Water tree characteristics in low-density polyethylene under power-frequency voltages with high-frequency components

For magnetic levitation railway systems using linear synchronous motors, pulse-width-modulation inverters and polymer insulated cables are used for driving cars and feeding electricity, respectively. This means that ac voltages with many harmonics induced by inverters are applied to cables. Water-tree characteristics have been investigated for low-density polyethylene using power-frequency ac voltages with high-frequency components. It was found that water trees grow very fast under high-frequency voltage. However, in the case of the superposed voltage, water trees do not grow so fast if the high-frequency component is not large. It is considered that the zero-crossing of the applied voltage plays an important role in the tree growth