A high-performance IGBT with new N+ buffer structure

We propose a new IGBT structure with a new N⁺ buffer, and confirm by experiments and numerical simulations that the new IGBT is superior to the conventional one. The following results were obtained. (1) According to our experiments, the new IGBT was able to decrease the total power loss, and the parallel operation became easier, compared with the conventional IGBT. Moreover, the short-circuit ruggedness of the new IGBT was almost the same as that of the conventional one by optimizing the ratio of the N⁺⁺ buried layer. (2) We clarified why the characteristics of the new IGBT were improved by numerical simulations. (a) When the new IGBT is on, holes injected from the P⁺ substrate flow through, remaining out of the N⁺⁺ buried layer. Also, the holes rapidly turn around in the N⁺⁺ buried layer when passing by, and the hole concentration becomes even. Because the lifetime of the new IGBT is designed to be long, the hole concentrations of the new IGBT increases in the N⁻ layer. Therefore, the saturation voltage of the new IGBT is lower than that of the conventional IGBT. (b) Since the lifetime of the N⁻ layer of the new IGBT can be extended, the temperature dependence of the lifetime becomes small, and IZTC of the new IGBT is improved. (c) In the turn-on state, the holes are injected through the N⁺ buffer layer with lower concentration from the P⁺ substrate, thus the turn-on speed of the new IGBT become quicker and the turn-on loss of the IGBT is reduced. (d) In the turn-off state, as the N⁻ layer is depleted completely, the carriers in the N⁺ buffer layer mainly influence the tail current. There are few carriers in the N⁺⁺ buried layer of the new IGBT, so the turn-off loss of the new IGBT is reduced. (e) Since the effect to prevent the holes being injected from the P⁺ substrate affects the N⁻ layer, the number of carriers of the N⁻ layer of the new IGBT is limited in the saturation current region. Therefore, the saturation current is also controlled, and the short-circuit ruggedness of the new IGBT is not diminished. © 1998 Scripta Technica, Electr Eng Jpn, 124(4): 37-46, 1998     

Characteristics of vertical earth potential difference signals

A modified measurement method to observe the electric earth potential difference is proposed in order to reduce the unnecessary signals induced by artificial and natural earth current sources. With this method, the potential difference is measured as the voltage between two electrodes that are buried in different vertical positions beneath the ground. We evaluated this method at four sites under different measurement conditions and we discuss the characteristic features of the three types of observed signals: (1) Horizontal earth potential difference signals appear according to the current induced by variations in geomagnetic fields. Vertical signals, however, do not appear because there is no induced vertical current. (2) After heavy rainfall the characteristic quasi-periodic pulse train signal occasionally appears. However, no such signal can be found in the horizontal direction. (3) Pulse signals appear with exactly the same period that could be expected to be caused by artificial current sources from structures and buildings. Consequently, in order to use this vertical observation method effectively, we should choose a location where the ground surface is as level as possible and which is as far as possible from current source points. © 1998 Scripta Technica, Electr Eng Jpn, 123(1): 23–31, 1998     

Peak load forecasting using multiple-year data with trend data processing techniques

This paper presents a regression-based daily peak load forecasting method using multiple-year data with trend cancellation and trend estimation techniques. Daily peak load heavily depends on daytime temperature and is influenced by the other weather factors such as humidity. Since the characteristic of the load is varying, peak loads just before a forecasting day are more significant for the forecasting. The regression model can represent relationships between these weather factors and peak loads. However, the forecasting model is sometimes not adequate for precise load forecasting. The regression model is well matched with the late data, but the model causes large forecasting errors in transitional seasons because of seasonal change of load characteristics. In order to forecast precisely through a year, a method of using seasonal or whole year data from past years is proposed. In this paper, two kinds of trend data processing techniques are described. The first is trend cancellation. The second is trend estimation. The trend cancellation technique removes annual load growth by means of division or subtraction processes with morning load on the forecasting day. The trend estimation technique estimates the trend between the forecasting year's load and the past year's load by using the variable transformation techniques. The performance of both techniques, verified with simulations on actual load data, is also described. © 1998 Scripta Technica, Electr Eng Jpn, 124(1): 7–16, 1998     

Homolog of a-factor receptor gene in Saccharomyces exiguus

Homologs of Saccharomyces cerevisiae STE3, a -factor receptor gene were detected from S. exiguus NFRI 3539 by low stringency Southern hybridization. This strain might have at least two types of homolog. One of these homologs, designated as e-STE3 was cloned. Its nucleotide sequence revealed 60% identity to STE3. The putative protein coding region consisted of 453 amino acid residues. The amino acid sequence identity between STE3 and e-STE3 was 62%, and that of the N-terminal 303 amino acid residues considered to be the pheromone binding domain was 79%. The e-STE3 sequence submitted to the DDBJ/EMBL/GenBank data libraries is available under Accession Number AB003086. © 1998 John Wiley & Sons, Ltd.     

