Faculty Profiles - KOJIMA Hiroki

Papers - KOJIMA Hiroki

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Surface Charge Accumulation and Partial Discharge Activity for Small Gaps of Electrode/Epoxy Interface in SF₆ gas Reviewed International journal

D. Mansour, H. Kojima, N. Hayakawa, F. Endo, H. Okubo

IEEE Transactions on Dielectrics and Electrical Insulation Vol. 16 ( 4 ) page： 1150 - 1157 2009.8

Language：English Publishing type：Research paper (scientific journal) Publisher：IEEE

The electrical insulation reliability of solid spacers in gas insulated switchgears (GISs) is an important issue to achieve a safe operation of such equipment. Among different phenomena, charge accumulation represents the most important matter that can degrade the overall performance of these insulation systems. For this respect, this paper discusses the contribution of partial discharge (PD) activity by ac voltage application to charge accumulation in the small gap at the electrode/epoxy interface as one of the weakest points in GIS solid spacers. The partial discharge inception voltages for non-accumulated charge case (PDIV0) and after exposing to PD activity (PDIVn) are measured among different gap lengths, simulating delamination at the electrode/epoxy interface. The PD activity is generated using applied voltage with 1.2Ã PDIV0 for all gap lengths examined in this study (50~500 Â¿m). In these measurements, PDIV increased with increasing the number of PD pulses as a result of accumulated charges. The accumulated surface charge density is estimated using the boundary equations and is compared for the different gap lengths. The accumulated charge density was larger for the smaller gap lengths. Comparing PD parameters with accumulated charge density enabled us to identify that the number of negative and positive PD pulses is the main parameter that corresponds to charge accumulation process.

DOI： 10.1109/TDEI.2009.5211869

Current Limiting Characteristics of Parallel-connected Coated Conductors for High-Tc Superconducting Fault Current Limiting Transformer (HTc-SFCLT) Reviewed International coauthorship International journal

K. Omura, H. Kojima, N. Hayakawa, F. Endo, M. Noe, H. Okubo

IEEE Transactions on Applied Superconductivity Vol. 19 ( 3 ) page： 1880 - 1883 2009.6

Language：English Publishing type：Research paper (scientific journal) Publisher：IEEE

We have been developing superconducting fault current limiting transformer (SFCLT) as a superconducting transformer with current limiting function. As the step-5 of the SFCLT project for the up-graded ratings of current and voltage, in this paper, we focused on the up-grading of current by parallel-connected YBCO coated conductors. Based on the asymmetric configuration of coated conductors, different parallel connections were tested and evaluated in terms of critical current level and current limiting characteristics under large AC current at 77 K in liquid nitrogen.

DOI： 10.1109/TASC.2009.2018067

Dynamic Thermal Characteristics of HTS Coil for Conduction-cooled SMES Reviewed International journal

H. Kojima, X. Chen, N. Hayakawa, F. Endo, H. Okubo

IEEE Transactions on Applied Superconductivity Vol. 19 ( 3 ) page： 2036 - 2039 2009.6

Authorship：Lead author,　Corresponding author Language：English Publishing type：Research paper (scientific journal) Publisher：IEEE

In this paper, we investigate the thermal runaway characteristics of a BSCCO HTS single pancake coil conduction-cooled with a 4 K-GM cryo-cooler system. We also evaluate the static and dynamic characteristics of the HTS coil by measuring the temporal evolution of the temperature and the voltage for steady current and the typical current patterns of the load fluctuation compensation at different ambient temperatures T0 in the range of 20-50 K. Moreover, we experimentally discuss the criterion for monitoring the conduction-cooled HTS SMES system during compensating operation for load fluctuations.

DOI： 10.1109/TASC.2009.2019238

Other Link： https://cir.nii.ac.jp/crid/1050845763730339840

Partial Discharge Activity in Electrical Insulation for High Temperature Superconducting (HTS) Cables Reviewed International journal

H. Okubo, H. Kojima, F. Endo, K. Sahara, R. Yamaguchi, N. Hayakawa

IEEE Transactions on Dielectrics and Electrical Insulation Vol. 15 ( 3 ) page： 647 - 654 2008.6

Language：English Publishing type：Research paper (scientific journal) Publisher：IEEE

This paper discusses electrical insulation characteristics of liquid nitrogen (LN2) / polypropylene (PP) laminated paper composite insulation system for the practical electrical insulation design of high temperature superconducting (HTS) cables. Partial discharge (PD) inception, propagation and breakdown (BD) characteristics under ac voltage application were obtained and discussed at atmospheric and pressurized conditions. Experimental results were analyzed quantitatively and revealed that voidtype discharge was categorized as an important form of discharge to characterize the transition of PD activity leading to BD. © 2008 IEEE.

