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The crosslinked polyethylene (XLPE) cable safety is affected by environmental factors and artificial defects during operation. This work proposes an improved locating method based on broadband impedance spectrum (BIS) to locate local defects in XLPE cables. The calculation process of the algorithm has been analyzed. The selection of the incident Gaussian signal and the peak recognition method have been discussed, where the pulse width of the Gaussian signal was found to be determined primarily by the upper limit frequency of the traveling wave transmitting in the cable. The centroid and function fitting methods were established to reduce the peak recognition error caused by the test sampling rate. This work verified the accuracy of the algorithm through experiments. A vector network analyzer (VNA) was used to test the BIS of the cable. A 20 m-long cable containing abrasion and an inserted nail with different depths was measured in the BIS test. It was found that the abrasion and the nail could be located. The locating deviation of abrasion was within ±1%, and the centroid and function fitting methods could effectively reduce the locating deviation. The locating deviation was within ±1% when the depth of the nail inserted into the cable accounted for less than 50% of the insulation thickness. When the depth exceeded 75% of the insulation thickness, the deviation of each method was more significant, and the maximum absolute value of the deviation was 4%. [ABSTRACT FROM AUTHOR]

Mechanical stresses generated during manufacturing and laying process of high voltage cables can result in degradation of insulation properties, affecting the stable operation of the transmission system. Traditional test methods for testing the effect of mechanical stress on the insulation properties of polyethylene still have some shortcomings to be explored and it is able to explain the changes of the insulation properties of polyethylene under mechanical stress from a microscopic perspective. In order to further study the effect of stress on the insulation properties of polyethylene, microstructural changes, the breakdown field strength, conductivity and charge distribution of polyethylene at different elongation rates are investigated by a combination of experimental and molecular dynamics simulations. The results show that the increase in stress leads to a decrease in crystallinity and microcrystalline size of the material decrease. The untwisting and orientation of the polyethylene molecular chains during the stretching process can create cavities, resulting in an uneven sample distribution and thickness reduction, leading to a reduction in the breakdown field strength. Meanwhile, some crystal regions are transformed into amorphous regions. The loose amorphous regions facilitate the directional migration of carriers, resulting in the increase of conductivity. When the elongation ratio is smaller, the distance between the molecular chains increases and the trap depth of the specimen becomes shallower. This facilitates the migration of ions and electrons and increases the rate of decay of the surface potential. When the stretch is further increased, new traps are created by broken molecular chains to limit the movement of charges, decreasing the decay rate of the surface potential and reducing the insulation properties of the polyethylene. Meanwhile, the molecular dynamics model of semi-crystalline polyethylene was developed to observe the microstructure and energy changes during the stretching process. The conclusions in terms of tensile tests were verified from a microscopic perspective. [ABSTRACT FROM AUTHOR]

Dielectric materials with high thermal conductivity and outstanding dielectric properties are highly desirable for advanced electronics. However, simultaneous integration of those superior properties for a material remains a daunting challenge. Here, a multifunctional epoxy composite is fulfilled by incorporation of boron nitride nanosheets (BNNSs) and mesoporous silica coated multi-walled carbon nanotubes (MWCNTs@mSiO2). Owing to the effective establishment of continuous thermal conductive network, the obtained BNNSs/MWCNTs@mSiO2/epoxy composite exhibits a high thermal conductivity of 0.68 W m−1 K−1, which is 187% higher than that of epoxy matrix. In addition, the introducing of mesoporous silica dielectric layer can screen charge movement to shut off leakage current between MWCNTs, which imparts BNNSs/MWCNTs@mSiO2/epoxy composite with high dielectric constant (8.10) and low dielectric loss (<0.01) simultaneously. It is believed that the BNNSs/MWCNTs@mSiO2/epoxy composites with admirable features have potential applications in modern electronics. [ABSTRACT FROM AUTHOR]

For better application of X-ray digital imaging technology in defect detection in Gas Insulated Switchgear (GIS), it is essential to investigate the typical defect and establish the defect database, which has not been adequately performed in previous work. Systematic experimental research is also needed to accumulate data and experience. In this research, an experimental platform, including Computed Radiography (CR) imaging system and a GIS model, is built, and extensive tests of different kinds of typical defects are studied. The influence X-ray irradiation on SF6 under different tube voltage levels is firstly examined, which proves that the withstand voltage of SF6 gas has not been affected and no dissociation has been found. Then, several kinds of defects are tested by X-ray digital imaging technology. The successful application examples of "visual" detection of defects further prove the practicability and validity of the X-ray digital imaging technique. Finally, the image database of typical defects inside of GIS is established and the defect risk is also analyzed in three levels, which would be useful for the defect severity diagnosis and risk assessment. [ABSTRACT FROM AUTHOR]