Conference Record of the 1988 IEEE International Symposium on Electrical Insulation, Boston, MA, June 5-81 1988 TEH QBERVATION OF EUCTRI-

- The starting point of AC electrical trees in polyethylene and the surrounding micro-structures were observed by a transmission electron microscope (TEN) for the first time. To minimize the size of tree paths the authors developed a new apparatus for preparing specimens in which the tree growth was suppressed to less than 10 pm in length. The tree was started from a needle electrode made of carbon-filled polyethylene. The results of TEM ovservation were as follows: (1)The AC tree was a hollow pipe of about 100 na in diameter immediately after its generation. There were constrictions inside the tree, resulting in a corrugated surface. (2)The starting point of the AC tree was a microscopic convex of several tens to one hundred nanometers, which was based on the cohesion of carbon particles of the electrode. (3)The AC tree path was virtually parallel to lamellae. lNTAODUCm When a polymer insulant with a thick insulation thickness suffers a dielectric breakdown, a local breakdown called an electrical tree will be generated. The tree is a hollow tube of several micrometers or under in diameter. The tree, once generated, will grow with internal discharges, and eventually bridge the electrodes, resulting in a complete breakdown. The authors have been investigating the initiation of AC electrical trees, using specimens which were prepared by molding a semiconducting needle electrode in low density polyethylene. The needle is made of carbon-filled polyethylene, so it gets to fit polyethylene insulant. The specimens simulate the interface between the electrode and the insulation layer of a plastic-insulated power cable. The investigation has revealed that the tree is generated at the interface of the electrode, and that there is a stage, in the early part of the tree's growth, in which the tree is less than 2-3 C(m.(l)