Hiroaki Cho

Preparation and characteristic evaluation of hydrophobic epoxy-based nanocomposites

Nanocomposite techniques are used to improve the electrical properties of epoxy materials, for example, insulation breakdown strength and partial discharge resistance. In the first step, we prepared epoxy-based nanocomposites by using hydrophobic epoxy as the base resin and 10wt% of titanium dioxide or 5wt% of layered silicate as nano fillers, and measured the partial discharge resistance and insulation breakdown strength. It was confirmed that the depth of the hole caused by partial discharge exposure was decreased to half and one-fourth and the insulation breakdown strength was increased by about 0.8% and 4% with the addition of titanium dioxide or layered silicate, respectively, as compared to the base resin. In the second step, we fabricated impregnated mica tapes, which have been widely used in high-voltage equipment, using nanocomposite epoxy as binder resins. We can see that, although the erosion depth was not changed significantly, the insulation breakdown strength was increased by about 35% and 8.7% in mica tapes containing titanium and layered silicate, respectively, compared with mica tape without nano-fillers.

Grounding effect on transient earth voltage signal induced by partial discharge in metal box model

This paper deals with advancement of insulation diagnosis technology for high voltage equipment by partial discharge (PD) detection using transient earth voltage (TEV) detection method. Our previous study showed that detected TEV signal contains both high-frequency and low-frequency components around 500 MHz and 30 MHz, respectively. To further investigate the principle of detecting the low frequency components, this paper presents experiment and finite difference time domain analysis of TEV signal propagation. Results revealed that the signal intensity detected by TEV sensor with grounding is about five to six times as much as that without the grounding. Furthermore, when grounded, the low frequency components tend to be strongly detected. In other words, it is suggested that PD current including the low frequency component is induced on the surface of the metal box via the ground line, and this component greatly influences the TEV detection intensity.

Influence of defect pressure on insulation characteristics of molded transformer model

This paper discusses the influence of pressure in a defect on time evolution of Partial discharge (PD) characteristics and lifetime characteristics of an insulation model of a molded transformer. Experimental results show that the time evolution of PD characteristics for a defect at atmospheric pressure is different from that for a defect at a reduced pressure. Secondly, the lifetime of a sample with an atmospheric pressure defect is short, since samples containing a defect at atmospheric pressure can lead to breakdown with a small amount of PD charge.

Application-oriented researches on nanocomposite insulating materials

In the past decade, the nanocomposite technology has made much progress rapidly. We also developed the epoxy-based nanocomposites which have the enhanced insulation properties. The advantages of the nanocomposite materials stimulate the efforts for practical use of them. Therefore, the applications of the nanocomposite materials are primary interests in current researches. In this paper, we briefly introduce the four kinds of the application-oriented researches on the nanocomposite insulating materials.