DOI： 10.1109/TDEI.2008.4543100

Quench-Induced Partial Discharge Characteristics of HTS Cables Reviewed International journal

N. Hayakawa, S. Ueyama, H. Kojima, F. Endo, Y. Ashibe, T. Masuda, M. Hirose

Journal of Physics: Conference Series Vol. 97 ( 1 ) page： 012053 2008.3

Language：English Publishing type：Research paper (scientific journal) Publisher：IOP Publishing

This paper discusses the partial discharge (PD) characteristics of HTS cables under quench condition, where the thermal stress due to quench would be transiently superposed on the electric field stress under the operating voltage of HTS cables. Such electrical insulation characteristics under thermal/electrical combined stress can be referred to as the "quench-induced PD characteristics" of HTS cables, which should be taken into account for the reliable insulation design of HTS cables. Experimental results using coaxial HTS cable samples revealed that the quench-induced PD could be generated, even under the intrinsic or static PD inception voltage (PDIVsta) without the quench. The criteria of the thermal/electrical combined stress level for quench-induced PD inception were quantified for different LN2 pressures and fault durations under sub-cooled condition. © 2008 IOP Publishing Ltd.

DOI： 10.1088/1742-6596/97/1/012053

Self-recovery Characteristics of High-Tc Superconducting Fault Current Limiting Transformer (HTc-SFCLT) with 2G Coated Conductors Reviewed International coauthorship International journal

H. Kojima, S. Itoh, N. Hayakawa, F. Endo, M. Noe, H. Okubo

Journal of Physics: Conference Series Vol. 97 ( 1 ) page： 012154 2008.3

Authorship：Lead author,　Corresponding author Language：English Publishing type：Research paper (scientific journal) Publisher：IOP Publishing

We have been developing Superconducting Fault Current Limiting Transformer (SFCLT) with both functions of superconducting transformer and superconducting fault current limiter. As the Step-IV of the SFCLT project, in this paper, we designed, fabricated and tested HTc-SFCLT with 2G YBCO coated conductors to be operated in liquid nitrogen at 77 K. The ratings of the HTc-SFCLT with 2G coated conductors are 3-phase, 100 kVA, 6600 V / 210 V. Especially, the possibility of self-recovery into superconducting state after the fault current limitation was investigated by the simple clearance of the fault, which is very important as a superconducting transformer and for power system operation. The criteria of self-recovery were quantified for different combination of load current before the fault and prospective fault current during the fault. © 2008 IOP Publishing Ltd.

DOI： 10.1088/1742-6596/97/1/012154

Breakdown Characteristics of N₂O Gas Mixtures for Quasi-uniform Electric Field under Lightning Impulse Voltage Reviewed International journal

H. Kojima, O. Kinoshita, N. Hayakawa, F. Endo, H. Okubo, M. Yoshida, T. Ogawa

IEEE Transactions on Dielectrics and Electrical Insulation Vol. 14 ( 6 ) page： 1492 - 1497 2007.12

Authorship：Lead author,　Corresponding author Language：English Publishing type：Research paper (scientific journal) Publisher：IEEE

From the viewpoint of mitigating global warming by SF6 gas, this paper discusses breakdown (BD) characteristics of different electronegative gas mixtures with N2O gas as SF6 gas substitutes for quasi-uniform electric field under lightning impulse voltage applications. Experimental results revealed the positive synergism in breakdown strength of binary N2O / CO2 and ternary N2O / CO 2 / O2 gas mixtures, respectively. Furthermore, N 2 gas as a retardant gas was also mixed with the electronegative gas mixtures in order to reduce the electron energy into the effective levels of electron attachment ability by the electronegative gas mixtures. As the result, ternary N2O /CO2 / N2 and quaternary N 2O / CO2 / O2 / N2 gas mixtures could exhibit the significant synergistic effect in breakdown strength. The optimum mixture rate of quaternary N2O / CO2 / O 2 / N2 gas mixtures was consistent with that estimated by assuming the independent contribution of component gases to the improvement of impulse BD characteristics. © 2007 IEEE.

