Muneaki Kurimoto

Application of functionally graded material for solid insulator in gaseous insulation system

Functionally graded materials (FGM) have spatial distribution of a material property in order to achieve efficient stress control. An application of the FGM to a solid insulator (spacer) for a gaseous insulation system, like gas insulated switchgear, is expected to improve electric field (E-field) distribution around the spacer. In this paper, we describe the applicability of the FGM spacer to gas insulated power equipment. In the FGM spacer, we gave spatial distribution of dielectric permittivity to control the E-field distribution inside and outside the spacer. This paper includes following key results for the applications of the FGM. Firstly, E-field simulation results when applying the FGM by a finite element method are presented, in which we show the effective reduction of the maximum field strength by applying the FGM. Next, a fabrication technique of the FGM spacer sample with not only step-by-step but also continuous changes of permittivity is presented by use of centrifugal force. Finally, dielectric breakdown tests using FGM samples which are accurately controlled the spatial distribution of permittivity are carried out under lightning impulse voltage applications. The test result indicates the increase of breakdown voltage (BDV). From these results, we verified the applicability and the fabrication technique of FGM spacer for improvement of the dielectric strength in the gaseous insulation system.

Application of functionally graded material for reducing electric field on electrode and spacer interface

For the size reduction and the enhancing reliability of electric power equipment, the electric field stress around solid insulators should be considered enough. For the relaxation of the field stress, the application of FGM (Functionally Graded Materials) with spatial distribution of dielectric permittivity (¿-FGM) can be an effective solution. In this paper, we investigated the applicability of ¿-FGM for reducing the electric field stress on the electrode surface with contact to the solid dielectrics, which was one of the important factors dominating a long-term reliability of the insulating spacer. Firstly, we carried out numerical simulation of electric field to confirm the reduction of the electric stress by U-shape permittivity distribution. Secondly, we investigated the fabrication feasibility of ¿-FGM with the U-shape distribution. Thirdly, we estimated the longterm electrical insulation performance of the ¿-FGM. Finally, we verified the applicability and the fabrication technique of the ¿-FGM to solid dielectrics for improvement of the electric stress and the long-term insulation performance in electric power equipment.

Permittivity characteristics of epoxy/alumina nanocomposite with high particle dispersibility by combining ultrasonic wave and centrifugal force

This paper proposes a novel technique to fabricate epoxy/alumina nanocomposites with nanoparticle composite process by combination of ultrasonic wave and centrifugal force. The particle dispersion effect of the nanoparticle composite process and its influence on dielectric permittivity were discussed quantitatively. Experimental results clarified that the combination of ultrasonic wave and centrifugal force was effective to increase dispersed nanoparticles and as well as to separate residual agglomerates. We verified that the improvement of particle dispersibility in the nanoparticle composite process by combination of ultrasonic wave and centrifugal force could bring about lower permittivity of the nanocomposites, especially than that of unfilled epoxy material.

Permittivity gradient characteristics of GIS solid spacer

For electrical insulation design of GIS spacer, we need to control electric field distribution around the solid spacer, especially around the triple junction (TJ). For this purpose, we proposed the application of functionally gradient materials (FGM), which has spatial distribution of dielectric permittivity, to the spacer of SF/sub 6/ GIS. In this paper, we discussed applicability of FGM with numerical simulation and fabrication conditions. We firstly investigated the field control effect of the FGM spacer with a conical shape, by finite element method (FEM). Secondly, we fabricated FGM spacer with continuously graded distribution of permittivity by applying the centrifugal force. As for the filler material to control the permittivity, we selected TiO/sub 2/ rutile crystal particle. In order to obtain the optimum permittivity distribution, centrifugal forces, their application duration, the diameter distribution of filler particles, volume ratio of filler versus resins and so on were controlled.

