Faculty Profiles - KOJIMA Hiroki

By using Kerr electro-optic technique, electric field in oil / pressboard (PB) composite insulation system, which is the typical insulation system of a transformer, was investigated at polarity reversal of HVDC voltage. When the applied voltage polarity was sharply reversed, the applied electric field was superimposed by the electric field of accumulated charges on PB, and very strong field appeared right after the polarity reversal. This could cause the discharge in the oil gap and on the PB surface. The critical value of the discharge initiation at the polarity reversal was verified to be determined by the sum of the applied electric field and the electric field of accumulated charges on PB. The discharge was observed by a high-speed video camera. It was clarified that the surface discharge on PB did not overlap with each other, which was explained by that the surface discharge could neutralize the charges on PB. ©2010 IEEE.

DOI： 10.1109/CEIDP.2010.5724003

Scopus

Other Link： https://www.webofscience.com/wos/woscc/full-record/WOS:000398821400069

Improvement of Cu-Cr Electrode Surface under Impulse Voltage Conditioning in Vacuum International conference

H. Kojima, H. Yasukawa, R. Nishimura, H. Okubo, H. Sato, H. Saito, Y. Noda

2010 International Conference on High Voltage Engineering and Application 2010.10.11 Mississippi State University and Chongqing University

Event date： 2010.10

Language：English Presentation type：Oral presentation (general)

Venue：New Orleans Country：United States

As vacuum attracts attention from a viewpoint of high insulation performance and environmental-friendliness, vacuum circuit breakers (VCBs) are expected to develop into higher voltage level. In this paper, we discuss the relations between the surface state and the conditioning mechanism of Cu-Cr electrode during impulse voltage application process in vacuum. The experimental results showed that the deposited layer at the cathode surface was formed by the anode material due to the impulse voltage application process. We finally clarified and concluded that under impulse voltage application process, characteristics of the cathode surface are changing to those of the anode material, and anode material could determine the final states of conditioning process.

DOI： 10.1109/ICHVE.2010.5640856

Scopus

CiNii Research

Electric Field Optimization of Floating Electrode Configuration in Vacuum Interrupter International conference

H. Kojima, K. Suzuki, K. Kato, H. Okubo

2010 International Conference on High Voltage Engineering and Application 2010.10.11 Mississippi State University and Chongqing University

Event date： 2010.10

Language：English Presentation type：Oral presentation (general)

Venue：New Orleans Country：United States

For environment-friendly vacuum interrupter (VI), higher voltage operation and more compactness are required. From this point of view, and to enhance the electrical insulation performance of VI, it is needed to introduce the field optimization techniques. In this paper, we propose a new optimization technique for electrode configuration with floating potential in VI. We applied this optimization technique to the edge configuration of center shield in VI with considering the arrangement of grounded tank. Consequently, the maximum electric field strength decreased to 70% of non-optimized shape. From the results, we verified the effectiveness of the proposed automatic numerical optimization technique for the improvement of the insulation performance in high voltage vacuum insulated equipment. ©2010 IEEE.

DOI： 10.1109/ICHVE.2010.5640872

Scopus

Other Link： http://hdl.handle.net/2237/14457

Positive Streamer Propagation and Breakdown Characteristics in Non-uniform Air Gap International conference

T. Iwata, H. Kojima, N. Hayakawa, F. Endo, N. Yanagita, T. Kato, T. Rokunohe, H. Okubo

2010 International Conference on High Voltage Engineering and Application 2010.10.12 Mississippi State University and Chongqing University

Event date： 2010.10

Language：English Presentation type：Poster presentation

Venue：New Orleans Country：United States

Discharge phenomena in a needle-plane electrode in air were studied with nano-second time resolution by means of synchronous measurement of streak image, light intensity, discharge current and fast-framing image. When positive impulse voltage was applied, a primary streamer initiated from the needle tip and propagated to the grounded plane electrode. A secondary streamer initiated after the primary streamer arrived at the plane electrode. There are close relations between the applied voltage, the primary and the secondary streamer current and the initiation time of the secondary streamer. The breakdown can start when the secondary streamer have arrived at the plane electrode. Moreover, the time to breakdown becomes shorter with the increase in the secondary streamer current.

DOI： 10.1109/ICHVE.2010.5640749

Scopus

Other Link： http://hdl.handle.net/2237/14456

Optimization of environment-friendly electric energy systems from the integrated viewpoint of diagnosis, monitoring, operation and apparatus development

M. Hanai, F. Endo, H. Kojima, N. Hayakawa, H. Okubo

2010.9.29

気中放電における1次及び2次ストリーマ進展モデルの基礎検討

柳田憲史，加藤達朗，六戸敏昭，岩田健，小島寛樹，早川直樹，遠藤奎将，大久保仁

平成22年電気学会基礎・材料・共通部門大会

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