Abstract :
It has been established that in service aged cable insulation, the polar components are directly related to the presence of water trees in the solid dielectric. R-C combinations, which represent water trees from an equivalent circuit point of view, have different time constants depending on the progression of the water tree. This paper describes an in-situ cable diagnosis method based on measurement of the return voltage. The cable is charged for several minutes by applying a DC high voltage. This step is followed by a soft discharge via a resistor for several seconds. During the third step the return voltage is measured, which results from the discharge of the polar components. The shape of the return voltage curve is characterized by the different time constants. While the applicability of the method seems to be proven in daily practice for PE and XLPE cables, many more tests and the creation of a database are necessary before a final decision regarding the applicability of the return voltage method for paper and PVC cables can be rendered
