Yoshihiko Murakami

Effects of nano-sized MgO-filler on electrical phenomena under DC voltage application in LDPE

A nanocomposite material, composed of nano-sized MgO-filler added to a low-density polyethylene (LDPE), was subjected to electrical property investigations under DC voltage application. The volume resistivity of LDPE, under 40-80 kV/mm DC field at 90 /spl deg/C, increases by the power of 10 due to the addition of only a few percent of nano-filler. The amount of space charge in LDPE under high electric field is reduced and the ratio of stress enhancement by space charge becomes less due to the addition of nano-sized MgO-filler. The DC breakdown strength of LDPE is increased by adding nano-sized MgO-filler. Based on these results, the mechanism where by the nano-sized MgO filler had improved DC characteristics of LDPE was discussed.

Variation of leakage current caused by hydrophobicity loss of silicone rubber in Dynamic Drop Test

Silicone rubber used for polymeric insulators has good hydrophobicity and recovery characteristic of it. CIGRE WG D1.14 aims to establish new test procedure, dynamic drop test. That can evaluate the property of hydrophobicity loss of silicone rubber easily and with small systems. So, we clarified the relation between processes of hydrophobicity deterioration or loss and leakage current waveform in dynamic drop test. The hydrophobic conditions were classified into three categories. In the condition of keeping hydrophobicity, pulse discharges were repeatedly observed at the time when droplets ran down. As hydrophobicity deteriorated, small water droplets remained on the surface and many pulse discharges were observed when the droplets ran down. The tendency of the current ingredient of 60 Hz became more dominant and discharge light started to be observed continuously regardless of the timing of the droplets just before when hydrophobicity lost. Then it was considered that a thin moisture layer was formed on the surface, although only droplets were observed with the naked eye. Finally, the leakage current flowed continuously.

Dielectric permittivity characteristic of mesoporous-alumina/epoxy composite

This paper presents an attempt to derive the dielectric permittivity characteristics of polymer composite filled with the metal oxide particle which has mesoporous structure. Experiments were carried out on the epoxy composites filled with alumina microparticles which have the mesoporous structure (mesoporous-alumina/epoxy composites) with different particle content. Measurement of the specific gravity of mesoporous-alumina/epoxy composites indicated that the porosity of mesoporous-alumina particle in the epoxy matrix was higher than that of nonporous-alumina particle. Furthermore, we evaluated relative permittivity of mesoporous-alumina/epoxy composites by measuring the capacitance of its specimens. As the results, we verified that the permittivity of mesoporous-alumina/epoxy composites was lower than that of nonporous-alumina/epoxy composites due to the particle porosity.

Round-robin test on repetitive PD inception voltage of twisted-pairs

Six laboratories in Japan have performed round-robin test (RRT) on measurement of repetitive PD inception voltage (RPDIV) under well-controlled experimental condition. They used common twisted-pair sample, common impulse generator and common data sheet with normalized air pressure and temperature. Scatterings of data were small except in relative humidity. Stability of PD occurrences against repetitive impulses was investigated using all record of PD data stored in PC. This paper reports the test procedures and the results of the RRT for RPDIV measurement.

Forming Fine V-Grooves on a Tungsten Carbide Workpiece with a PCD Electrode by EDM

Polycrystalline diamond (PCD) exhibits a thermal conductivity similar to that of the electrically conductive chemical vapor deposition diamond (EC-CVD diamond) found to function as zero-wear electrodes at short pulse duration. In this study, PCD was used as electrodes applied to EDM on tungsten carbide. Two kinds of PCD (CTB-010 and CTH-025) with a flat surface were used. The wear of the PCD electrodes was about 1.5% for very short pulse duration such as te=1μs, but it was zero wear at te=30μs, though the wear of a Cu-W electrode was 10% even on the machine recommended conditions for the low wear. EDM experiment using a V-shaped PCD electrode with an included angle of 45° was also carried out and the performance was compared with the case using a V-shaped Cu-W electrode. Under the conditions of a no load voltage of 60V, a set peak current of 2A, and a medium pulse duration of te=15μs, there was no wear on PCD electrodes when observed under the SEM, whereas a 50μm-deep wear on the Cu-W electrodes even under the machine recommended condition for the low wear was observed.

Influence of interfacial condition between film and lower electrode on space charge measurement

One electrical insulation evaluation technology is the pulsed electroacoustic method, which detects the acoustic pressure due to the space charge in the bulk. To improve its reliability, a measurement system for the apparent acoustic impedance of the sample is introduced into the space charge measurement system. The apparent acoustic impedance of a polyethylene terephthalate film increases as the applied mechanical pressure increases. The position of the lower electrode signal shifts under a lower mechanical pressure and magnitude of the lower electrode signal decreases. The larger silicone oil layer lead to a charge shift due to interfacial polarization and a smaller magnitude of the pressure wave generated from the interface between the silicone oil layer and the sample.

