Water Tree Propagation in a Wide Temperature Range: Insight into the Role of Mechanical Behaviors of Crosslinked Polyethylene (XLPE) Material

Abstract

To understand the propagation characteristics of water trees at a wide temperature range, this paper presents the effect of mechanical behaviors on the sizes of water trees. An accelerated water tree aging experiment was performed at −15 °C, 0 °C, 20 °C, 40 °C, 60 °C, and 80 °C for crosslinked polyethylene (XLPE) specimens, respectively. Depending on the micro observations of water tree slices, water tree length is not always increasing with the increase in temperature. From 0 °C to 60 °C, water tree length shows a trend from decline to rise. Above 60 °C, water tree length continues to reduce. Dynamic mechanical analysis (DMA) shows that the glass transition temperature of the new XLPE specimen is about −5 °C, and the α-relaxation is significant at about 60 °C. With the increase in temperature, the XLPE material presents different deformation. Meanwhile, according to the result of the yield strength of XLPE at different temperatures, with the increase in temperature, the yield strength decreases from 120 MPa to 75 MPa, which can promote the water tree propagation. According to the early stage in the water tree propagation, a water tree model was constructed with water tree branches like a string of pearls to calculate electric field force. According to the results of electric field force at different expansion conditions, with the increase in temperature, due to expansion of the water tree branches, the electric field force at water tree tips drops, which can suppress the water tree propagation. Regardless of high temperature or low temperature, the water tree propagation is closely related to the mechanical behaviors of the material. With the increase in temperature, the increased deformation will suppress the water tree propagation, whereas the decreased yield strength will promote water tree propagation. For this reason, at different temperatures, the promotion or suppression in water tree propagation is determined by who plays a dominant role.