Study of the experimental literature regarding nanodielectrics indicates numerous inconsistencies in the results obtained. In many cases, the likely cause is due to a lack of quality control during nanocomposite processing. By examining examples from the literature along with an alumina/polyamideimide nanocomposite and silica/crosslinked polyethylene nanocomposite, this contribution seeks to shed light on some of the likely causes for these inconsistencies. Measurements of the dielectric breakdown strength and voltage endurance confirm that poor dispersion can lead to poor material performance and the use of quantitative techniques is highlighted. Good dispersion alone is not sufficient to achieve improved properties. The addition of nanoparticles can alter the resulting structure of the nanocomposites, can introduce water into the system resulting in cavity formation and can also result in degradation of the polymer if the processing parameters are not carefully selected. In this review, understanding the effects of nanoparticle addition requires not only characterization of relevant dielectric properties, but also careful control of the processing parameters and characterization of changes in polymer structure, particle dispersion, and water content.

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A review on the importance of nanocomposite processing to enhance electrical insulation

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 solid insulator in gaseous insulation system

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.

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

The electric field distributions along gas-solid interfaces determine the reliability, lifetimes and sizes of gaseous insulated switchgears/pipelines (GIS/GIL), which also affect the reliability of power systems. In this study, the characteristics of steady and transient electric field distributions were first introduced, followed by the concept of functionally graded materials (FGMs). The development histories of FGM applied in electrical engineering and the related optimisation methods were described in detail. The field regulation effect and fabrication technology of different FGM insulators were also compared. To overcome the limitations of traditional FGM insulators, the design of surface FGM (SFGM) was proposed, together with their preparation methods and electrical performance. Furthermore, the future development prospects of FGM and SFGM insulators for compact GIS/GIL were summarised.

https://ietresearch.onlinelibrary.wiley.com/doi/pdf/10.1049/hve.2019.0327

Promising functional graded materials for compact gaseous insulated switchgears/pipelines

Functionally graded materials (FGMs) are advanced composite materials that are used to solve a number of engineering problems, as well as in the biomedical implant applications for the replacement of human tissues. These materials are used to eliminate the stress singularities that occur, as a result of the property mismatch in the constituent materials in a composite. There are different types of FGMs that are used today, depending on the type of application, for which the material is intended. In this chapter, the different types of FGMs are presented. The areas of application of this novel material are also explained.

Types of Functionally Graded Materials and Their Areas of Application

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.

Fabrication and experimental verification of permittivity graded solid spacer for GIS

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.

Permittivity gradient characteristics of GIS solid spacer

For applications of nanocomposite materials to solid insulator for electric power apparatus, we have to consider various characteristics from mechanical, thermal and electrical points of view. In particular, the agglomerate of nanoparticles would decline the excellent properties of nanocomposite. In this paper, we investigated the influence of dispersibility of nanoparticles on electrical properties. Firstly, we fabricated epoxy/alumina nanocomposites with application of ultrasonic wave and centrifugal force. Secondly, we evaluated the dispersibility of nanoparticles by image analysis of agglomerate diameter from scanning electron microscopy (SEM) and the measurement of filler contents. Then, we examined the dielectric properties such as permittivity and dielectric loss of the nanocomposite. Finally, we verified the influence of the dispersibility of nanoparticles on the dielectric characteristics.

Dielectric Properties of Epoxy/Alumina Nanocomposite Influenced by Particle Dispersibility

For the application of nanocomposite materials to solid insulator in electric power apparatus, we investigated the influence of the dispersibility of nanoparticles on dielectric property. Experiments were carried out in epoxy/alumina nanocomposites with the particle dispersion techniques using ultrasonic wave and centrifugal force. As the result, we verified the specific characteristics of dielectric permittivity in nanocomposites with good dispersibility.

H. Okubo

Papers

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

Fabrication and experimental verification of permittivity graded solid spacer for GIS

Permittivity gradient characteristics of GIS solid spacer

Dielectric Properties of Epoxy/Alumina Nanocomposite Influenced by Particle Dispersibility

Dielectric properties of epoxy/alumina nanocomposite influenced by particle dispersibility and filler content