A Study of Characteristics of Water Tree Growth by Electric Field Analysis

Three-dimensional electric fields analysis was applied to the tip of water trees in XLPE cable. First, pre-breakdown detection was curried out on “needle-shaped” water trees. The results were analyzed by the three-dimensional electric field F.E.M. The water tree was simplified by a spheroid in the analysis. The position of the trees, length, tip radius were read from the microphotographs. The analysis was done on all 11 examples. The test results could be explained well when the conductivity of the water tree region was assumed to be 5×10-7S/m. Next, the electric fields of tip of three kinds of water tree (“blue”, “needle-shaped” and “white” water tree) were analyzed. Three kinds of water tree were expressed by changing the conductivity of water tree region. And, a distance from water trees to inside half conductor was made to change in three kinds. These were analyzed about the cable (insulation thickness: 3, 6, 9 mm) of 6.6, 22, 66 kV respectively. It was found out that “blue” and needle-shaped tree in the 66 kV cable and “blue” tree in the 22 kV cable may cause a breakdown under the operation voltage. As for other cases, the tree may propagate without making breakdown until it bridges the insulation. And, the possibility that the growth of “white” water tree declined rapidly in 66, the 22 kV cable when it touches inner semi-conducting layer so that the tip electric fields of the tree are the same as the average electric fields of the cable was suggested.