A new insight into the influence of mechanical orientation on water tree propagation in abnormal water tree shapes.

To understand abnormal water tree shapes under mechanical stresses, this paper reveals the mechanism of mechanical orientation for the effect of water tree propagation behaviors. A heated metal needle was used to produce pinhole defects in two groups of XLPE bar samples (group A and group B) to simulate the mechanical orientation of the XLPE material. The samples in group A were subjected to orientation analysis by an infrared spectrum (IR) method, and the samples in group B were subjected to an accelerated water tree aging experiment. According to the results of IR analysis, the XLPE material around the pinholes was oriented after the insertion of the heated metal needle. The microscopic observation results show that there are abnormal water tree shapes in the samples in group B. The abnormal water tree shapes are very similar to the morphology of the stress strips observed in the samples in group A. Based on the mechanical orientation theory, after the mechanical orientation, the XLPE material becomes anisotropic, and some performances of the material will change in the direction of orientation and in the transverse direction. As a result, in the direction of orientation, water tree propagation is promoted, whereas water tree propagation is inhibited in the direction perpendicular to orientation. Mechanical orientation is the main reason for the abnormal water tree shapes. A similar phenomenon is also verified in bent cable samples. The types, directions, and intensities of mechanical stresses determine the behaviors of mechanical orientation in cables, which can further influence the behaviors of water trees. [ABSTRACT FROM AUTHOR]