We have investigated the effect of white discharge trace remaining on pressboard (PB) surface on the impulse creeping discharge characteristics in oil/PB composite insulation system. This paper deals with the effect of the white trace on the partial discharge inception voltage (PDIV), the number of discharge pulses, the discharge charge amount, and the time delay of discharge occurrence under the standard lightning impulse voltage application. Then, we also investigated the mechanism how the white trace affected the creeping discharge characteristics.

Recently, additive manufacturing techniques have been attracted in various fields of engineering including electrical insulating. However, the influence of the inner structure of additive manufactured polymer on electrical insulating property is not revealed. In this paper, the electrical breakdown strength of the horizontally-built and vertically-built photopolymer plate made by the inkjet-type 3D printer was measured. The results demonstrated that the electrical breakdown strength of the horizontally-built photopolymer plate was larger than that of vertically-built photopolymer plate. Moreover, the distribution and shape of voids in the both types of photopolymer plates was analyzed by scanning electron microscope and image processing. It is shown that building direction of photopolymer plate affects the electrical breakdown strength and anisotropy of voids inside the specimens.

The thermal conductive plate in a device like power module for an automobile requires materials with higher thermal conductivity and acceptable electric breakdown strength. To develop a composite material with both a higher thermal conductivity and acceptable breakdown strength, polymethylmethacrylate (PMMA)/hexagonal boron nitride (h-BN) composite materials in which the flaky surface of h-BN oriented parallel to the electric field and the thermal conductivity measurement (= the sample thickness) direction were produced by an electrostatic adsorption method. The breakdown strength of the PMMA/h-BN composite material showed the increasing trend with the increase of the PMMA particle size. On the other hand, the thermal conductivity of the PMMA/h-BN composite material also showed the decreasing trend with the increase of the PMMA particle size. The h-BN content and/or the orientation rate of the flaky surface of h-BN to the electric field (= the thermal conductivity measurement) direction determine these properties.

Recently, the use of ester-based insulating fluids in electrical equipment has been increased for reducing the environmental impact. However, the fault diagnostic techniques have not been established as those by DGA for mineral oil immersed electrical equipment. In our present study, in order to estimate the overheating temperature, the characteristics of gas generations from three types of locally heated ester-based insulating fluids (PFAE, FR3 and MIDEL7131) were investigated, with a temperature ranging from 300 to 700℃. The results reveal that the gas generation characteristics for ester-based fluids are greatly different from that of mineral oil. In addition, it was found that the gas generation pattern also changes with the type of ester oil. We succeeded in estimating the overheat temperature of oil filled electrical equipment with high accuracy by investigating and establishing the relationship between relative gas ratio and temperature.

The development of electrical tree is one factor that the breakdown occur in solid insulator. So, the study concerning the tree has been proceeded. When the silicone rubber gel made by changing the degree of cross-linking agent was used as a sample, the tree named pearl-chain-type was generated. The pearl-chain-type tree consisted of two parts: string and bubble. Branching occurred at bubble parts. In this study, the pattern of pearl-chain-type tree was observed under different frequency of applied voltage using a high speed camera. Main branche and side branch was also considered.

A thin film common-mode filter (CMF) for the cell phone at UHF band was fabricated and evaluated. The thin film CMF made by the thin-film technology, and it consisted of plural MIM capacitors and a coupled inductor. The authors fabricated two kinds of the thin film CMF for 0.70 - 1.0 GHz band and 1.8 - 2.0 GHz band. A size of the 0.70 - 1.0 GHz band type CMF is 1,140×1,260×10.5 µm³. On the other hand, a size of the 1.8 - 2.0 GHz band type CMF is 1,060×1,060×10.5 µm³. The volume of both thin film CMF is less than half of that of the commercial chip common-mode choke coil (CMC). In addition, both thin film CMFs has an insertion loss in differential mode less than 3 dB, and a common mode attenuation more than 28 dB at each band. Their characters are substantially equal to the characteristic of the commercial chip CMC.

In 2010, the international commission on non-ionizing radiation protection (ICNIRP) guidelines provided the reference levels of contact currents in the frequency range from 1 Hz to 10 MHz. These restrictions are based on the threshold levels of electric shocks of human volunteers in response to sinusoidal contact current stimuli except transient currents due to electrostatic discharges (ESDs), despite the fact that such transient currents should also cause the electric shock. In this study, concerning the contact currents causing electric shocks, we compared the charge amounts from an electrostatically charged human and from an induction charging conductor exposed to a power-frequency electric field, which were measured in volunteers' experiments by our group and by Reilly, respectively. The results showed that both the charge thresholds have almost the same level. We then conducted a human safety evaluation of a 2-kV discharge current for ESD immunity testing prescribed in the standard of the international electrotechnical commission (IEC), integrating the ratio η of the frequency spectrum of the current to the ICNIRP reference level (named conformity index), in view of the conformity criterion of simultaneous exposure to multiple frequency fields. As a result, we found that the integrated ratio or conformity index η is significantly greater than 1. This means that the 2-kV discharge current exceeds the ICNIRP reference level of contact currents. It was also found that charge voltages to provide the transient currents not exceeding the reference level from η < 1, are 30 V and 70 V for general public exposure and occupational environment, respectively, which match the safety limit of electric shock from direct-current voltages with a short time interval.

Less deterioration in magnetic properties of laminated cores, produced in the manufacturing process by automatic interlocking die, is important in order to reduce energy loss in motors. This report evaluates the influences of shrink fitting on magnetic properties of interlocked ring cores. Core losses increases because of the deformation caused by interlocking and shrink fitting. And then it was found that the eddy current losses of the ring cores increased due to both the compressive stress in magnetizing direction and the electrically short-circuit between sheets in the interlocking parts, through comparison with the single sheet core losses under the applied compressive stress. Eddy current losses under the compressive stress were discussed using the magnetic domain model.