Role of voltage zero-crossing in propagation of water trees

Water tree characteristics have been investigated for low-density polyethylene by applying power-frequency ac voltages with high-frequency components, which are simulated output voltages of a pulse-width-modulation inverter. If we compare the water-tree length among single-frequency voltages, the water tree grows faster if the frequency is higher. However, if we superpose a power-frequency voltage onto the high-frequency voltage, the tree growth is suppressed. The tree length becomes shorter if the superposed power-frequency voltage is higher. This surprising result is explainable by considering that the zero-crossing of the applied voltage plays an important rule for the tree propagation. In case that the high-frequency voltage is superposed onto a dc voltage, the de voltage seems not to have any influence to the tree growth. This result can be also explained by considering the number of voltage reversals at the tree tip.