DOI： 10.1109/TDEI.2007.4401233

Electrical Insulation Characteristics of HTS Cables under Quench-Induced Thermal Stress Condition Reviewed International journal

N. Hayakawa, S. Ueyama, H. Kojima, F. Endo, T. Masuda, M. Hirose

IEEE Transactions on Applied Superconductivity Vol. 17 ( 2 ) page： 1660 - 1663 2007.6

Language：English Publishing type：Research paper (scientific journal) Publisher：IEEE

This paper discusses the electrical insulation characteristics of high temperature superconducting (HTS) cable under quench condition exposed to thermal and electrical combined stress. We applied a high voltage of 60 Hz ac to the sample electrode, and injected a large current of 60 Hz ac into Bi2223/Ag tapes to induce quench. Experimental results revealed that partial discharges (PD) were induced by the transient thermal stress due to quench under the high voltage condition. A criterion of the combined stress for PD inception was experimentally obtained. © 2007 IEEE.

DOI： 10.1109/TASC.2007.899968

Thermal Runaway Characteristics of Bi2212 Coil for Conduction-cooled SMES Reviewed International journal

H. Kojima, N. Hayakawa, S. Noguchi, F. Endo, N. Hirano, S. Nagaya, H. Okubo

IEEE Transactions on Applied Superconductivity Vol. 17 ( 2 ) page： 1959 - 1962 2007.6

Authorship：Lead author,　Corresponding author Language：English Publishing type：Research paper (scientific journal) Publisher：IEEE

In this paper, we investigated the thermal runaway characteristics of a conduction-cooled Bi2212 HTS coil with a 4 K-GM cryocooler system under compensating operation for load fluctuations. We measured the temporal evolutions of temperature and voltage distributions of the HTS solenoidal coil for typical current patterns of load fluctuation compensation at different ambient temperatures. On the basis of these experimental results, we constructed a thermal analysis model, and analysed the thermal runaway characteristics during and after the load fluctuation compensation. Based on the results of our experiments and the numerical simulations, we proposed criterion for the monitoring of an HTS SMES system during compensating operation for load fluctuations.

DOI： 10.1109/TASC.2007.898492

High-Tc Superconducting Fault Current Limiting Transformer (HTc-SFCLT) with 2G Coated Conductors Reviewed International coauthorship International journal

H. Okubo, C. Kurupakorn, S. Ito, H. Kojima, N. Hayakawa, F. Endo, M. Noe

IEEE Transactions on Applied Superconductivity Vol. 17 ( 2 ) page： 1768 - 1771 2007.6

Language：English Publishing type：Research paper (scientific journal) Publisher：IEEE

We developed a 3-phase, 100 kVA, 6600 V/210 V high temperature Superconducting Fault Current Limiting Transformer (HTc-SFCLT) with functions of both superconducting transformer and fault current limiter. The HTc-SFCLT is characterized by the application of 2G coated conductors with the higher current limiting performance and flexibility than that of the HTc-SFCLT with 1G Bi2212/CuNi composite bulk material. Fundamental tests of the HTc-SFCLT were carried out, and the design parameters as a superconducting transformer and as a superconducting fault current limiter were verified. © 2007 IEEE.

DOI： 10.1109/TASC.2007.897813

Synergy Effect in Electrical Insulation Characteristics of N₂O Gas Mixtures Reviewed

N. Hayakawa, O. Kinoshita, H. Kojima, F. Endo, M. Yoshida, T. Ogawa, H. Okubo

IEEJ Transactions on Power and Energy Vol. 126 ( 11 ) page： 1164 - 1170 2006.11

Language：Japanese Publishing type：Research paper (scientific journal) Publisher：The Institute of Electrical Engineers of Japan

This paper discusses partial discharge (PD) and breakdown (BD) characteristics of N<sub>2</sub>O gas and N<sub>2</sub>O gas mixtures as SF<sub>6</sub> substitutes as a function of gas pressure and N<sub>2</sub>O content for quasi-uniform and non-uniform electric field under lightning impulse and ac voltage application. Experimental results revealed the significant positive synergism in breakdown strength of N<sub>2</sub>O30%/CO<sub>2</sub>70% and N<sub>2</sub>O30%/CO<sub>2</sub>50%/O<sub>2</sub>20% gas mixtures under quasi-uniform electric field. Furthermore, corona stabilization effect in N<sub>2</sub>O30%/CO<sub>2</sub>70% gas mixtures under non-uniform electric field was discussed and suggested to be activated due to the combination of electronegative N<sub>2</sub>O and CO<sub>2</sub> gases.

