In AC/DC converter transformers under DC application, the electric field distributions are distorted due to the deposited charge on pressboard. However, the time dependent characteristics of DC electric field stress in oil has not yet been clarified in detail. In this paper, we investigated the time and electric field space transition characteristics of DC electric field distributions in oil-PB composite insulation systems. As a result, we quantitatively clarified that DC electric field stress strongly depends on the elapsed time and the location in the electric field space in oil. In particular, it was found that the most severe electric field stress in oil may emerge during the time transition process.

We develop a micro-gap electrode device on quartz substrate to investigate the dielectric breakdown resistance on constrained layer of silica/epoxy interface. After application of stress, the device is cut at the middle of the micro-gap by laser dicing to observe the cross-section, and the cross-section shows where the breakdown occurs in the vicinity of the silica/epoxy interface at sub-micron scale. These results give us a clue to understand the effect of nano-fillers on the dielectric breakdown of nano-composite materials.

The stator coil end of a rotating machine has the structure of electric field grading system (SG system) for the purpose of preventing partial discharge. Light emission of partial discharge at such SG on a model bar system under operating temperature is measured by a photomultiplier tube and a digital camera equipped with an image intensifier. It is found that light emission of PD on SG system decreases under operating temperature. Small PD which occurs at lower voltage under high temperature may prevent the occurrence of the subsequent PD.

With the proliferation of wearable terminals and internet of things (IoT) equipment, electrostatic discharges (ESDs) due to electrification of humans and objects can cause electromagnetic malfunction to these devices, which is called intra-EMC (Electromagnetic Compatibility). This type of the ESD/EMC issues would come from a steep transient electromagnetic near fields generated by spark discharges between micro-gaps. To clarify the underlying mechanism of micro-gap sparks, it is indispensable to quantitatively grasp the spark phenomena of such micro-gap ESDs. For this purpose, different two spark-resistance laws proposed by Toepler and Rompe-Weizel have often been used so far; however, the applicability to micro-gap ESDs is not being fully understood. In this study, a spark current is derived for an electric dipole sparking model in closed forms from the above spark resistance formulae to show the characteristics of spark discharges theoretically. Measurements of spark currents due to the sparks between parallel disk electrodes with a micro-gap are conducted at charge voltages from 300 V to 7000 V in order to verify the validity and applicability of Toepler's and Rompe-Weizel's formulae. As a result, spark constants derived from the measured results can be independent of spark lengths, and agree fairly with those empirically obtained for both the formulae. In comparison with the Toepler's formula, however, the Rompe-Weizel's formula can better explain the charge voltage dependence of spark characteristics such as nominal duration times, peaks and frequency spectra of spark currents.

Transient magnetic field caused by micro-gap ESD was measured to examine the discharge current of a dipole radiation model. The magnetic field measured by a shielded loop probe, a conversion factor from an output voltage of the probe to the magnetic field strength was derived as a probe correction factor (magnetic-field antenna factor) obtained by the three-antenna method. The magnetic field strength calculated by the conversion factor considering the frequency characteristic of probe correction factor using a process of FFT and IFFT. As a result, the peak value of magnetic near field strength was about 500 A/m - 700 A/m in discharge voltage of 600 V.

Immunity testing of electronic equipment for electrostatic discharge (ESD) is prescribed as IEC 61000-4-2 by the International Electrotechnical Committee (IEC), in which contact discharge current testing from an ESD generator is specified with only the following four parameters: the rise time of the first peak current, the current value of the first peak, the current values at 30 ns and at 60 ns. To reduce the variations in test results by different ESD generators, the Japanese national committee of IEC SC77B once proposed a time width of 60% of the first peak current as an additional specified parameter in the IEC standard, while the proposal was not adopted since a round robin test was unable to confirm the effectiveness. In the 2008 revised version, a theoretical calibration current waveform based on the Heidler’s formula was shown as the typical contact discharge current waveform from an ESD generator. In this paper, using different eight models of commercially available ESD generators manufactured by six manufacturers from the market, we compare the waveforms of discharge currents measured for contact discharges with the calculated waveform from the Heidler's formula. In addition to the IEC specified parameters, the previously proposed time width (1.5 ns to 3.5 ns) of 60% of the first peak current is also examined. Results show that almost all the ESD generators except for one generator meet the specifications, and that there are only four generators whose time widths of 60% of the first peak currents satisfy the suggested time width. Furthermore, all the frequency current spectra at frequencies lower than 30 MHz approximately agree with the current spectrum calculated from the Heidler’s formula, whereas they vary depending on the ESD generators at frequencies higher than 30 MHz, even though ESD generators meet both the IEC specified requirements and the assumed time width of 60% of the first peak.

