Physical properties of single water trees extracted from field aged cables

The long-term degradation phenomenon called water treeing is still of concern in polymeric insulated medium voltage cables. To get a better understanding of this ageing effect we investigated the physical properties of single water trees extracted from field aged cables. We will present measurements of the permittivity, the water content and the I-V-characteristics of single vented trees. The real part of the permittivity of single vented trees is strongly enhanced compared to untreed material and shows an asymmetry in respect to the field direction in the cable. The enhancement can mainly be explained by additional water present within the tree volume, typically about 1%. The current-voltage characteristic is highly nonlinear with the applied field corresponding to a voltage dependent loss mechanism. The microscopic results form single vented trees can be used as a basis to improve the understanding of water treeing