Dielectric properties of single water trees

An experimental approach involving the study of the electrical properties of single trees extracted from field-aged cables has been applied. The authors present first results on the measurement of the permittivity parallel and perpendicular to the tree growth direction, i.e. to the field direction in the cable. Capacitance measurements on single water trees, extracted from field-aged cables, show a large permittivity enhancement compared to untreed material. The parallel and perpendicular measurements show an asymmetry of at least 1.3. With the help of a simple model, the water content and length of the substructure in a water tree are estimated. It is shown that during longer measurements the above difference disappears. These findings suggest that: (a) the morphology of a water tree consists of elongated structures, filled at least partially with water, and (b) the permittivity enhancement disappears either due to the collapse of the microstructure or simply due to the outdiffusion of water.<
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