At joint and terminal sections of electric power cables, there is possibility to occur partial discharges (PDs) in defects of insulating materials. Therefore, PD measurement is useful from a viewpoint of assessing insulation deterioration of electric power cables. There are some measurement systems to locate PDs, however, it is quite difficult and complicated in the case where cable lines include branched lines because the PD current pulse separately flows at the branch line. To establish a PD location system, the investigation of propagation characteristics of PD current pulse for branched lines is necessary. Based on EMTP simulations and experiments, we evaluated how a PD current pulse propagates at a Y-branch. The results showed that the separation of a PD current pulse is dependent on frequency component of the PD current pulse and the cable impedance after branching. The frequency components of a PD current pulse, which are higher than the resonant frequency of lines, tend to flow into the longer cable side.

The International Electro-technical Commission (IEC) specifies air discharge immunity testing for electronic equipment in the standard 61000-4-2 under the climatic conditions of relative humidity from 30 to 60% and ambient temperature from 15 to 35 degrees Celsius. As aiming to improve the testing reproducibility, to clarify effects of the above climatic parameters on air discharge testing, we previously measured air discharge currents from an electrostatic discharge (ESD) generator with an approach speed of 80 mm/s under 6 combinations of relative humidity and temperature. The result showed that the same absolute humidity provides almost similar waveforms of the discharge currents despite different relative humidity and temperature. In this study, to further examine such combined effects, we measure air discharge currents from the ESD generator at a test voltage of 15 kV with three different approach speeds of 20 mm/s, 50 mm/s and 80 mm/s under 9 combinations of relative humidity (RH) and temperature inside and outside the IEC specified climatic ranges. As a result, air discharge current behavior is affected by not only absolute humidity (AH) but also saturation deficit (SD), or the difference between the AH and the saturated water vapor amount. The humidity dependence of peak currents differs according to the IEC specified and non-specified climatic ranges, which can better be explained by the SD. Under the IEC specified conditions over 30% RH, at faster approach speeds, the peak currents of air discharges have maximum values at AH7.70 g/m³ and SD15.36 g/m³, whereas they are 1.2 to 1.4 times larger compared to those at almost the same AH7.71 g/m³ and smaller SD5.14 g/m³. In the IEC non-specified climatic range less than 20% RH, however, regardless of the approach speeds, the current peaks noticeably reach maximum values at AH3.81 g/m³ and SD19.26 g/m³, which are 3.6 to 5.2 times larger than those at roughly the same AH3.96 g/m³ and larger SD35.64 g/m³. The above finding indicates that, under the same AH conditions, the maximum current peaks tend to be higher as the SD increases under over 30% RH conditions, whereas they rapidly increase as the SD decreases under less than 20% RH conditions.

This paper presents an effect of transient-electromagnetic fields (or pulsed electromagnetic fields) on the hatching of silkworm eggs. In our experiment, transient-electromagnetic fields by spark discharges were applied to silkworm eggs. The pulse width of discharge current (800 A) and the temperature were set to 3µs and 20°C, respectively. The application period was set to 3, 6, and 9 days both for two conditions; 5 and10 times application in a day. As a result, we found that the transient-electromagnetic fields increased the hatching rate of the applied silkworm eggs, resulting in the maximum relative differences between the applied samples and the control being 1.8 to 2.8 times.