水树老化,老化结束后采用快速超低频介损设备(very-low frequency dielectric loss detector,iFDS)在1mHz~1kHz范围内测试不同水树老化状态的XLPE电缆的介电频谱和不同频率下U-I滞回曲线。结果发现:水树老化后的电缆段在超低频段(1mHz~0.1Hz)的介损值显著增大,并在0.002和0.1Hz附近出现两个明显的松弛损耗峰。老化越严重的试样,U-I滞回曲线形变越严重。水树老化后电缆绝缘中产生了新的松弛极化过程,相较于单一频率0.1Hz下的介损值,超低频介电谱能够提供更丰富的绝缘状态信息。在1mHz下,U-I滞回曲线特性的偏转角变化率与曲线形变率均随超低频介损值的上升而增大,能够更加准确地评估电缆绝缘的水树老化程度。将超低频介损结合U-I滞回曲线可以有效地评估电缆绝缘的水树老化程度。

To assess the influence of water tree on the insulation performance of cross linked polyethylene(XLPE)cable,a new method combining very-low frequency(VLF)dielectric loss(tand)and U-I hysteresis curve is proposed in this paper.Three groups of XLPE samples are subjected to an accelerated water tree aging for 0,96 and 168h,respectively.Afterwards the dielectric spectra from 1 mHz to 1kHz and the U-I hysteresis at different frequencies of XLPE cables with different water-tree aging time are measured via fast VLF test equipment(iFDS).It is found that the tanδof the water-tree aged samples at the frequency below 0.1Hz(1mHz~0.1Hz)is increased dramatically.Meanwhile,two typical dielectric loss peaks at around 0.002Hz and 0.1Hz can be found,and the U-I hysteresis curves present an obvious deformation for the severely aged samples.It is analyzed that new relaxation polarizations are resulted from the water-tree ageing process.Compared with the value of tanδat the single frequency of 0.1Hz,dielectric loss spectra at very-low frequency range can provide more information about XLPE insulation,which is more accurate to assess the deterioration degree of XLPE cable insulation. Moreover, the rate of deflection angle (RD) and thecurvilinear deformation rate (RC) are both enhanced with theincrease of tanδ at 1mHz. It is found that the condition ofwater-tree aged XLPE cables can be effectively assessedcombining VLF and U-I hysteresis curves, which provides anew route for XLPE cable performance characterization.