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The growth and partial discharge (PD) characteristics of electrical tree in crosslinked polyethylene (XLPE) needle-plate electrode samples are investigated under constant dc voltage at different temperatures. The results reveal that under the experimental conditions, the electrical trees are all branch trees, and the discharge intensity fluctuates with time. The increase in temperature promotes the growth of electrical tree as well as the intensity of PD. Five texture features of PD ${q}$ - ${t}$ histogram are extracted and used as the elements to evaluate the insulation status of XLPE needle-plate electrode under dc voltage based on the gray correlation degree. The area ratio and lateral tree ratio of electrical tree is extracted based on the gray histogram. The lateral tree ratio increases with the increase in temperature, which is closely related to the intensity of PD. Gray correlation theory is used to evaluate the status of XLPE insulation. The results reveal that the evaluated insulation statuses based on the gray correlation degree are the same as those based on experience, which indicates that the proposed diagnosis method of XLPE insulation status based on the gray correlation degree with ${q}$ - ${t}$ histogram texture features as elements has a high accuracy. [ABSTRACT FROM AUTHOR]

In order to investigate the effect of nano-doping on partial discharge (PD) and electrical tree growth in cross-linked polyethylene (XLPE), samples of 0.5 wt% magnesium oxide MgO/XLPE were prepared and an AC experiment was carried out. A color-coded diagram for PD signal was developed to describe the PD activity during the whole process of treeing. The results show that the PD intensity in positive half cycle of voltage (PHC) is larger than that in negative half cycle of voltage (NHC), and the PD magnitude in MgO/XLPE is lower than that in XLPE. Moreover, the tree length and morphology in MgO/XLPE are similar to those in XLPE. Furthermore, a finite element simulation based on the bipolar charge transport model was applied to investigate the effect of space charge on PD under AC voltage. Simulation results suggest that the characteristics of PD and tree growth may be associated with the accumulated charge around the needle tip. The distribution of negative charges in NHC would be wider than that of positive charges in PHC, which would weaken the electric field in NHC and lead to the less PD in NHC. Moreover, the range of charge shield in MgO/XLPE may be larger and is equivalent to increasing the effective radius of curvature of needle tip, which would lead to smaller electric field and lower PD magnitude in MgO/XLPE. However, the addition of nano-MgO reduces the crystallinity of material based on the differential scanning calorimetry (DSC) result. Hence, although the PD magnitude in MgO/XLPE is lower, the MgO/XLPE researched in this paper does not show significant resistance to AC tree growth. [ABSTRACT FROM AUTHOR]

This study introduces silicone elastomers with micro and nano-sized boron nitride (BN) and silicon carbide (SiC) particles with various doping levels to improve thermal properties and partial discharge resistance. The effect of micro and nano-fillers on thermal conductivity, coefficient of thermal expansion (CTE), and thermal stability are investigated. Polarization and depolarization current and partial discharge are measured to investigate the non-linear conductivity, trap energy density distribution, and partial discharge resistance. Experimental results show higher thermal conductivity, lower CTE, and better thermal stability than the original silicone elastomer. Large size fillers dominate the thermal conductivity when the doping level is low, whereas the composite microstructure plays a significant role in the thermal conductivity when the doping level is high. The combination of different filler type and size has less effect on thermal stability and CTE compared to the effect of the doping level. According to the depolarization current, the composite's trap depth is generally shallower than that of the original silicone elastomer. With increased doping level, the shallow trap density increases, providing hopping sites for carriers, whereas the deep trap density decreases, reducing the number of trapped charges. The partial discharge inception voltage is higher than that of the original silicone elastomer, and it increases with the doping level, which may be because the electrostatic field generated by diffused charges reduces the local electric field near the high voltage electrode. [ABSTRACT FROM AUTHOR]

Experimental studies on partial discharges (PD) occurred in oil-insulated devices using ultra-high frequency (UHF) sensors to detect PD signals and using wavelet analysis to process the acquired UHF signals are presented, and a threshold selecting method adapting to various noise circumstances when de-noising UHF signals using wavelet transform is proposed in this paper. The threshold method was based on numerical fitting of standard deviations and manual setting thresholds for brush-fire samples from a large scale. Horizontal comparisons between different geometries and vertical comparisons between different intensities of partial discharges in oil were also discussed and the mathematical tool was wavelet analysis. Multi-resolution analysis (MRA) results show the significant differences that can be used to recognize the discharge pattern and to judge the intensity. Particularly attention was given to needle-plate discharge geometry, which was often used by researchers focusing on the mechanism of pre-breakdown phenomena in liquid dielectrics. The results of needle-plate model show good agreement with electronic theory of liquid pre-breakdown mechanism. [ABSTRACT FROM AUTHOR]