Studies on the initiation and growth of electrical trees from water trees

Experimental data on initiation from nonbridging and bridging water trees are presented. It is shown that the field strength at which electrical trees are initiated by water tree tends to decrease with greater water tree length. Electrical trees are produced at the boundary between water trees and healthy insulation. As a result of water tree tests of sheet samples, it was found that even if the water trees should bridge the insulation, that alone would not bring about breakdown; insulation integrity was maintained for a certain period of time. In several of the bridging water trees, the formation of electrical trees was observed, while no changes were observed in the case of nonbridging water trees. This shows that the damaging effects of the bridging water trees (i.e., the ease with which they induce electrical trees) are considerably greater than that of the nonbridging trees