Characteristics Analysis of Water Needle Induced Electrical Trees in XLPE

In this paper, electrical trees initiated from water and metal needle are investigated, and the inception and morphological characteristics of water needle electrical trees are analyzed. The existing literature shows that electrical trees can be initiated from the metal needle or transformed from the water trees. But in this study, it is found that the electrical tree can be directly induced by water needle in addition to the above cases. Moreover, the comparative results also indicate that the electrical trees initiated from water needle and metal needle have different inception features and morphological characteristics. To obtain the water needle electrical trees initiation and development mechanism, the water needle tips are observed before and after the sample degas process. Besides, the electric field distribution of the metal needle tip, the water needle tip before and after pretreatment, and of tree branch tip during the electrical tree development are calculated. The observations and calculations results show that before the degas pretreatment, a micro-gas cavity is formed at the water needle tip due to the tension of the water surface. The cavity at the needle tip may cause and support partial discharge (PD), causing accumulative damage and material defects. Then the density of defects becomes sufficiently large, and a tree branch can be formed. After the branch formation, the water flows into the tree branch, and the micro-gas remains at the branch tip, which may facilitate the PD, and trees propagation, resulting in the diversified morphological characteristics of the water needle electrical trees.