A diagnostic method based on RVM (return voltage measurement) for condenser bushings with oil–paper insulation systems

The bushing of a transformer is one part of the transformer system, but is as vital as the transformer itself because it forms a part of the main circuit. Deterioration and abnormal condition of the bushing possibly cause its destruction, shortage of the main circuit to the Earth, and fire in the systems, so that diagnosis of the bushings is thus very important. At present, mainly visual inspection from outside of the bushing is carried out. Inspection of the inside condition of the bushing by dissolved gas analysis of the inside oil is done only as sampling for selected bushings because it is difficult due to the sealed construction of the bushings. Therefore, a simple diagnostic method for bushings has been expected for a long time. In Eastern Europe, the RVM (Return Voltage Measurement) method based on scanning the polarization spectra was applied to the oil–paper insulation systems inside the transformer as a diagnostic method. In this paper we investigate theoretically the principle of the RVM method applied to bushings with the same oil–paper insulation systems as the transformer by using the equivalent circuit, and the possibility of using this novel diagnostic method for condenser‐type bushings is demonstrated by a model experiment. © 1999 Scripta Technica, Electr Eng Jpn, 130(1): 21–29, 2000     

Breakdown mechanism of capacitively coupled RF discharges

Measured breakdown characteristics of capacitively coupled RF discharges are compared with numerical simulations performed under different boundary conditions at the electrode. When a pair of glass‐covered aluminum disks are used in the breakdown measurement of N₂ and Ar gases at RF frequency of 13.56 MHz, the peculiar bend in the left branch of the minimum of V‐shaped breakdown curves is not observed, suggesting the absence of secondary electron emission. For the treated boundary condition of the electron density n, the simulation with the zero gradient gives satisfactory agreement with the experimental V‐shaped curves, while the simulation with n = 0 is shown to be effective in the region of small electric fields. The development of RF breakdown is explained based on the simulation for spatiotemporal profiles of n. © 1999 Scripta Technica, Electr Eng Jpn, 130(1): 8–14, 2000     

Rate enhancement of amines in the photopolymerization of methyl methacrylate under oxygen

The photopolymerization of methyl methacrylate was accelerated by amines such as N,N-dimethylbenzylamine, triethylamine, and diethylamine under nitrogen, in which additional initiating radicals were generated by the reaction of the amines with excited MMA. Enhanced rates were observed under oxygen for this photopolymerization in the presence of amines. Because UV spectra of the amines under oxygen indicated that the amines form charge-transfer complexes with oxygen, the rate enhancement was ascribed to photodecomposition of complexes that yield radical species. Molecular weights of polymers obtained in the photopolymerization in the presence of amines decreased under oxygen, supporting the assumed mechanism. © 1998 Society of Chemical Industry     

Frequency stabilization of a semiconductor laser under direct FSK modulation: The modulation method and evaluation of stability

The frequency stability of a semiconductor laser deteriorated under direct frequency shift keying (FSK) when stabilization depended entirely upon an external frequency reference. Therefore, the PEAK method was devised for the purpose of improving frequency stability. This method requires two distinct frequency components and, therefore, will not work effectively in a communications system that produces a succession of identical frequencies, as does the FSK method on occasion. This paper, then, explains the results of our comparative analysis of the two modulation methods. The evaluation of frequency stability requires us to use a beat note between two stabilized laser beams, referred to here, as “signal” and “reference,” lasers. The reference laser is stabilized by a method that takes advantage of the magneto-optical effect. This laser restrains the tendency of the beat note frequency to broaden in other stabilization methods, and even makes it possible to predetermine the beat frequency to be employed. The improvement in frequency stability obtained is about one order of magnitude. © 1998 Scripta Technica, Electr Eng Jpn, 125(2): 44–51, 1998     

Loop current characteristics of a thyristor-controlled series capacitor

This paper presents the results of an investigation on the loop current of a thyristor-controlled series capacitor (TCSC). The knowledge of loop current characteristics is important for understanding analytically the equivalent impedance of TCSC, estimating harmonics produced by TCSC, and determining ratings of the TCSC circuit components. However, little has been written on the loop current of TCSC circuit. Therefore, using a laboratory-scale TCSC installed on a laboratory power system, the TCSC currents and voltages were measured and analyzed to investigate the behavior of loop current. A mathematical model of TCSC was also developed to compare to the experimental results and further clarify the relationship between loop current and steady-state impedance characteristics, including effects of TCSC circuit component size on the characteristics. Capacitor voltage harmonics and their impact on the power system were also investigated. ©1998 Scripta Technica, Electr Eng Jpn, 125(1): 37–46, 1998     

Proposal of new iterative scheme in GMRES

In the paper, GMRES is applied to various large matrices appearing in 3D field analyses. By changing the condition of the matrices, effective preconditioning of the matrix is sought. Also studied is a relationship between the residual norm and the precision evaluated during GMRES. The paper shows that the residual norm, which is indirectly computed in GMRES and has so far been used as a measure of precision, cannot be used as an indicator of the solution's accuracy. After discussing deterioration of precision, GMRES(m1), which solves matrices with a modified iterative scheme, is proposed. It is to be confirmed that GMRES(m1) is faster and more accurate than any of GMRES and its modified version, GMRES(m). © 1998 Scripta Technica, Electr Eng Jpn, 123(4): 15–24, 1998     

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     

Robot vision robust to changes of lighting conditions using a wide-dynamic-range visual sensor

We have developed a robust vision system for 2D positioning of industrial parts stable to changes of lighting conditions. A wide dynamic range vision sensor that we had developed previously was used to avoid the saturation of object images. Additionally, a gray scale pattern matching technique was employed for robust image processing. Performance of the system with a wide dynamic range vision sensor was investigated experimentally in comparison with that of a system with a conventional video camera. The probability of correct positioning of an object under changing lighting conditions, simulating those in a factory, was 100% for the wide dynamic range vision sensor and 83% for the conventional video camera. The experimental results confirm the effectiveness of the developed system, revealing that dynamic range expansion of the video cameras is very effective for realizing robust robot vision systems. © 1997 Scripta Technica, Inc. Electr Eng Jpn, 120(2): 34–40, 1997