Faculty Profiles - KOJIMA Hiroki

Presentations -

Division display 61 - 80 of about 805 ／ All the affair displays >>

Flashover Voltage of Permittivity Graded Materials (ε-FGM) Spacer with Metallic Particles in SF₆ Gas International conference

N. Hayakawa, H. Kato, T. Kato, H. Kojima, S. Kamohara, Y. Hoshina

23rd International Symposium on High Voltage Engineering 2023.8.29 The Department of Electronic and Electrical Engineering of the University of Strathclyde

Event date： 2023.8 - 2023.9

Language：English Presentation type：Poster presentation

Venue：Glasgow Country：United Kingdom

Functionally graded materials with spatial distribution of permittivity (ε-FGM) are promising for the compact, efficient and environmentally benign GIS spacers. In this paper, for the practical insulation design and installation of ε-FGM spacers to GIS, the risk of metallic particles on ε-FGM spacers is investigated. Experimental results using the fabricated cone-type ε-FGM spacers revealed that the metallic particles in permittivity-grading area on convex surface were the most harmful and reduced the flashover voltage in SF6 gas by about 40 %, compared with that without the particles. The flashover voltage was almost equal to that of the conventional spacers with a uniform permittivity distribution and the metallic particles in the maximum electric field area on concave surface. Thus, the risk of metallic particles on ε-FGM spacers can be almost the same as that of the conventional spacers.

DOI： 10.1049/icp.2024.0618

Scopus

Non-destructive Measurement of Permittivity Distribution in Permittivity Graded Materials (ε-FGM) by X-ray Computed Tomography International conference

N. Hayakawa, T. Kato, H. Kojima, H. Yanase, K. Okamoto

23rd International Symposium on High Voltage Engineering 2023.8.29 The Department of Electronic and Electrical Engineering of the University of Strathclyde

Event date： 2023.8 - 2023.9

Language：English Presentation type：Oral presentation (general)

Venue：Glasgow Country：United Kingdom

Toward the compact gas insulated switchgears (GIS), we have been investigating a novel electric field grading technique around the GIS spacer by “functionally graded materials (FGM)” with spatial distribution of permittivity (ε-FGM). We have already designed and fabricated the cone-type ε-FGM spacer samples with the relative permittivity (εr) of 10.0-4.0 for an actual 245 kV-class GIS as well as those with εr of 12.7-4.0 for its 1/4-sized model and revealed the improvement of flashover voltage in SF6 gas through experiments and calculations. It is important to develop a non-destructive method to measure and verify the permittivity distribution inside the spacer to realize the practical application of ε-FGM spacers. In this paper, in order to establish the non-destructive measurement method for ε-FGM spacers with the relative permittivity distribution, ZH focused on X-ray computed tomography (CT). We obtained the relationship between the permittivity and the X-ray transmission intensity by analysing different disc-shaped samples (or =12.2-4.2) and verified the measurement possibility of the relative permittivity distribution inside ε-FGM spacer by comparison to the target distribution at the fabrication of ε-FGM spacer.

DOI： 10.1049/icp.2024.0517

Scopus

多端子直流送電システムの超電導限流器動作時における液体窒素の動的絶縁破壊電界の推定

西澤大将，小島寛樹，早川直樹

令和五年度電気・電子・情報関係学会東海支部連合大会 2023.8.28 電気学会東海支部，電子情報通信学会東海支部，情報処理学会東海支部，照明学会東海支部，映像情報メディア学会東海支部，日本音響学会東海支部，IEEE名古屋支部

真空中コンディショニングにおけるギャップ長と最適放電電荷量の関係

北直樹，小島寛樹，福田英昭，山村健太，早川直樹

令和五年度電気・電子・情報関係学会東海支部連合大会 2023.8.29 電気学会東海支部，電子情報通信学会東海支部，情報処理学会東海支部，照明学会東海支部，映像情報メディア学会東海支部，日本音響学会東海支部，IEEE名古屋支部

真空中高周波交流コンディショニングにおける多重絶縁破壊の時間間隔の印加電圧依存性

小名木良太，小島寛樹，飯塚伸介，小林将人，早川直樹

真空中沿面放電の初期進展過程における絶縁物表面への電界入射方向と2次電子なだれの関係

杉浦拓真，小島寛樹，早川直樹

電動推進航空機用モータ絶縁に適用する潤滑油の絶縁破壊特性の流量依存性

中澤尚輝，小島寛樹，有賀信雄，卯辰清志，渡邉瞬也，早川直樹

ε/σ-FGM（誘電率/導電率傾斜機能材料）による直流GIS用絶縁スペーサのフラッシュオーバ電圧向上効果

杉本遼次，小島寛樹，早川直樹

アルミ箔/油浸紙複合絶縁系における部分放電特性のアルミ箔電極ズレ幅依存性

林恭平，永木雄也，小島寛樹，吉田昌展，早川直樹

Electrode Material Adhesion between Anode and Cathode in Spark Conditioning in Vacuum International conference

N. Kita, H. Kojima, H. Fukuda, K. Yamamura, N. Hayakawa

30th International Symposium on Discharge and Electrical Insulation in Vacuum 2023.6.26 Technical Committee of Electrical Dicharges, Plasma and Pulsed Power, IEEJ

