热老化对配电电缆绝缘等温松弛电流及阈值电场的影响

Effect of thermal ageing on isothermal relaxation current and threshold electric field of distribution cable insulation

针对等温松弛电流法在配电电缆绝缘状态评估中的问题及难题,本文采用国产未老化的10 kV配电电缆新样作为检测对象,通过检测140℃下加速热老化288 h过程中电缆段的等温松弛电流和绝缘切片的高场电导,分析了热老化过程中表征绝缘状态的电流松弛分量、老化因子(A)和阈值电场(Et)变化趋势及范围。结果表明:等温松弛电流中存在3个明显的极化松弛峰,时间常数τ_(1)、τ_(2)和τ_(3)分别在7~12 s、31~39 s和210~536 s范围;热老化过程中等温松弛电流峰2和峰3对应的材料陷阱改变较大,其中峰3的改变最为明显;基于等温松弛电流的老化因子A在1.72~3.17范围,随老化时间的增加老化因子A先降后升,而对应的电导阈值电场Et呈先升后降的趋势,这表明阈值电场(Et)和老化因子(A)存在明显的关联,均是绝缘材料内部分子结构的宏观外在表象;国产电缆未老化时的老化因子A为1.91,已近德国标准DIN VDE 0276的“老年”状态,而热老化48 h时的老化因子(A)降至1.72,达到德国标准的“较好”状态,表明国产电缆老化因子A的偏高可能与电缆交联副产物量的残留相关,应完善国产电缆生产中的脱气工艺。

In view of the problem and difficulty of isothermal relaxation current method in evaluating the insulation state of distribution cables,a new sample of domestic unaged 10 kV distribution cable was used as the test object in this paper,and the isothermal relaxation current of the cable section and the high-field conductance current of the insulation slice during the 288 hours of accelerated thermal ageing process at 140℃were detected.The variation trend and range of current relaxation component,ageing factor A,and threshold electric field Et during thermal ageing were analyzed.The results show that there are three obvious polarization relaxation peaks in the isothermal relaxation current,and the time constantsτ_(1),τ_(2),andτ_(3) are in the range of 7-12 s,31-39 s,and 210-536 s,respectively.During the thermal ageing process,the material traps corresponding to isothermal relaxation current Peak2 and Peak3 change greatly,and the change of Peak3 is the most obvious.The ageing factor A based on the isothermal relaxation current is in the range of 1.72-3.17.The A decreases at first and then increases with the increase of ageing time,and the corresponding conductance threshold electric field Et shows the opposite trend of rising at first and then decreasing,which indicates that there is a clear correlation between the threshold electric field Et and the ageing factor A,they are both the macroscopic appearances of internal molecular structure of the insulating material.The ageing factor A of the domestic cable before ageing is 1.91,which is close to the"aged"state of the German standard DIN VDE 0276,and the A drops to 1.72 after ageing for 48 h,which reaches the"better"state of the German standard.The high A of the domestic cable may be related to the residues of cross-linking by-products of cables,so the degassing process should be improved in the production of domestic cables.