Insight into the influence of mechanical orientation on water tree propagation according to abnormal water tree shapes

In this paper, the effect of mechanical orientation is presented to understand abnormal water tree shapes. For the XLPE samples, the possible mechanical orientation of XLPE insulation was simulated by inserting a heated metal needle into the samples. After that, one XLPE sample (comparison sample) was selected to be observed for stress strips (the macroscopic morphologies of oriented XLPE materials) using a polarizing microscope, and the other samples were subjected to accelerated water tree aging experiment for water tree analysis. The results show that there are abnormal water tree shapes in the XLPE samples, which are very similar to the morphologies of stress strips observed in the comparison sample. Because of the anisotropy in oriented XLPE materials, in orientation direction, the propagation of water trees is promoted, while in the perpendicular direction, the propagation of water trees is inhibited As a result, abnormal water tree shapes were observed. The results indicate that the mechanical orientation of XLPE materials plays a leading role in the propagation direction of water trees to a large extent.