DOI： 10.1541/ieejpes.126.1164

Simulation of Electrical and Thermal Behavior of High Temperature Superconducting Fault Current Limiting Transformer (HTc-SFCLT) Reviewed International coauthorship International journal

C. Kurupakorn, H. Kojima, N. Hayakawa, F. Endo, N. Kashima, S. Nagaya, M. Noe, H. Okubo

Journal of Physics: Conference Series Vol. 43 page： 950 - 953 2006.7

Language：English Publishing type：Research paper (scientific journal) Publisher：IOP Publishing

Superconducting Fault Current Limiting Transformer (SFCLT) is expected to perform functions both of transformer in the normal operating condition and of fault current limiter in the system fault condition. As the Phase-3 of the SFCLT project, we have been developing SFCLT based on Bi2212/CuNi bulk coils at LN2 temperature and verified its technical feasibility. In this paper, we developed a numerical model for evaluation of the electrical and thermal behavior of HTc-SFCLT such as current limitation and recovery characteristics. This model took into account E-J characteristics of Bi2212/CuNi bulk coil and its electrical and thermal transient phenomena during the operation of HTc-SFCLT. The simulated current agreed well with the experimental data with the error of less than 5%. The excellent current limitation and self recovery characteristics obtained by the experiments could also be reproduced. With the numerical model, current and thermal behavior of HTc-SFCLT was simulated for different parameters of conductor configuration, which would be useful for the future design and optimization of HTc-SFCLT. © 2006 IOP Publishing Ltd.

DOI： 10.1088/1742-6596/43/1/232

Electrical and Thermal Characteristics of Bi2212/Ag HTS Coils for Conduction-cooled SMES Reviewed International journal

N. Hayakawa, S. Noguchi, C. Kurupakorn, H. Kojima, F. Endo, N. Hirano, S. Nagaya, H. Okubo

Journal of Physics: Conference Series Vol. 43 page： 813 - 816 2006.7

Language：English Publishing type：Research paper (scientific journal) Publisher：IOP Publishing

In this paper, we investigated the electrical and thermal performance of conduction-cooled Bi2212/Ag HTS coils with 4K-GM cryocooler system. First, we measured the critical current Ic for different ambient temperatures T0 at 4.2 K - 40 K. Experimental results revealed that Ic increased with the decrease in T0 and was saturated at T0 < 10 K. We carried out thermal analysis considering heat generation, conduction and transfer under conduction-cooling condition, and reproduced the electrical and thermal characteristics of the conduction-cooled HTS coil, taking account of temperature dependence of specific heat and thermal conductivity of the materials. We also measured the temperature rise of Bi2212/Ag HTS coil for different continuous current levels at T0 4.8 K. Experimental results revealed the criterion of thermal runaway, which was discussed in terms of heat generation and propagation in the test coil. © 2006 IOP Publishing Ltd.

DOI： 10.1088/1742-6596/43/1/198

Impulse and AC Partial Discharge Inception and Breakdown Characteristics of LN₂/Polypropylene Laminated Paper Composite Insulation System Reviewed International journal

H. Okubo, H. Kojima, F. Endo, K. Sahara, N. Hayakawa

IEEE Transactions on Dielectrics and Electrical Insulation Vol. 13 ( 3 ) page： 664 - 669 2006.6

Language：English Publishing type：Research paper (scientific journal) Publisher：IEEE

Partial discharge (PD) inception and breakdown (BD) characteristics of liquid nitrogen (LN2) / polypropylene (PP) laminated paper composite insulation system for high temperature superconducting (HTS) cables are presented. In this paper, we measured PD inception characteristics under lightning impulse and ac voltage applications by electrical, optical and acoustic methods. The impulse PD inception characteristics were compared with those under ac voltage application, and the PD generation mechanisms are discussed. Experimental results reveal that PD detection sensitivity under lightning impulse voltage application is as small as 6 pC. PD inception at the lower stress was verified to be generated not only in the butt gaps but also in micro gaps between PP laminated paper layers under both lightning impulse and ac voltage applications. After PD inception, PD activity could be developed in the butt gaps at the higher stress, finally leading to BD. © 2006 IEEE.

