Kohji Ajiki

Water-tree characteristics in low-density PE under simulated inverter voltages

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 on the high-frequency voltage, 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 role in tree propagation.

Dielectric breakdown of low-density polyethylene under simulated inverter voltages

Breakdown characteristics have been investigated for low-density polyethylene immersed in silicone oil by applying power-frequency ac voltages with high-frequency components, which are simulated output voltages of a pulse-width-modulation inverter. It was found that the breakdown voltage decreases as the high-frequency component increases. However, the number of zero-crossings of the applied voltage, which is very influential in water-tree growth, does not seem to play an important role in the dielectric breakdown process.

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.

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.