Single-winged and bow-tie water trees-model concept and growth behaviour

A careful examination of the shapes, length distributions, and concentrations of water trees grown in medium-voltage cables during service revealed the existence of single-winged and symmetric bow-tie trees. In some cases nonsymmetric bow-tie trees were found. To analyze the growth behavior of water trees, additional aging of cable specimens under accelerated conditions in the laboratory was performed. The results show that both types of water trees were initiated and started to grow. At a certain length the single-winged trees transform into bow-tie trees. Locally different micromechanical properties of the polymer are the reason for the initiation of the different types of water trees, while the transformation into two winged trees at a certain length was in accordance with predictions drawn from an electro-physical model concept involving dielectrophoresis and precipitation