DOI： 10.1109/TDEI.2006.1657982

Temperature Dependence of Critical Current at 4.2 K-55 K of Conduction-cooled Bi2212/Ag Wires for SMES Reviewed International journal

H. Kojima, S. Noguchi, C. Kurupakorn, N. Hayakawa, M. Goto, N. Hirano, S. Nagaya, H. Okubo

IEEE Transactions on Applied Superconductivity Vol. 15 ( 2 ) page： 2550 - 2553 2005.6

Authorship：Lead author,　Corresponding author Language：English Publishing type：Research paper (scientific journal) Publisher：IEEE

In this paper, fundamental characteristics of conduction-cooled Bi2212/Ag wires and coils for HTS SMES were investigated, in terms of critical current at different temperatures at 4.2 K-55 K. Experimental results revealed that the critical current did not always increase with the temperature decrease, but was saturated or even decreased, depending on the test sample condition. Thermal analysis considering temperature dependence of specific heat and thermal conductivity of the materials could explain the critical current-temperature characteristics, and suggest the possibility to optimize the operating temperature of SMES. © 2005 IEEE.

DOI： 10.1109/TASC.2005.847646

Dielectric Characteristics of HTS Cables Based on Partial Discharge Measurement Reviewed International journal

N. Hayakawa, M. Nagino, H. Kojima, M. Goto, T. Takahashi, K. Yasuda, H. Okubo

IEEE Transactions on Applied Superconductivity Vol. 15 ( 2 ) page： 1802 - 1805 2005.6

Language：English Publishing type：Research paper (scientific journal) Publisher：IEEE

We have discussed dielectric characteristics of liquid nitrogen (LN 2)/polypropylene (PP) laminated paper composite insulation system for the practical electrical insulation design of high temperature superconducting (HTS) cables. Focusing on the partial discharge (PD) inception characteristics and mechanism, the volume effect and its saturation on PD inception electric field strength (PDIE) was evaluated at atmospheric and pressurized condition. PD inception characteristics under lightning impulse voltage application were also investigated using an electrical and optical PD measuring system, and compared with those under ac voltage application. © 2005 IEEE.

DOI： 10.1109/TASC.2005.849292

Recovery Characteristics after Current Limitation of High Temperature Superconducting Fault Current Limiting Transformer (HTc-SFCLT) Reviewed International coauthorship International journal

C. Kurupakorn, H. Kojima, N. Hayakawa, M. Goto, N. Kashima, S. Nagaya, M. Noe, K.-P. Juengst, H. Okubo

IEEE Transactions on Applied Superconductivity Vol. 15 ( 2 ) page： 1859 - 1862 2005.6

Language：English Publishing type：Research paper (scientific journal) Publisher：IEEE

Superconducting Fault Current Limiting Transformer (SFCLT) is one of the promising superconducting power apparatus, since it has both functions of superconducting transformer in normal condition and superconducting fault current limiter in fault condition. In this paper, current limiting and the recovery tests of high temperature SFCLT (HTc-SFCLT) with MCP-BSCCO2212 bulk coils were performed at liquid nitrogen temperature. Experimental results revealed that the HTc-SFCLT could successfully recover into the superconducting state by itself immediately after the fault clearance. Moreover, the recovery criterion of HTc-SFCLT as a function of load current was clarified. © 2005 IEEE.

DOI： 10.1109/TASC.2005.849314

Formation of Deep Potential Well and its Structural Transition in Small Tokamak Device International journal

Y. Fukuzawa, H. Kojima, T. Okada, N. Ohno, S. Takamura

Europhysics Conference Abstracts Vol. 28G page： P-5.075 2004.7

小型トカマク放電の高機能化に関する研究 Reviewed

小島寛樹

名古屋大学大学院工学研究科博士学位論文 ( 甲第6265号 ) 2004.3

Formation and Dynamics of Very Deep Negative Potential Well in the Small Tokamak Device CSTN-IV Reviewed International journal

H. Kojima, Y. Fukuzawa, T. Manabe, S. Takamura, T. Okada, N. Ohno

Czechoslovak Journal of Physics Vol. 53 ( 10 ) page： 895 - 902 2003.10

Authorship：Lead author,　Corresponding author Language：English Publishing type：Research paper (scientific journal) Publisher：Springer Nature

We achieved a potential well more than fifty times deeper than the plasma electron temperature, down to -0.3 kV with the electrode biasing, by the cross-field radial arcing current of up to 150 A which flows between the small electron-emitting electrode (LaB6)inserted to the plasma center and the stainless-steel wall of vacuum chamber. The empirical scaling of the radial resistance of plasma Rr on the radial arcing current Ib and the toroidal magnetic field BT is found to be Rr ∼ Ib-1.2 BT1.0. Structural dynamics of negative potential well formation is observed: the potential dip propagates from the plasma core to the periphery. The ion saturation current also increases, starting from the inner region. The fluctuation frequency in I sat changes abruptly from 200 kHz to 30 kHz in a time of around 0.25 ms after negative biasing.

DOI： 10.1023/A:1026392406717

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