Effects Of Cross-linking Method On Insulation Properties Of Cross-linked Polyethylene

XLPE insulated power cables are widely used in urban power system because of the excellent physical, mechanical and dielectric properties of the insulation, the safety of the grid is deeply dependent on the insulation reliability. It has been revealed that the inner layer and out layer of PE are subjected to different cross-linking temperatures or cross-linking time, which in turn leads to the alteration of the chemical and physical structures. It possibly influences the properties of XLPE, which may lead to serious breakdown of the insulation that damages the system. Therefore, from the safety point of view, it is very necessary to gain a good understanding of the effect of cross-linking method on the properties of XLPE.First of all, fourier transform infrared spectrum, differential scanning calorimetry, polarization microscope, surface potential decay methods are respectively employed to investigate the chemical structure, crystallinity and space charge decay phenomena of XLPE. Obtained results show that with the increase of the cross-linking temperature, the crystallinity decreases obviously, both of the concentration of acetophenone and the decay rate of space charge initially decrease then tend to increase. It is proposed that the variation of charge decay behavior is attributed to the combined effect of acetophenone and moisture.Secondly, the effect of cross-linking method on the dielectric breakdown behavior and the mechanical strength has been investigated. Obtained results show that with the increase of the cross-linking temperature, both the withstand voltage and hold-off time increase initially then appear to decrease. This is attributed to the competition between cross-linking inhibited thermal breakdown and the acceleration of electric breakdown caused by the thermal decomposition of DCP agent and the decrement of crystallinity. Furthermore, with the increase of the test temperature, both the withstand voltage and hold-off time decrease. Such decrement becomes serious as the temperature exceeds 90℃. The mechanical strength increases with the cross-linking temperature because of the generation of 3-D grid in XLPE.Finally, the effect of cross-linking method on the characteristics of electrical tree has been investigated by using a digital microscope. Six patterns of trees are observed in XLPE, e.g. branch tree, pine branch tree, bine branch tree, bush tree, bush-pine mixed tree and branch-bine mixed, in which branch tree and branch-bine mixed tree propagate with highest rate as well as bush tree with lowest speed. With the increase of the test temperature, the patterns of the trees decrease. At 170℃,the mean propagation speed of the tree is lowest. Such behavior is considered to be originated from the variation of crystallinity caused.