Dielectric properties of single water trees

摘要

An experimental approach involving the study of the electricalnproperties of single trees extracted from field-aged cables has beennapplied. The authors present first results on the measurement of thenpermittivity parallel and perpendicular to the tree growth direction,ni.e. to the field direction in the cable. Capacitance measurements onnsingle water trees, extracted from field-aged cables, show a largenpermittivity enhancement compared to untreed material. The parallel andnperpendicular measurements show an asymmetry of at least 1.3. With thenhelp of a simple model, the water content and length of the substructurenin a water tree are estimated. It is shown that during longernmeasurements the above difference disappears. These findings suggestnthat: (a) the morphology of a water tree consists of elongatednstructures, filled at least partially with water, and (b) thenpermittivity enhancement disappears either due to the collapse of thenmicrostructure or simply due to the outdiffusion of water