Research On Insulation Diagnosis And Detection Criterion Of 10kV XLPE Cable Based On VLF-TD

The distribution cables often have water tree degradation in the insulation layer,causing frequent power outages and posing challenges to grid security.The very low frequency dielectric loss(VLF-TD)testing technology widely used in foreign countries has just started in China.The IEEE Std 400.2-2013 VLF-TD criterion is not fully applicable to China's localized XLPE cable.Therefore,for the practical problem of this project,the 10 kV XLPE cable is taken as an example to carry out theoretical analysis,experimental research,field testing and data processing of VLF-TD,and the effectiveness and detection criteria of VLF-TD detection are improved.It provides a reference for the formulation of the domestic distribution cable dielectric loss state evaluation procedures.Specific research content includes:(1)The relationship between the degree of moisture and the VLF-TD characteristics is analyzed by establishing a circuit model of the cable joint under different degrees of moisture.When the formation of water trees is not penetrated,the numerical simulation method is used to analyze the variation of water tree length,water tree density,the proportion of the damp section length,dielectric constant and electrical conductivity of cable XLPE after damp and the correlation with VLF-TD.It provides a theoretical basis for the VLF-TD detection of XLPE insulated water tree aging.(2)Based on the water needle electrode method,an accelerated water tree platform was built in the laboratory.Different moisture levels were measured by setting different pressurization times to obtain the water tree size and VLF-TD of XLPE insulation with different moisture levels,which provides a basis for the VLF-TD detection of cable insulation water tree aging.(3)The on-site VLF-TD field testing was carried out,classify the 252 sets of data according to the IEEE Std 400.2-2013 international dielectric loss standard,analyze the dielectric loss variation law,and propose the new characteristic quantity reverse dielectric loss change rate(RDTD)and extreme difference of dielectric loss(ETD).(4)Comprehensive IEEE VLF-TD three criteria,Korean dielectric loss Skirt and the newly proposed RDTD and ETD,using state distribution models such as Weibull distribution,lognormal distribution and exponential distribution,fitting the field testing data,determining the optimal distribution model,and obtaining the four-level evaluation threshold of the cable operation state according to the Pareto rule,and finally obtaining the VLF-TD evaluation guide for the 10 kV XLPE cable in a certain area.Which provide a basis for the development of the domestic distribution cable dielectric loss state evaluation procedures in the era of big data.