Faculty Profiles - KOJIMA Hiroki

For the downsizing of gas insulated power apparatus, we have proposed the electric field grading technique by 'functionally graded materials' with spatial distribution of permittivity (\varepsilon -FGM). In this paper, we fabricated post-type and truncated cone-type \varepsilon -FGM by a novel and practical technique 'flexible mixture casting method' (FMC method). We confirmed that \varepsilon -FGM by FMC method has the discharge inception voltage improvement effect by both experiment and simulation.

DOI： 10.1109/CEIDP.2018.8544770

Scopus

Combined Effect of Temperature and Humidity of Magnet-Wires on Partial Discharge Inception Voltage under Inverter-Surge Voltage International conference

T. Kaji, H. Asai, H. Kojima, N. Hayakawa

2018 IEEE Conference on Electrical Insulation and Dielectric Phenomena 2018.10.24 IEEE DEIS

Event date： 2018.10

Language：English Presentation type：Poster presentation

Venue：Cancun Country：Mexico

We investigated the combined effect of temperature and humidity on partial discharge inception voltage (PDIV) of magnet wires under repetitive surge-wave voltages. As the results, the PDIV increased as the temperature increased under uncontrolled humidity, which is peculiar under surge-wave voltage. On the other hand, the PDIV decreased lineally as the temperature increased under controlled humidity. From these results, we found that the PDIV increase under uncontrolled humidity was attributed to the effect of the relative humidity decrease corresponding to the temperature increase. Furthermore, the PDIV was also influenced by the moisture amount in insulation film. Under uncontrolled humidity, the PDIV during the temperature increase was lower than that during the temperature decrease with the hysteresis. We found two reasons for the PDIV hysteresis. One reason is the humidity increase in the air gap due to moisture evaporation from the film. Another reason is the electron emission from the film with the lower first ionization energy. Based on the above reasons, the generation probability of initial electrons during the temperature increase is higher than that during the temperature decrease, which decreases PDIV.

DOI： 10.1109/CEIDP.2018.8544745

Scopus

Estimation of Partial Discharge Inception Voltage under Repetitive Inverter Surge Voltage by Volume-Time Theory International conference

N. Hayakawa, Y. Daicho, T. Kaji, H. Kojima

2018 IEEE Conference on Electrical Insulation and Dielectric Phenomena 2018.10.24 IEEE DEIS

Event date： 2018.10

Language：English Presentation type：Poster presentation

Venue：Cancun Country：Mexico

We measured the repetition frequency dependence of partial discharge inception voltage (PDIV) of twisted-pair test samples with different specifications under repetitive surge voltages with different waveforms. We also extended the PDIV estimation method with Volume-Time theory under single shot of surge voltage to that under repetitive surge voltage, in consideration of the effect of the repetition frequency on the initial electrons generation probability. The estimated and measured PDIV agreed well under repetitive surge voltage.

DOI： 10.1109/CEIDP.2018.8544814

Scopus

Electric Field Grading by Functionally Graded Materials (FGM) for HVDC Gas Insulated Power Apparatus International coauthorship International conference

N. Hayakawa, R. Oishi, H. Kojima, K. Kato, N. Zebouchi

2018 IEEE Conference on Electrical Insulation and Dielectric Phenomena 2018.10.23 IEEE DEIS

Event date： 2018.10

Language：English Presentation type：Oral presentation (general)

Venue：Cancun Country：Mexico

Functionally graded materials (FGM) have been investigated as innovative field grading and control techniques for gas insulated power apparatus such as gas insulated switchgears (GIS) and gas insulated transmission lines (GIL). We have so far verified that FGM with the spatial permittivity distribution can control the electric field distribution around GIS spacers under AC and impulse voltage stresses. In this paper, we proposed the new application of FGM for HVDC power apparatus (DC-FGM) and investigated the technical feasibility of the electric field control by DC-FGM. The calculation results revealed that DC-FGM with the spatial conductivity distribution in the spacer bulk can control the electric field distribution around the spacer in consideration of the temperature distribution under DC voltage stress.

DOI： 10.1109/CEIDP.2018.8544834

Scopus

Effect of Initial Electrode Surface and Breakdown Charge on Spark Conditioning in Vacuum under Non-Uniform Electric Field International conference

H. Kojima, M. Noda, H. Fukuda, M. Sakaki, N. Hayakawa

28th International Symposium on Discharges and Electrical Insulation in Vacuum 2018.9.25 Leibniz Institute for Plasma Science and Technology and University of Greifswald – Institute of Physics

Event date： 2018.9

Language：English Presentation type：Poster presentation

Venue：Greifswald Country：Germany

We discuss the effect of initial cathode surface and breakdown (BD) charge on spark conditioning in a rod-plane electrode system, especially with suppressed BD charge. In the case of Cu rod cathode vs. stainless steel plane anode with a large BD charge, the saturated 50% BD field strength at the cathode tip was independent of the initial cathode surface condition, because the large BD charge led to sufficiently large melting of the cathode surface. However, in the case with suppressed BD charge not enough to remove the large weak points, the saturated BD field strength depended on the initial cathode surface condition. On the other hand, in the case of stainless steel rod cathode vs. stainless steel plane anode, the saturated BD field strength was independent of the initial cathode surface condition. In this case, the anode heating would mainly initiate BD. Since the melting of anode was the starting point of BD, the saturated BD field strength would not depend on the initial cathode surface condition regardless of BD charge.

DOI： 10.1109/DEIV.2018.8537140

Scopus

Mechanism behind Increasing Electrostatic Charging Tendency in Aged Mineral-Oil-Immersed Pressboards International conference

H. Morooka, A. Yamagishi, K. Kawamura, H. Kojima, N. Hayakawa

2018 International Conference on Condition Monitoring and Diagnosis 2018.9.25 Curtin University

Event date： 2018.9

Language：English Presentation type：Oral presentation (general)

Venue：Perth Country：Australia

The surface chemical properties of aged pressboards (PBs) were evaluated to investigate the mechanism behind the increasing tendency of electrostatic charges for diagnosing power transformers. The PBs were taken from several aged transformers, and their surface elemental composition and chemical structures were measured by X-ray photoelectron spectroscopy. The PBs properties were measured after being washed by an organic solvent to remove impregnated mineral oil. Carbon (C) and oxygen (O) atoms were mostly observed on virgin PBs surfaces; these are the atoms that compose cellulose polymer, the main component of kraft pulp. In contrast, sulfur (S) and calcium (Ca) atoms were observed on the surfaces of aged PBs in addition to the C and O atoms. Moreover, the surface densities of the Ca atoms had a good correlation with those of the S atoms. While we detected no S atoms inside the aged PBs, we observed almost the same amount of Ca atoms as in the virgin PBs. Mineral oils mainly consist of hydrocarbons, with very small amounts of components such as sulfur compounds derived from crude oil. Heat stress degrades these sulfur compounds. On the other hand, PBs made of kraft pulp contain a very small amount of metal ions, such as Ca ions. We conclude that S atoms observed on aged PB surfaces are a part of the aged sulfur compounds adsorbed from the mineral oil. The adsorption of aged sulfur compounds induced the migration of Ca ions from the inside of the PBs to the surface, which caused the surface density of the Ca ions to increase. This suggests that the accumulation of Ca ions along with the sulfur compounds led to the increasing tendency of electrostatic charges in aged PBs.

DOI： 10.1109/CMD.2018.8535704

Scopus

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