计及双界面的电缆绝缘水树缺陷时域反射解析模型

A Dual Interface Analytical Model for Time-Domain Reflectometry of Water Tree Defects in Power Cables

配电网中中压电缆的敷设条件相对较差,在环境水分和工作电压等因素的共同作用下容易在绝缘中生成水树缺陷,影响电缆绝缘的性能。时域反射法(TDR)可用于检测电缆故障,近来也有研究将其用于检测电缆绝缘局部缺陷,但检测结果与绝缘缺陷情况的关联尚不明晰。针对此问题,该文基于等效电路原理推导了同轴绝缘含水树缺陷时的等效介电常数,在此基础上结合传输线理论建立计及双界面反射的时域解析模型,可用于分析与预测电缆绝缘局部存在水树缺陷时TDR实验的反射波形。通过制作含模拟水树的电缆试样并进行TDR实验,考察了模拟水树缺陷的含水量、缺陷段长度和电导率等因素对TDR实验结果的影响。结果显示,模拟水树的TDR实验结果与解析模型的计算结果具有较好的对应性,初步验证了该模型用于分析与预测电缆绝缘局部缺陷反射特性的有效性。预期该解析模型有助于为TDR在电缆绝缘水树检测中的应用提供参考。

Due to the poor operating environment,medium voltage cables in distribution networks are prone to water tree aging under the combined effect of moisture and voltage,which can degrade the insulation performance.The time-domain reflectometry(TDR)method can be used to detect cable faults,and some recent studies have used it to detect the aging of the local insulation of cables.However,the relationship between the detection results and the aging condition of the insulation is not clear.To address this problem,an equivalent permittivity for the coaxial insulation with water tree defects is derived.Moreover,a time-domain analytical model considering the reflection of dual interfaces is developed to analyze and predict the results of TDR experiments on cables with local water tree defects.First,the equivalent permittivity of the coaxial insulation with water trees was derived based on circuit theory,and the effectiveness of using the equivalent permittivity to characterize the reflection characteristics of water tree defects in cable insulation was verified through finite element simulation.Then,a time-domain dual-interface reflection model considering the interfaces at both ends of the defect was established based on transmission line theory and the equivalent permittivity.This model was used to analyze the relationship between the reflection of water tree defects and the condition of water treeing as well as the characteristics of the input signal(such as amplitude and waveform).Finally,by making cable samples with mimic water tree defects and conducting TDR tests,the effects of factors such as water content,length,and conductivity of the defects on TDR test results were investigated.The results showed that the TDR test results of the mimic water tree defects agreed well with the calculated results of the proposed analytical model,preliminarily validating the effectiveness of the proposed model for analyzing and predicting the reflection characteristics of local water tree defects in cable insulation.The following conclusions can be drawn from this study:(1)The equivalent permittivity can characterize the wave reflection characteristics of local defects in cable insulation.The proposed analytical model considering multiple reflections at both ends of the defect can more accurately predict the reflected waveform generated by local insulation defects.(2)The TDR experiments on the mimic water tree defects showed that the reflection magnitude and waveform were closely related to the water content(quantity of the mimic water trees)and length of the defective section.For example,in this experiment,adding 5 small holes to the defective section increased the amplitude of the reflected wave to 2.6 times the original value,while doubling the length of the defective section increased the amplitude of the reflected wave to 1.6 times the original value.This preliminary result indicates that TDR measurements can provide a basis for assessing the extent and range of similar insulation defects(such as localized moisture and water treeing).The analytical model established in this study helps to link the water treeing condition of cable insulation with TDR test results,and it is expected to provide a reference for the application of TDR testing in detecting water treeing in cable insulation.