Faculty Profiles - KOJIMA Hiroki

Presentations -

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真空中スパークコンディショニングにおけるデコンディショニング

上野楓真，小島寛樹，福田英昭，山村健太，早川直樹

令和7年電気学会全国大会 2025.3.18 電気学会

宮路仁崇，石川裕卓，大竹泰智，小島寛樹，早川直樹

電気学会誘電・絶縁材料/放電・プラズマ・パルスパワー/高電圧合同研究会 2025.1.23 電気学会誘電・絶縁材料/放電・プラズマ・パルスパワー/高電圧技術委員会

Electrode Material Dependence of Dielectric Strength Improvement in High Frequency AC Conditioning in Vacuum International conference

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

7th International Conference on Electric Power Equipment – Switching Technology 2024.11.12 Xi'an Jiaotong University, Xiamen University of Technology

Event date： 2024.11

Language：English Presentation type：Oral presentation (general)

Venue：Xiamen Country：China

Spark conditioning by repeated breakdowns (BDs) with AC voltage application is an effective method for improving dielectric strength in vacuum. During AC conditioning, the multiple BDs occur successively in a half cycle of AC voltage application. It is considered that the multiple BDs are harmful for conditioning and triggered due to the previous BD in the same half cycle. In our previous research, we have confirmed that high frequency AC conditioning suppress the multiple BDs and improves dielectric strength for the Cu electrodes. In this paper, we discuss the electrode material dependence of the dielectric strength improvement in high frequency AC conditioning in vacuum. We compare high frequency AC conditioning effect for the stainless steel (SUS) electrodes with that for the Cu electrodes. Experimental results have revealed that the high frequency AC conditioning improved the dielectric strength by 16% for the Cu electrodes, but did not almost improve for the SUS electrodes. It is considered that the harmful effects of the multiple BDs are significant for electrode materials with high electrical and thermal conductivities such as Cu, and the dielectric strength improvement by the suppression of the multiple BDs due to high frequency AC conditioning is expected to be useful for such electrode materials.

DOI： 10.1109/ICEPE-ST61894.2024.10792561

Scopus

真空沿面帯電・放電進展メカニズムおよび傾斜機能材料による三重点電界緩和 Invited

小島寛樹

第14回アドバンストSEMセミナー 2024.10.29 日立ハイテクアドバンストSEMテクノロジ特別共同研究事業

Partial Discharge Progress and Breakdown in Oil-Impregnated Paper Insulation System of Capacitor Voltage Transformers International conference

Y. Nagaki, K. Hayashi, H. Kojima, M. Yoshida, N. Hayakawa

10th International Conference on Condition Monitoring and Diagnosis 2024.10.24 The Korean Institute of Electrical Engineers

Event date： 2024.10

Language：English Presentation type：Poster presentation

Venue：Gangwon-do Country：Korea, Republic of

In this paper, towards the insulation diagnosis of capacitor voltage transformers (CVTs) based on partial discharge (PD) measurements, PD progress and breakdown (BD) characteristics in aluminum foil/oil-impregnated paper composite insulation system were discussed, especially in the transition from creeping PD to penetrating PD. PD extended in the creeping direction of oil-impregnated papers with bubbles, eroded and penetrated the 1st layer of oil-impregnated paper. The penetrating PD with large charge, long length and red light emission extended in H2-based gases. PD sequentially penetrated the laminated oil-impregnated papers and led to BD.

DOI： 10.23919/CMD62064.2024.10766131

Scopus

Partial Discharge Characteristics of Aluminum Foil/Oil-Impregnated Paper Composite Insulation System with Misaligned Aluminum Foils Invited International conference

N. Hayakawa, K. Hayashi, Y. Nagaki, H. Kojima, M. Yoshida

10th International Conference on Condition Monitoring and Diagnosis 2024.10.22 The Korean Institute of Electrical Engineers

Event date： 2024.10

Language：English Presentation type：Oral presentation (invited, special)

Venue：Gangwon-do Country：Korea, Republic of

Capacitor voltage transformers (CVTs) composed of aluminum foil/oil-impregnated paper composite insulation system have been operated over 30 years in substations. The insulation diagnosis of CVTs are expected to be based on the measurement of partial discharge (PD) leading to breakdown (BD). The misalignment of aluminum foil electrodes in the fabrication and/or operation of CVTs may cause PD. This paper discussed the PD characteristics of misaligned aluminum foil electrodes toward the insulation diagnosis of CVTs. PD progressed in the creeping direction of oil-impregnated papers within the misalignment width of aluminum foil electrode peripheries. The creeping PD eroded the surface of the 1st layer of oil-impregnated paper and penetrated the other layers, leading to BD.

DOI： 10.23919/CMD62064.2024.10766108

Scopus

Concept of Condition Monitoring for Capacitor Voltage Transformers International conference

H. Isaji, Y. Nagaki, M. Yoshida, H. Kojima, N. Hayakawa

10th International Conference on Condition Monitoring and Diagnosis 2024.10.22 The Korean Institute of Electrical Engineers

Event date： 2024.10

Language：English Presentation type：Poster presentation

Venue：Gangwon-do Country：Korea, Republic of

It is important to establish methods for detecting internal abnormalities and diagnosing the accuracy of the transformation ratio of instrument transformers. In the case of a capacitor voltage transformer (CVT) with multiple oil-impregnated paper capacitors connected in series, partial discharge (PD) occurs due to the deterioration of capacitor elements. When PD is extended, the capacitor electrode is short-circuited and the transformation ratio changes. Namely, the diagnostic of transformation ratio is effective for detecting internal abnormalities of CVT. The change in the voltage division ratio was investigated by experimentally reproducing the phenomena from the generation of PD to a short circuit in the capacitor elements. On the other hand, the effect on the transformation ratio due to the difference in the position and number of short-circuited capacitor elements in the CVT was simulated. In a digital substation, secondary voltage values of several CVTs are obtained simultaneously using intelligent electronic devices (IED). Therefore, using programmable devices supported by the IEC 61850 communication protocol, data can be acquired from IEDs and CVT condition monitoring can be achieved with simple calculations.

DOI： 10.23919/CMD62064.2024.10766186

Scopus

回転機絶縁システム適用に向けたセルロースナノファイバー（CNF）成形板の電気絶縁特性

木村啓韻，小島寛樹，早川直樹，古田尚，奥村浩史，安食厚志，山中雄太，岩永拓実，岡本徹志

第55回電気電子絶縁材料システムシンポジウム 2024.9.27 電気学会基礎・材料・共通部門誘電・絶縁材料技術委員会

電動推進航空機用モータの高出力密度化に向けた流動潤滑油の絶縁破壊特性

中澤尚輝，野口真平，小島寛樹，有賀信雄，卯辰清志，渡邉瞬也，早川直樹

第55回電気電子絶縁材料システムシンポジウム 2024.9.26 電気学会基礎・材料・共通部門誘電・絶縁材料技術委員会

多端子直流送電システムにおける超電導限流器の限流協調

西澤大将，小島寛樹，中村綾花，岩田幹正，早川直樹

電気学会電力技術/電力系統技術合同研究会 2024.9.20 電気学会電力技術/電力系統技術委員会

Improvement of Flashover Voltage by Functionally Graded Materials in Gas Insulated Power Apparatus Invited International conference

H. Kojima

2024 China-Japan Forum on The Development of Electrical Materials 2024.9.7 State Key Laboratory of Electrical Insulation and Power Equipment, Department of International Cooperation & Exchanges, Xi'an Jiaotong University