Single-winged and symmetrically grown water trees

To improve the understanding of the water treeing phenomenon, the authors evaluated the size distributions of water trees in a set of service-aged cables. Careful microscopic analysis revealed the existence of single-winged and more or less symmetrically grown bow-tie trees. It is noted that, by comparing the data from cables produced by different manufacturers, the data points can be split into two groups, indicating the use of two different raw materials. Comparing the mean lengths of the two types of water trees in the service aged cables examined, the mean growth of bow-tie trees seems to be about four times that of single-winged trees. Furthermore, a `starting length´ of about 20 μm seems to exist. Using an apparatus for the accelerated generation of water trees in medium voltage cables, the growth of the different types of water trees with time was examined. The results are discussed with regard to the consequences for the model concept of the water treeing phenomenon