Faculty Profiles - KOJIMA Hiroki

22nd International Symposium on High Voltage Engineering 2021.11.23 Xi'an Jiaotong University, State Key Laboratory of Electrical Insulation and Power Equipment, China Electrotechnical Society, Engineering Dielectrics Technical Committee, China Society for Electrical Engineering, Chinese National Committee of CIGRE, High Voltage Technical Committee and The International Council on Large Electric Systems

Event date： 2021.11

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

Venue：Xi'an Country：China

Toward the compact gas insulated switchgears (GIS) and transmission lines (GIL), 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 distribution between 10 and 4 for 245 kV class GIS and obtained the higher flashover voltage (FOV) than that of the spacer samples with uniform permittivity. In this paper, in order to verify the estimated FOV of the ε-FGM spacer samples, we estimated the theoretical discharge inception voltage (TDIV50) by the analytical approach with the extended Volume-Time theory in consideration with the conductor surface roughness. The estimated TDIV50 of the ε-FGM spacer sample was verified to be consistent with the measured FOV and higher by 28 % in SF6 gas at 0.6 MPa-abs than that of the conventional spacer sample.

DOI： 10.1049/icp.2022.0260

Scopus

Inverse Analysis of Optimum Permittivity Distribution for FGM Spacer in Consideration with Multiple Objective Functions in Gaseous Insulation System International conference

K. Kato, H. Kojima, N. Hayakawa, H. Masui, H. Yanase, K. Okamoto, H. Okubo

22nd International Symposium on High Voltage Engineering 2021.11.24 Xi'an Jiaotong University, State Key Laboratory of Electrical Insulation and Power Equipment, China Electrotechnical Society, Engineering Dielectrics Technical Committee, China Society for Electrical Engineering, Chinese National Committee of CIGRE, High Voltage Technical Committee and The International Council on Large Electric Systems

Event date： 2021.11

Language：English Presentation type：Poster presentation

Venue：Xi'an Country：China

For relaxation of electric field (E-field) stress around solid insulation spacer in HV-GIS (Gas Insulated Switchgears), we have proved the effectiveness of an application of functionally graded materials (FGM) with spatial distribution of permittivity (ε-FGM). For this purpose, we have proposed a technique for inverse analysis of the permittivity distribution in an ε-FGM spacer. The technique can provide us optimum permittivity distribution with the maximum E-field stress relaxation with less computation time. In this paper, we developed the new technique of the inverse analysis under the consideration of multiple objective functions on the insulation performances. In our study, we focus on three kinds of functions: (a) dielectric strength in SF6 gas gap, (b) dielectric strength on the spacer surface, and (c) dielectric strength on the embedded electrode/epoxy spacer interface. Under the consideration of three objective functions, the optimum permittivity distribution in ε-FGM spacer are modified full-automatically. Through the application results to GIS spacer, we obtain the inverse analysis of optimum permittivity distribution by only 11 times of iterations. As a result, we obtained 25% improvement of insulation performance around ε-FGM spacer compared with the conventional spacer with uniform permittivity distribution.

DOI： 10.1049/icp.2022.0245

Scopus

Time Transition of Gas Conductivity and Charge Behavior in SF6-Epoxy Composite Insulation System Under DC Partial Discharge International conference

R. Nakane, H. Kojima, N. Hayakawa

Event date： 2021.11

Language：English Presentation type：Poster presentation

Venue：Xi'an Country：China

The charge accumulation at the interface between gas medium and solids in DC gas insulated switchgear (DC-GIS) and DC gas insulated line (DC-GIL) is crucial in their electrical insulation design and operation. In this paper, the charging process on solid insulator surface under DC partial discharge (DC-PD) in SF6 gas is reported. A needle-epoxy plane electrode system is installed in SF6 gas at atmospheric pressure. The measured surface potential distribution on epoxy plate is compared with the simulation considering charge behavior in SF6 gas such as ion generation due to DC-PD, drift and diffusion. As a result, we quantitatively clarify the charge accumulation process under DC-PD as the main source of charge carrier in terms of the activated conductivity distribution in SF6 gas. The charging process under DC-PD is attributed to the time transition of activated conductivity distribution in SF6 gas gap, which is approximately 1010 times higher than that in non-activated conductivity space. This means that the activated conductivity distribution in SF6 gas is the crucial parameter that determines the charging process for DC-GIS/GIL under DC-PD occurrence.

DOI： 10.1049/icp.2022.0474

Scopus

Application of SiC-filled Permittivity and Conductivity Graded Material (ε/σ-FGM) in HVDC GIS Spacer International coauthorship International conference

Rachmawati, H. Kojima, K. Kato, N. Zebouchi, N. Hayakawa

Event date： 2021.11

Language：English Presentation type：Oral presentation (general)

Venue：Xi'an Country：China

The size reduction of HVDC GIS is needed for reduction of SF6 gas use. The challenge concerning the insulating material in HVDC GIS is to reduce the high electric field strength in gas around the solid insulators (spacers). Our previous works have shown the effectiveness of the functionally graded material (FGM) with graded permittivity (ε) and conductivity (σ) in controlling the high electric field strength under various DC operating conditions. However, ε/σ distributions were independent of each other. For practical implementation of FGM to HVDC GIS, the flexible mixture casting (FMC) method so far allows us to create a material whose ε/σ distributions are interdependent. In this paper, based on the measured dependence between ε and σ in SiC-filled epoxy composites, the electric field relaxation effect is evaluated under three different ε/σ distributions (U-type, grading-to-higher conductivity (GHC)-type and grading-to-lower conductivity (GLC)-type) applied to a 320 kV HVDC GIS spacer model. The results show that GLC-type ε/σ-FGM has effectively and constantly reduced the maximum electric field strength under the time-varying voltage application from DC-on to DC-steady state and against temperature differences between the high voltage and ground sides up to 70 K.