Influence of DC prestress on impulse breakdown strength of insulating paper-liquid nitrogen composite system

Breakdown characteristics of PPLP (Polypropylene laminated paper)® impregnated in liquid nitrogen (LN2) are presented. PPLP has three-laminated structure of kraft paper (KP), polypropylene film (PP film) and KP and has been used as an electrical insulating material in high-temperature superconducting cables. In this report, we investigated the influence of DC prestress on impulse breakdown strength of KP, PP film and PPLP in LN2. We performed DC prestress test in which DC voltage of 10 kV was applied as prestress and then impulse breakdown voltage was superimposed for the breakdown test. By the application of DC prestress of homo polarity with impulse voltage, the impulse breakdown strength of KP and PP film was found to be increased. On the other hand, by the application of DC prestress of hetero polarity, the breakdown strength of KP was found to be increased, which was unique to paper material. We proposed the negative charge injection model into KP to explain this result. Furthermore, the breakdown strength of PPLP was almost the same under the application of DC prestress of 10 kV. The influence of DC prestress on impulse breakdown strength appeared to be different depending on the specimen, and the possible model at this moment was discussed.

Electrical breakdown of dielectric elastomer and its lamination effect

Dielectric elastomers have potential as materials for artificial muscle actuators due to their excellent properties such as contractility. To understand the breakdown mechanism, breakdown tests of the dielectric elastomers were performed under different applied field waveforms (dc, ac, or impulse). In addition, the influence of the lamination number on the breakdown strength was investigated. Regardless of the applied field waveform, the breakdown strength decreases as the temperature increases. For a given temperature, the breakdown strength has the following order: impulse breakdown strength > dc breakdown strength > ac breakdown strength. Heating due to the dielectric loss under ac voltage application is considered to reduce the ac breakdown strength displayed at the RMS value, suggesting that a thermal process is responsible for breakdown. Under all applied waveforms, the breakdown strength increases as the lamination number increases.

DC prestressing effect on breakdown characteristic of insulating paper in liquid nitrogen and insulating oil

Currently, high temperature superconducting (HTS) cables are developed mainly as AC cables with PPLP®-liquid nitrogen composite insulating system. On the other hand, solar and wind power are expected as alternative energy of fossil fuels in the future. DC transmission using superconducting technology having low transmission loss and environmental burden is most suitable for such new energies. However, DC HTS cable may raise the problem that charge injection may degrade the insulating performance, which is the same as the problem in existing DC oil immersion cables. Moreover, the influence of charge injection on breakdown characteristics of PPLP® in liquid nitrogen (LN2) is not clarified yet. In this paper, DC prestressing effects on impulse breakdown strength (Fb) of PPLP® was investigated, and the charge injection model in LN2 was discussed from the conventional knowledge to charge injection in DDB oil. As a result, the opposite polarity relations in impulse Fb and the prestressing effects in LN2 appeared compared with the results in DDB oil. The DC prestress effect on impulse Fb in DDB oil can be explained based on the charge injection model in LN2 with the opposite polarity charge injection. Therefore, the difference of breakdown characteristics of PPLP® in DDB oil and in LN2 are influenced by the polarity of the charge that is easy to inject into KP layer and can explain using the charge injection model developed for the breakdown characteristics in LN2.

Space charge formation and conduction current of MgO/LDPE nanocomposite under DC ramp field

To understand the space charge formation of nano-sized magnesium oxide (MgO)/low-density polyethylene (LDPE) nanocomposite materials, the space charge and the conduction current up to the breakdown has been investigated under increasing DC ramp field. A negative charge was dominated in the 0 phr sample. A positive charge was dominated in nanocomoposite sample. The field increment (= the maximum field - the average field) of the 0 phr increased with increasing average field and then was saturated with a few rises and falls before the breakdown. In the 1 phr sample, after showing the peak, the field increment turned to decrease with further increase of the average field. In the 2 phr and more, the clear decrease of the field increment was not observed due to a few positive charges. The conduction current of 0 phr sample and 5 phr sample increased monotonically with a increase of the average field. The conduction current of the 1 phr and the 2 phr showed great increase under one average field. From these results, it was considered that the homo space charge effect at the anode brought by charge trapping effect of MgO nano-filler controlled the space charge formation and the space charge influenced to the conduction current under a high field.