The Influence of Different Degrees of Mechanical Bending on Water Tree Propagation

In order to understand the influence of cable mechanical bending on the propagation of water trees, the propagation characteristics of water trees were compared in different bending degrees of XLPE cables. An accelerated water tree aging experiment was performed in unbended, smaller, and greater bending degree for XLPE cables respectively. After 28 days of aging, the three groups of samples were sliced, the morphologies of water trees were observed via an optical microscope, and the sizes of water trees were measured. Meanwhile, the experimental results were explained based on the mechanical orientation theory of XLPE materials. Optical microscope observation shows that with the increase in cable bending degree, water tree morphology changes from circular to cone on the outer side of the samples, and the difference between water tree length and width becomes larger. The greater the bending degree of cable, the water tree becomes wider than long. When the cable is bent to a certain extent, the local strain on the outer side of the insulation will exceed the yield strength of XLPE, which can cause the mechanical orientation of molecular chains. Once orientation occurs, the material will become anisotropic, along the orientation direction, water tree propagation will be promoted, while in the direction perpendicular to the orientation, water tree propagation will be inhibited, which further results in the formation of conical water trees.