Faculty Profiles - KOJIMA Hiroki

We have clarified that the primary and secondary streamers have a significant impact on breakdown mechanism in air. In this paper, we focused on the highly-conductive channel progress breakdown, and considered the initiation condition of highly-conductive channel from the viewpoint of the secondary streamer initiation. We revealed that the highly-conductive channel initiation voltage in the case with the grounded electrode covered by a dielectric barrier was higher than that without the barrier. We suggested that the electric field relaxation effect of the residual positive space charges formed by the primary streamer is greater than that by the secondary streamer. The highly-conductive channel is initiated from the region where the electric field relaxation is weak.

DOI： 10.1109/CEIDP.2014.6995797

Scopus

Other Link： https://www.webofscience.com/wos/woscc/full-record/WOS:000352763100026

Electric Field Control in Coaxial Disk-Type Solid Insulator by Functionally Graded Materials (FGM) International conference

J. Ishiguro, M. Kurimoto, H. Kojima, K. Kato, H. Okubo, N. Hayakawa

2014 IEEE Conference on Electrical Insulation and Dielectric Phenomena 2014.10.22 IEEE Dielectrics and Electrical Insulation Society

Event date： 2014.10

Language：English Presentation type：Oral presentation (general)

Venue：Des Moines Country：United States

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.

DOI： 10.1109/CEIDP.2014.6995799

Scopus

Other Link： https://www.webofscience.com/wos/woscc/full-record/WOS:000352763100129

Conditioning Development in Vacuum Based on Breakdown Probability Distribution International conference

H. Kojima, Y. Otake, M. Sakaki, H. Saito, K. Hasegawa, N. Hayakawa

2014 International Symposium on Discharges and Electrical Insulation in Vacuum 2014.9.29 Bhabha Atomic Research Center and Indian Vacuum Society

Event date： 2014.9 - 2014.10

Language：English Presentation type：Oral presentation (general)

Venue：Mumbai Country：India

Spark conditioning is important for the enhancement of insulation strength in vacuum. In order to enhance the insulation performance of vacuum circuit breakers, we have to clarify the conditioning effect and improve a conditioning procedure. In this paper, we discuss the conditioning process of Cu and stainless steel rod electrodes based on breakdown (BD) probability distribution. We applied a negative impulse voltage to a rod-plane electrode configuration. We adopted two procedures for the voltage application, i.e. up-down and 3 times methods. For the Cu rod with the up-down method, BD voltage increased by the conditioning effect, however, the minimum BD voltage did not increase or even decreased due to the damage by BD. A moderate conditioning procedure, e.g. 3 times method, could increase the minimum BD voltage as well as the stabilization of insulation strength. The Weibull analysis of the development of a BD probability distribution has enabled us to evaluate the damaging effect. Based on these results, we can discuss a suitable conditioning procedure.

DOI： 10.1109/DEIV.2014.6961607

Scopus

Other Link： https://www.webofscience.com/wos/woscc/full-record/WOS:000369858100005

Influence of Heat Treatment in Atmosphere for Surface Flashover Characteristics on Alumina in Vacuum International conference

N. Asari, T. Shioiri, J. Sato, K. Matsuo, H. Kojima, N. Hayakawa

2014 International Symposium on Discharges and Electrical Insulation in Vacuum 2014.9.30 Bhabha Atomic Research Center and Indian Vacuum Society

Event date： 2014.9 - 2014.10

Language：English Presentation type：Poster presentation

Venue：Mumbai Country：India

Alumina Ceramics are widely used for vacuum insulation in devices such as vacuum interrupters. In development and design of vacuum interrupters, it is very important to improve the flashover characteristics on alumina ceramic surfaces in a vacuum. Thus, it is necessary to clarify the flashover mechanism on alumina ceramics in a vacuum. From past research, it is well-known that these characteristics are influenced by the insulation material, its surface condition, surface charging of the insulator and so on.(1) This paper presents the results of measurements of surface flashover characteristics and the cathode luminescence (C.L.) spectra after heat treatment in the atmosphere. These results are compared with those without heat treatment in the atmosphere. It is thus found that oxygen defects on an alumina ceramic surface decreases and the flashover voltage is improved by heat treatment in the atmosphere.

DOI： 10.1109/DEIV.2014.6961626

Scopus

Other Link： https://www.webofscience.com/wos/woscc/full-record/WOS:000369858100024

Transient Charging for Impulse Surface Flashover Development in Vacuum International conference

Y. Nakano, H. Kojima, K. Tsuchiya, N. Hayakawa

2014 International Symposium on Discharges and Electrical Insulation in Vacuum 2014.9.29 Bhabha Atomic Research Center and Indian Vacuum Society

Event date： 2014.9 - 2014.10

Language：English Presentation type：Oral presentation (general)

Venue：Mumbai Country：India

Surface discharge on solid insulator in vacuum is one of the significant issues in the development of vacuum circuit breakers and vacuum interrupters. By measuring induced current in electrostatic probes behind alumina dielectrics due to explosive electron emission (EEE) at surface flashover inception, we studied transiently formed charge characteristics on the alumina dielectrics in the development process of surface flashover. From experimental results, we found that EEE affects the flashover development through two effects of the transient charge: one is the suppression of the EEE current by the relaxation of the electric field below the cathode, the other is promotion of the secondary electron emission avalanche. In the latter effects, because of negative charge on the alumina dielectrics below the cathode, the secondary electron can fly longer along the alumina surface. As a result, the current channel on the alumina dielectrics is formed more easily.

DOI： 10.1109/DEIV.2014.6961618

Scopus

Other Link： https://www.webofscience.com/wos/woscc/full-record/WOS:000369858100016

Planning Maintenance Strategy for Power Apparatus and T&D System with Intelligent Grid Management System (IGMS) International conference

H. Kojima, K. Fukaya, M. Hanai, N. Hayakawa, H. Okubo

International Conference on Condition Monitoring and Diagnosis 2014