环境湿度对配电电缆绝缘等温松弛电流的影响

Effect of Environmental Humidity on Isothermal Relaxation Current of Distribution Cables Insulation

环境湿度是配电电缆绝缘等温松弛电流测试的重要干扰因素之一,对基于松弛电流的绝缘状态评估影响显著。以退役三芯户外终端为试样,测试了不同湿度环境下的等温松弛电流,并开展了绝缘状态评估误差分析。结果表明:在检测系统直流电源输出1 kV电压后,随着环境湿度增加,A、B和C相电缆的等温松弛电流和电缆线芯实际分压均呈现整体快速下降的趋势,这表明等温松弛电流的下降主要与电缆绝缘子伞裙沿面电阻Rh下降相关。A、B和C相电缆等温松弛电流均可通过三阶指数模型模拟,计算出的老化因子A均大于德国判据中“劣化”等级线2.10,且各相A值存在明显区别,这表明其在投后的各相电缆绝缘的老化程度不同,其可能与各相线路的负荷量大小不同导致的绝缘层电热老化进程相关。环境相对湿度在49%以下时,不同湿度等温松弛电流法计算出的老化因子A误差在6%以内,但当湿度进一步提高后,极化电场不足导致老化因子A误差过高,准确性较低,因此在实际应用中应注意环境湿度的影响,建议电缆接头周围的环境相对湿度控制在49%以下。

Humidity is one of the most important interference factors during the measurement of isothermal relaxation current for the field distribution cable,and it may significantly affects the evaluation of insulation state based on relaxa-tion current.In this paper,the retired three-core outdoor terminal was taken as a sample.The isothermal relaxation current of sample was detected at different humidity environments and the error analysis state evaluation was carried out.The results show that,after the detection system DC power supply outputs 1 kV,the isothermal relaxation currents of A,B and C phase cables and the actual voltage Vc of the cable cores present an overall rapid decrease trend with the increase of ambient humidity,which indicates that the decrease of the isothermal relaxation current was mainly related to the decrease of the creeping resistance Rh of insulator umbrella skirt.Isothermal relaxation current in A,B and C phase cables can all be fitted by the third-order exponential model.The calculated aging factor A are all larger than the“deterioration”grade line 2.10 in the German criterion.In addition,there are obvious differences in aging factor A values of each phase,which indicates that the aging degree of the cable insulation of each phase is different.This phenomenon may be related to the electrothermal aging process of the insulation layer caused by the different load of each phase line.When the ambient humidity is below 49%,the error of the aging factor A calculated by the isothermal relaxation current method with different humidity is within 6%.However,when the humidity is further increased,the insufficient polarization electric field will lead to an excessively high error of the aging factor A and low accuracy.Therefore,attention should be paid to the influence of environmental humidity in practical applications.It is recommended that the relative humidity of the environment around the cable joint be controlled below 49%.