Yoshinobu Nakamichi

Fundamental study on energy storage system for DC electric railway system

The methods to stabilize power sources have been important issues in DC feeding circuits. A number of applications of energy storage have been investigated so far. In recent years, the development of energy storage media has been remarkable for drive-power supplies of electric vehicles. Therefore, the authors produced an energy storage prototype for a DC electric railway system that consists of electric double-layer capacitors, diode bridge rectifiers, a chopper system and PWM power converters. This paper describes its characteristics and experimental results of the prototype.

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.

Observation of water trees using terahertz spectroscopy and time-domain imaging

Terahertz measurements were carried out to detect water trees grown in low-density polyethylene sheets. Water absorbs light at terahertz frequencies, fairly strongly at about 5.0 THz and rather weakly from 0.1 to 1.0 THz. Using the absorption at these frequencies, observation of water trees was tried according to the following procedures. First, we made a model sample, consisting of a polyethylene sheet, a water layer, and a copper plate, and terahertz light was irradiated to this sample vertically. The waveform and intensity of electric field of the terahertz light reflected by the sample clearly pointed out the presence of water layer beneath the polyethylene sheet by the reflection peak appearance time and the phase of reflected electric field. Secondly, water trees were grown in a polyethylene sheet, and terahertz light was scanned over the sheet. As a result, the intensity distribution of terahertz light reflected by the sample was in good agreement with the shape of the water trees. Observation of terahertz image was also carried out using the same polyethylene sheet with water trees over which a polyvinyl chloride sheet or a carbon-loaded polyethylene sheet was put to simulate the structure of a real cable. An image of water trees was also successfully observed. These results indicate that the terahertz spectroscopy can be a new characterization tool to observe the presence of water trees in a test sample taken from an aged cable.

Role of number of consecutive voltage zero-crossings in propagation of water trees in polyethylene

Effects of superposition of a low-frequency (0.1 to 50 Hz) voltage and a high-frequency 2 kHz voltage on the growth of water trees in polyethylene are examined. It has become clear that both the number of total voltage zero-crossings during the whole voltage application period and the number of consecutive voltage zero-crossings in the instant of the polarity reversal of the low-frequency voltage play important roles in the length and shape of water trees. Namely, the water tree length becomes longer as the number of total zero-crossings increases. Furthermore, among the voltages with the same number of total zero-crossings, the length becomes even longer with a prolonged shape in the direction of electric field as the number of consecutive zero-crossings increases. Effects of superposition of a high-frequency voltage onto a dc voltage are also examined. In this case, the dc voltage seems not to have any influence to the tree growth. By assuming the presence of space charge around the water-tree tip, the effective zero-crossings become only sensitive to the high-frequency component. Therefore, this result also indicates that the number of voltage reversals at the tree tip plays a crucial role in the growth of water trees.

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.

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.

Role of the number of consecutive voltage zero-crossings on the water tree growth in polyethylene

With the progress in power electronics, ac voltages with various harmonic components are often applied to insulating materials of power apparatus. Water treeing is the most-observed degradation pattern in polyethylene-insulated power cables, and much work has been done. Nevertheless little is known about their dielectric properties under above-mentioned particular ac voltages. In this paper, the authors examine the influence of superposition of a low-frequency ac voltage (0.1 to 5 Hz) and a high-frequency 2-kHz ac voltage on the growth of water trees in polyethylene. It has become clear that the number of consecutive voltage zero-crossings in the instant of the polarity reversal of the low-frequency voltage plays an important role in the length and shape of water trees.