水树老化电缆的绝缘评估图谱特征

Characteristics of Insulation Assessing Map of Water Tree Aging Cables

针对现有基于极化–去极化电流(polarization and depolarization current method,PDC)法的绝缘评估方法无法综合反映老化电缆特征参数变化特性这一问题,提出了绝缘评估图谱来多维度表现电缆老化状态。绝缘评估图谱中包含特征参数图谱和支路辨识图谱。水树老化是对电缆绝缘结构影响较大的老化形式之一,对水树老化电缆和退运电缆进行PDC测试后分析了其绝缘评估图谱的变化特征,并利用其他理化分析手段更加深入地分析了退运电缆的异常老化特征,结果表明:随着水树老化时间增加,电缆的电导率σ未出现明显变化趋势,而单位化去极化电量Qn和介质损耗因数最大值tanδmax参数则先减小后增大,这导致水树老化电缆的特征参数图谱面积先减小后增大;电缆发生水树老化后,其支路辨识图谱形态会发生显著变化,且随着水树老化进行支路辨识图谱上移;若电缆交联聚乙烯(cross-linked polyethylene,XLPE)绝缘中存在树枝通道状的不同质结构,会导致去极化电流中存在违背指数衰减规律的异常增大的电流分量,在支路辨识图谱中表现为拟合系数Ai为负数。

In order to solve the problem that the existing insulation assessing methods based on polarization and depolarization current method(PDC)cannot comprehensively reflect the variation characteristics of parameters of aging cables,we proposed insulation assessing maps to show aging states of cables in multiple dimensions.An insulation assessing map includes a characteristic parameter map and a branch identification map.Water tree aging is one of the aging forms that have significant influences on cable insulation structure.After PDC tests of water tree aging cables and retired cables,the changing characteristics of insulation assessing maps were analyzed,and the abnormal characteristics of the returned cables were further analyzed by using other physical and chemical analysis methods.The results show that,with the increasing of water tree aging time,the conductivityσof the cables does not show an obvious changing trend,while the unitized depolarizing quantity Qn and the largest dielectric loss factor tanδmax decrease first and then increase,which enables the area of characteristic parameter maps of water tree cables to possess the same trend.After the water tree aging of cables,the shape of branch identification maps will change significantly.Besides,the branch identification maps will move upward with the water tree aging.If some inhomogeneous structures exist in XLPE,there will be an abnormal increasing current component in depolarization currents which violates the exponential attenuation rule of depolarization currents,causing a negative fitting coefficient Ai in branch identification maps.