Event date： 2023.6

Language：English Presentation type：Oral presentation (general)

Venue：Okinawa Country：Japan

We discuss the electrode material adhesion between cathode and anode during spark conditioning process in vacuum, and its effect on dielectric strength in terms of breakdown (BD) charge. We focus on the conditioning effects under rod-plane electrodes with different combination of electrode materials. The material of the rod (cathode) is Cu and that of the plane (anode) is stainless steel (SUS), and their gap length is 1 mm. We observe the electrode surface before and after conditioning experiments. As a result, when BD charge is smaller than 2.0 mC, the anode material (SUS) is found on the cathode (Cu) surface and the dielectric strength becomes high. With increasing BD charge, the adhesion ratio of anode material on the cathode surface decreases, and the dielectric strength also tends to decrease. In the case where BD charge is larger than 2.0 mC, on the contrary, the cathode material (Cu) adheres to the anode (SUS) surface and the dielectric strength is low, which is consistent with the conditioning characteristics when both the cathode and anode are made of Cu. These results indicate that up to a certain BD charge, the anode material adheres to the cathode surface, and above this BD charge the cathode material adheres to the anode surface. When the cathode (Cu) surface is excessively melted, i.e., damaged, the cathode material Cu can adhere to the anode SUS surface. In other words, too large BD charge can not only fails to improve the dielectric strength due to the adhesion of the anode material SUS to the cathode Cu surface, but also causes a critical decrease in the dielectric strength, or deconditioning.

DOI： 10.23919/ISDEIV55268.2023.10201112

Scopus

CiNii Research

Multiple Breakdown Induced by Previous Breakdown in Higher Frequency AC Conditioning International conference

R. Konagi, H. Kojima, S. Iitsuka, M. Kobayashi, N. Hayakawa

30th International Symposium on Discharge and Electrical Insulation in Vacuum 2023.6.29 Technical Committee of Electrical Dicharges, Plasma and Pulsed Power, IEEJ

Event date： 2023.6

Language：English Presentation type：Poster presentation

Venue：Okinawa Country：Japan

We have confirmed that the multiple breakdowns (BDs) occur in a half cycle of AC voltage application in the AC conditioning process in vacuum. In previous research, it has been experimentally confirmed that the higher frequency AC conditioning suppresses the multiple BDs and improves the conditioning effect. In this paper, we discuss how the multiple BDs are induced. By the BD light emission images during AC conditioning, we found that the multiple BDs occurred in close to the previous BD position. This suggests that the multiple BDs were induced by the previous BD. Two possible causes of the multiple BDs are investigated: (a) metal vapor generated by the previous BD and (b) metal particles generated by the previous BD. The diffusion time of metal vapor is much shorter than the time interval for the multiple BDs. Therefore, the multiple BDs may not be induced by metal vapor. On the other hand, for particles of 10 μm order in diameter, the time of collision to the counter electrode matches the time interval of the multiple BDs. Particles of the order of 10 μm in diameter were found on the electrode surface after AC conditioning. These results suggest that the multiple BDs were initiated by metal particles generated by the previous BD and do not contribute to protrusion melting, but damage the electrode surface. Therefore, the higher frequency AC conditioning to suppress the multiple or harmful BDs can be promising for AC conditioning process in vacuum.

DOI： 10.23919/ISDEIV55268.2023.10201218

Scopus

CiNii Research

Applicability of Lubricating Oil Insulation in Higher Power Density Motors for Electric Powered Aircraft International conference

N. Hayakawa, H. Kamada, H. Kojima, N. Aruga, K. Utatsu, S. Watanabe

22nd IEEE International Conference on Dielectric Liquids 2023.6.27 IEEE DEIS

Event date： 2023.6

Language：English Presentation type：Oral presentation (general)

Venue：Worcester Country：United States

Higher power density motors for electric powered aircraft require higher voltage. The authors aim to apply a lubricating oil used for cooling motor windings to an insulating material to improve the dielectric strength of motors. In motor insulation, it is required to prevent the occurrence of partial discharge (PD) and understand the insulating characteristics of the lubricating oil. In this paper, we first measured and compared the breakdown voltage (BDV) of the lubricating oil and mineral oil in a fundamental sphere-sphere electrode system, and discussed the basic breakdown characteristics. The results suggest that the lubricating oil has superior dielectric characteristics to the mineral oil. Next, PD inception voltage (PDIV) of an arrow pair of enamel-coated rectangular wires in the lubricating oil was measured and compared with the estimated PDIV by electric field analysis. As a result, the measured PDIV was generally consistent with the estimated PDIV. Therefore, the lubricating oil insulation is expected to improve the dielectric strength in the motor windings for electric powered aircraft.

DOI： 10.1109/ICDL59152.2023.10209296

Scopus

CiNii Research

多端子直流送電システムの線路故障時における超電導限流器の限流効果および過渡安定度向上効果

村上雅紀，西澤大将，小島寛樹，早川直樹

令和5年電気学会全国大会 2023.3.17 電気学会