Fabrication and experimental verification of permittivity graded solid spacer for GIS

Functionally gradient materials (FGM) have a spatial distribution of permittivity in order to achieve an efficient stress control. The application of FGM to solid spacers for gas insulated switchgear (GIS) is expected to improve electric field distribution around the spacer. In this paper, we discussed the applicability of FGM with experimental results and fabrication conditions. We firstly investigated the field control effect of the double-layer distribution FGM spacer by dielectric breakdown experiments under the application of lightning impulse voltage. Secondly, we confirmed the effect of FGM on the enhancement of electric field utilization experimentally. Thirdly, we succeeded in the fabrication of continuously graded distribution of permittivity by applying the centrifugal force technique.

Dielectric properties of epoxy/alumina nanocomposite influenced by control of micrometric agglomerates

Introduction of metal oxide nanoparticles to polymer material is known to have unique dielectric behavior and significant advantages in electrical insulation performance in power apparatus. This paper presents an attempt to clarify the influence of dispersibility of nanoparticles, especially focusing on agglomerates, on dielectric properties of a nanocomposite system by changing particle dispersion processes. Experiments were carried out in epoxy/alumina nanocomposites with the particle dispersion techniques by applying ultrasonic wave and centrifugal force. For the dispersibility control of nanoparticles, we changed the duration of ultrasonic wave and centrifugal force. The experimental results clarified the effect of centrifugal force on the separation of agglomerates and the effect of ultrasonic wave on the disruption of agglomerates. Next, we examined the dielectric properties such as relative permittivity and tan δ of the nanocomposites. As the result, we verified that the permittivity of epoxy/alumina nanocomposites became low due to separation and disruption effects of the agglomerates.

DC dielectric breakdown characteristics of mesoporous-alumina/epoxy composite

This paper presents an attempt to derive the dc dielectric breakdown characteristics of polymer composite filled with the metal oxide particle which has mesoporous structure. Mesoporous-alumina/epoxy composite was fabricated by compounding epoxy resin and mesoporous-alumina particles which had the micrometric particle-diameter and the nanometric pore-size. 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. From the breakdown test with using McKeown electrode, DC breakdown strength of mesoporous-alumina/epoxy composites was obtained, which was compared with DC breakdown strength of the nonporous-alumina/epoxy composites. These results suggested that the nanometric pore with wormhole structure inside mesoporous particle could not be critical defect for DC dielectric breakdown characteristics in low filler concentration.

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.

Dielectric breakdown mechanism of polypropylene laminated paper in liquid nitrogen

Breakdown characteristics of PPLP (Polypropylene laminated paper) impregnated with LN2 for its application to high-temperature superconducting cable are presented. In this paper, we investigated the breakdown characteristics of PPLP, which was the three-laminated structure of kraft paper (KP), polypropylene film (PP film) and KP, in LN2 by comparison with those of the papers with different laminated structure, e.g., single-layer of KP, single-layer of PP film and the 2 layer laminated paper of KP and PP. The breakdown characteristics under the application of impulse voltage, ac voltage and dc voltage were obtained, and the breakdown mechanisms were discussed. Experimental results revealed that the breakdown strength of PPLP under the application of impulse voltage and dc voltage was higher than that of the single-layer material. This could be attributed to the dispersion effect of the streamer by KP layer of PPLP. In order to obtain the unified explanation for the breakdown mechanism with the different voltage polarity and the different laminated structure, we suggested the injection model of the negative charge into the KP layer of PPLP and the less-injection model of the charge into the PP.

Electric field control in coaxial disk-type solid insulator by functionally graded materials (FGM)

We have been investigating functionally graded materials (FGM) with the purpose of controlling and relaxing electric field in solid insulator of electric power apparatus. FGM is characterized by the spatial distribution of dielectric permittivity in a solid insulator, and can control the surrounding electric field distribution in power apparatus such as gas insulated switchgears (GIS) and epoxy mold transformers. In this paper, we fabricated a coaxial disk-type GLP (grading to lower permittivity)-FGM for the GIS spacer. Experimental results revealed that we could fabricate GLP-FGM with relative permittivity distribution from 8.2 to 5.3 in the coaxial disk-type spacer. According to the numerical analysis, the maximum electric field at the HV-electrode/spacer interface was reduced by 12 % for the coaxial disk-type GLP-FGM than that of the conventional spacer with uniform permittivity.