DOI： 10.1049/icp.2022.0264

Scopus

Discrimination of Discharge Pattern in Cut Model of Vacuum Interrupter International conference

H. Kojima, T. Donen, T. Kimura, K. Kokura, N. Hayakawa

29th International Symposium on Discharges and Electrical Insulation in Vacuum 2021.9.28 Consorzio RFX

Event date： 2021.9

Language：English Presentation type：Oral presentation (general)

Venue：Padova Country：Italy

Electrical insulation of a vacuum interrupter (VI) is a composite system composed of vacuum, metal, and solid insulator. Detection of weak points in insulation is important for insulation design of VI. However, solid insulators of VI are mostly made of opaque ceramics, and it is very difficult to directly visually observe discharge sites and patterns inside VI. Previously, we have proposed a discrimination method of discharge patterns in VI by extracting features of applied voltage, discharge current, and shield potential change. In this study, we aim to verify that our proposing method can be applied to an actual structure of VI. We prepared a cut model of VI, whose shield electrode and alumina ceramic insulators have been partly cut for observing the discharge site. In this VI cut model, we simulated the weak points in insulation by additional needle electrodes. We applied a negative standard lightning impulse voltage between contact electrodes, and measured the applied voltage, the shield potential, and the anode current. In various settings, we almost succeeded to discriminate the initiation pattern of discharge, even in the case of complex discharge pattern with gap and/or surface discharge. In this verification, we found the new discharge pattern, in which the first gap discharge induced another gap discharge due to the diffusion of particles from the first discharge. However, by modifying the discrimination method based on the above physical mechanism of new discharge pattern, we got the prospects that the method can be applied to actual VI structure.

DOI： 10.1109/ISDEIV46977.2021.9586961

Scopus

Anode Material Adhesion to Cathode during Spark Conditioning Process in Vacuum International conference

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

29th International Symposium on Discharges and Electrical Insulation in Vacuum 2021.9.27 Consorzio RFX

Event date： 2021.9

Language：English Presentation type：Poster presentation

Venue：Padova Country：Italy

Spark conditioning is an effective method to improve electrical insulation strength of vacuum circuit breakers. In the conditioning process, defects on cathode surface are removed by repetitive breakdowns (BDs). However, we have elucidated that at the same time, micro-protrusions can be generated on the cathode surface by the adhesion of melted particles of anode material. In this paper, we discuss the relationship between the anode material adhesion to cathode and the conditioning saturation from an experimental approach. We set rod-plane electrodes made of Cu and stainless steel (SUS) in a vacuum chamber. We conduct spark conditioning by repetitive impulse voltages for several pairs of Cu and SUS electrodes. To one pair, the voltage is applied until the conditioning saturation. To the others, the smaller number of voltages before conditioning saturation is applied. After that, we observe the surface of rod cathodes with a digital microscope. As a result, we observe protrusion-like adhesion of the anode material to the cathode surface after conditioning saturation. On the other hand, we confirm coat-like adhesion of the anode material to the cathode surface before conditioning saturation. From these results, it is considered that coat-like adhesion mainly occur up to the saturation voltage, and then protrusion-like adhesion occur due to BD at the saturation voltage level. In other words, the generation of micro-protrusions can be a dominant factor of conditioning saturation process in vacuum.

DOI： 10.1109/ISDEIV46977.2021.9586907

Scopus

Dielectric Strength Improvement by Higher Frequency AC Conditioning International conference

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

29th International Symposium on Discharges and Electrical Insulation in Vacuum 2021.9.28 Consorzio RFX

Event date： 2021.9

Language：English Presentation type：Oral presentation (general)

Venue：Padova Country：Italy

We discuss the effects of AC conditioning with higher frequency on the dielectric strength in vacuum. We have confirmed that multiple breakdowns occur in a half cycle of AC voltage application in the AC conditioning process. It is considered that the multiple breakdowns are triggered by metal vapor due to the previous breakdown in the same half cycle. Such breakdowns can not be attributed to a micro protrusion on the electrode surface. Therefore, multiple breakdowns are considered not effective, but harmful for conditioning. In this paper, we carried out the AC conditioning with the higher frequency than the commercial power frequency in the sphere-plane electrode system. We experimentally confirmed that the conditioning effect is improved by increasing the frequency of AC conditioning in the sphere-plane electrode system, where multiple or harmful breakdowns were suppressed during AC conditioning. These results suggest the feasibility of higher frequency AC conditioning for the dielectric strength improvement in vacuum.

DOI： 10.1109/ISDEIV46977.2021.9586929

Scopus

アルミ箔/油浸紙複合絶縁系における部分放電開始・進展・絶縁破壊特性

脇田周太，永木雄也，小島寛樹，吉田昌展，早川直樹

第52回電気電子絶縁材料システムシンポジウム 2021.9.14 電気学会誘電・絶縁材料技術委員会

Theoretical Investigation on Discharge Inception Voltage Improvement of SiC-filled Permittivity and Conductivity Graded Material (ε/σ-FGM) for HVDC GIS International coauthorship

Rachmawati, H. Kojima, K. Kato, N. Zebouchi, N. Hayakawa

52nd Symposium on Electrical and Electronic Insulating Materials and Applications in Systems 2021.9.14 IEEJ Technical Committee on Dielectrics and Electrical Insulation

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