The paper describes a new selective detection method for water trees developed in consideration of the electrophysical concept of the water treeing phenomenon. As most of the liquid water is not chemically bound in the water trees it can move out or in, depending on the actual physical conditions such as the solubility of water in the surrounding polymer. An increase of the insulation temperature finally leads to a drying out of water trees and the water tree correlated loss tangent decreases. This change is very specific and characteristically different from that of cable insulations without water trees. Hence the differential variation of loss tangent and capacitance in dependence on the temperature can be used as a measure of water tree degradation of polymer insulated medium voltage cables.
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