Influence of Mechanical Responses of XLPE Cables at Different Temperatures on Water Tree Propagation

To further understand the influence of mechanical stress on water tree propagation in XLPE cables under different temperatures, XLPE sheet samples were subjected to mechanical stress at a low temperature and high temperature, respectively. The propagation characteristics of water trees in the samples were investigated, and the relative theoretical analysis was proposed. Pinhole defects were made in two groups of XLPE samples under two conditions of cold/heat insertion, respectively. The two groups of the samples were then subjected to an accelerated water tree aging experiment, and the morphology and sizes of water trees in the two groups of aged samples were observed by a microscope. In addition, the orientation condition in un-aged samples under two conditions of cold/heat insertion was detected by a polarizing microscope and an infrared spectroscopy (IR). The mechanical response of the XLPE materials under different temperatures and its influence on water tree propagation were investigated. The results show that there are oriented inclined stress strips on one pinhole side of the heat insertion un-aged samples. Similarly, there are inclined water tree branches on one side of the heat insertion aged samples, the morphology of the inclined water tree branches is similar to that of the stress strips in heat insertion unaged samples, and the average length of the inclined water trees is significantly higher than that of the water trees in the cold-insertion aged samples. Based on the mechanical orientation theory in polymer physics, the mechanical orientation of XLPE materials is the main reason for the formation of inclined water trees. As the temperature rises, the yield strength of XLPE materials decreases and it is easier for the materials to be oriented. After the orientation of the materials, the propagation direction of water trees can depend on the orientation direction to a large extent, and water tree propagation can be promoted in orientation direction, thus resulting in inclined water trees with larger sizes. © 2018, High Voltage Engineering Editorial Department of CEPRI. All right reserved.