Effect of the applied voltage frequency on the water tree shape in polyethylene

We have been examining the effects of superposition of a high-frequency voltage to various voltages such as dc, low-frequency (0.1 to 5 Hz), and power-frequency voltages on the development of water trees in polyethylene. We have made clear that the number of voltage zero-crossings is a decisive factor in the length of water trees. In the present research, the water tree shapes grown under the various superposed voltages were carefully observed. As a result, we have confirmed that the water tree tends to become a hand-like shape if the frequency of the lower-frequency component is between 0 and 0.5 Hz, and become spherical if the frequency is higher than 1.0 Hz. This result is explained by assuming that the growth of water trees is governed by the effective number of consecutive voltage zero-crossings at the tip of the water tree.