Influence of water and ion diffusion on generation and progress of bow‐tie tree

Abstract

SUMMARYBow-tie tree (BTT) generated from contaminant, such as metal, carbon, amber (overcured resin) or void is a deterioration factor of XLPE cable. In particular, BTT in contact with inner or outer semiconductive shield could significantly lower residual AC breakdown voltage of HV power cables. To evaluate the influence of water and ion diffusion on generation and progress of BTT, we investigated the relationship between water content of XLPE and the generation of BTT under various accelerated aging conditions. The number of BTT in XLPE samples with accelerated aging under the open condition, involving evaporation of water in which samples were immersed, was very large compared with the closed condition. Furthermore, when samples were intermittently immersed in water, the number of BTT in samples was large compared with samples immersed continuously. In these experiments the generation of BTT seemed to have nothing to do with changes in water content before and after accelerated aging. Therefore, it was suggested that diffusion of ions rather than water in XLPE played an important role in the generation of BTT.

Low Participation of Oxygen in the Generation and Progression of Bow‐Tie Trees

“…Therefore, there is a possibility that degassing and the opposite processes taking place immediately after voltage application result in an increase and acceleration of the diffusion of water (steam) and ions toward the voids, especially near the void walls. As previously reported , we can assume that the generation of BTTs requires an environment in which ions move inside the XLPE in certain directions for certain periods of time. With degassing interpreted as a factor creating such environment, the experimental results can be explained by the promotion of BTT origination and development, even under decompression (low oxygen partial pressure).…”

“…We previously investigated differences in BTT behavior in various submersion experiments using cross‐linked polyethylene (XLPE) samples. As already reported , we assumed that BTT generation requires an environment in which ions move inside the XLPE in certain directions for certain periods of time. In addition, in heavy water submersion experiments , we assumed that prevention of metal contamination contributes to BTT inhibition, and that water‐tree deterioration phenomena are related to ions rather than water.…”

Experimental Study on Dependence of Bow‐Tie Trees on Nuclei

“…Thus, we next consider the reason for the smaller number of BTTs associated larger nuclei in terms of an important role of water content and ion diffusion in the origination and development of water trees. As reported earlier , it was assumed that BTTs originate in an environment in which ions move inside XLPE in a certain direction for certain time. In such an environment, one can easily assume that the directional motion of ions is disturbed in the vicinity of larger nuclei, which thus are likely to cause stagnation.…”

“…We previously investigated the behavior of BTTs by various immersion voltage tests using XLPE (cross‐linked polyethylene) samples. As reported earlier it was assumed that BTTs originate in an environment in which ions move inside the XLPE bulk in a certain direction for a certain time. In addition, ions rather than water were assumed to be responsible for water‐tree deterioration phenomena, and some unknown reactions involving metal ions were assumed.…”

“…These results suggested that BTT development is promoted by easily diffusible ions . Such ions contributing to the origination and development of BTTs include those initially existing in the bulk of XLPE, and those penetrating from outside (water) . Therefore, in order to understand the differences in behavior of BTTs originating from nuclei of various shapes, one must consider water and ions in both the nuclei and the XLPE bulk.…”