Recent advance in fine structure of water trees by confocal microscopy

High voltage power cables insulated with polymeric materials such as polyethylene, experience water treeing degradation produced by electrical stress in the presence of moisture. In cable insulation, the breakdown stress level is strongly affected by the size of water trees even though the treed region can be considered as insulation material. Although it is easier to observe water trees by epifluorescence with a standard microscope, the use of a confocal laser scanning microscope gives images of a better quality and allows the three dimensional reconstruction of the microchannels. Microchannels are found to grow along erratic entangling with respect to one another in three dimensions, likely according to the presence of a crystallite boundary or of heterogeneous electric field value. These diameters are less that 0.6 μm and sometimes less than 0.2 μm. On the basis of observations, the authors suggest that microchannels are not formed by the juxtaposition of isolated cavities, within the limit of the 0.2 μm resolution