Development of Nb-Ti cable with ultrafine filament and high-resistivity matrix for ac use

Superconducting ac machines such as transformers and reactors are expected to have an important role in future electric power transport lines. In these machines, superconducting coils are wound with superconducting cables that have low ac loss, stable ac quenching current, and high normal resistivity. We have developed Nb-Ti superconducting cables with ultrafine filaments and high-resistivity matrix for these coils. One such cable is a double-stranded round structure using 0.2-mm strands with 0.14-μm filaments and Cu-30wt%Ni as a matrix material. The 50-Hz quenching current without external magnetic field exceeds 1400 Arms. The ac loss is 15 kW/m³ at a transverse external magnetic field of 0.5 T, 50 Hz, and the normal resistivity is 0.21 Ω/m at 0 T, 10 K. © 1997 Scripta Technica, Inc. Electr Eng Jpn, 120(2): 8–18, 1997     

Effect of Ga2O3 on Sintering and Phase Stability of Sc2O3‐Doped Tetragonal Zirconia Prepared via Aqueous Solution Route

The effects of the presence of Ga₂O₃ on low‐temperature sintering and the phase stability of 4, 5, and 6 mol% Sc₂O₃‐doped tetragonal zirconia ceramics (4ScSZ, 5ScSZ, and 6ScSZ, respectively) were investigated. A series of zirconia sintered bodies with compositions (ZrO₂)_0.99?x(Sc₂O₃)_x(Ga₂O₃)_0.01, x = 0.04, 0.05, and 0.06 was fabricated by sintering at 1000°C to 1500°C for 1 h using fine powders that were prepared via the combination of homogeneous precipitation method and hydrolysis technique using monoclinic zirconia sols synthesized through the forced hydrolysis of an aqueous solution of zirconium oxychloride at 100°C for 168 h. The presence of 1 mol% Ga₂O₃ was effective in reducing sintering temperature necessary to fabricate dense bodies and enabled to obtain dense sintered bodies via sintering at 1100°C for 1 h. The phase stability, that is, low‐temperature degradation behavior of the resultant zirconia ceramics was determined under hydrothermal condition. The zirconia ceramics codoped with 1 mol% Ga₂O₃ and 6 mol% Sc₂O₃ (1Ga6ScZ) fabricated via sintering at 1300°C for 1 h showed high phase stability without the appearance of monoclinic zirconia phase, that is the tetragonal‐to‐monoclinic phase transformation was not observed in the 1Ga6ScZ after treatment under hydrothermal condition at 150°C for 30 h.     

Synthesis of deoxy(thiosulfato)chitin as the precursor for noncatalytic photoinduced graft copolymerization

Photoinduced graft copolymerization of vinyl monomers onto deoxy(thiosulfato)chitin (S₂O₃–chitin) has been studied. Chitin was first tosylated and subsequently transformed into S₂O₃–chitin. S₂O₃–chitin has good solubility over tosyl–chitin. Graft copolymerization of S₂O₃–chitin proceeded very easily by ultraviolet irradiation without catalyst. Photolysis of S₂O₃ groups was confirmed by infrared spectra. But the photolysis occurred only in quartz, not in a Pyrex tube. Methyl methacrylate (MMA) and acrylonitrile showed good grafting activities. In the case of acrylic acid and acryl amide, homopolymer formation was predominant, and the degree of grafting was low. We chose the MMA monomer for further information. The grafting rate of S₂O₃–chitin using MMA was much faster than those of chitin and O‐acetyl–chitin. Under the appropriate conditions, the degree of grafting reached 600% only in 2 h, and the grafting efficiency was over 75% in any monomer concentration. But addition of DMSO into the polymerization system decreased the degree of grafting. © 1999 John Wiley & Sons, Inc. J Appl Polym Sci 71: 189–195, 1999     

Control schemes and transient characteristics of a high‐speed phase shifter for power flow control in power transmission systems

In recent years, attention has been paid to the concept of FACTS (Flexible AC Transmission Systems), along with significant progress in power electronic technology. A high‐speed phase shifter, which is one of the most promising devices in the FACTS concept, has the potential of power flow control and/or voltage stability in power transmission systems. In this paper, theory and experiment reveal that conventional high‐speed phase shifters may cause power swings in a transient state as a result of coupling between instantaneous active and reactive power control loops. Thus, two new control schemes for a high‐speed phase shifter are proposed to achieve both power flow control and power swing damping. The second proposed control scheme is based on the control scheme of an already proposed series active filter. Simulated and experimental results agree well with analytical results, not only in steady states but also in transient states. © 1999 Scripta Technica, Electr Eng Jpn, 128(2): 74–82, 1999     

Investigation of partial discharge mechanism by simultaneous measurement of current waveform and light emission

We observed current pulse waveforms of partial discharge (PD) in SF₆ gas so as to investigate the PD mechanism. We also measured light intensity and light emission image of PD simultaneously under different conditions of applies voltage and SF₆ gas pressure. From these experiments, we found that the “double-peak current waveform” appeared at high pressure and high voltage conditions. We also analyzed the mutual correlation of waveforms between a single current and the light emission. Moreover, we obtained experimental evidence of filmentlike light image appearing at the PD tip under the same condition with double-peak current waveform. From the electric field analysis around the needle electrode tip, we believe that the filamentlike light image expands beyond the critical electric field of SF₆ gas. Thus, we concluded that these current waveforms with double peaks showed evidence of leader-type PD, leading to breakdown. Finally, we could point out that leader-type PD should be distinguished and measured for the diagnosis of GIS insulation performance. © 1999 Scripta Technica, Electr Eng Jpn, 129(4): 58–65, 1999     

Simple cellular automaton‐based simulation of ink behaviour and its application to Suibokuga‐like 3D rendering of trees

Suibokuga is a style of monochrome painting characterized by the use of Chinese black ink (sumi), a complex interaction between brush, ink and paper, and such visual features as Noutan (shade), Kasure (scratchiness), and Nijimi (blur). In this paper we present a simple behavioural model of water and ink particles based on a 2D cellular automaton computational model, and its application to a Suibokuga‐like rendering of 3D trees. Copyright © 1999 John Wiley & Sons, Ltd.