Faculty Profiles - KOJIMA Hiroki

Papers - KOJIMA Hiroki

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Resin Injection Behavior in Process of Manufacturing Insulation Spacer with Permittivity Gradient Using Computational Fluid Dynamics Reviewed International journal

T. Imai, Y. Hoshina, H. Kojima, N. Hayakawa

IEEE Transactions on Dielectrics and Electrical Insulation Vol. 26 ( 5 ) page： 1669 - 1677 2019.10

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

The advent of new technology for designing insulation materials with desired properties stimulates our interest in development of insulation spacers in gas-insulated switchgear (GIS). Tailoring of two kinds of epoxy-based composite materials (resins) with different relative permittivities enables development of insulation spacers with graded permittivity that decrease the diameter of a GIS tank. Computational fluid dynamics for the resin injection process based on the properties of resin mixtures at various mixing ratios between two resins with high/low relative permittivity enables us to estimate the injection behavior and distribution of resins in the manufacturing of insulation spacers. Simulation results demonstrated the gradient of resins in the insulation spacers by this resin injection method using two resins. In particular, the insulation spacer formed by two or four injection inlets have a more favorable distribution of resins than that achieved by one injection inlet.

DOI： 10.1109/TDEI.2019.008239

Dynamic Breakdown Characteristics of Pancake Coil Model for Resistive-Type Superconducting Fault Current Limiters Reviewed International journal

N. Hayakawa, M. Mimbu, H. Kojima, S. Isojima, M. Kuwata

IEEE Transactions on Applied Superconductivity Vol. 29 ( 5 ) page： 1 - 6 2019.8

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

Toward the reliable and rational insulation design of resistive-type superconducting fault current limiters (RSFCL), we have been investigating not only the intrinsic breakdown (BD) characteristics of liquid nitrogen (LN2) without bubbles but also the dynamic BD characteristics of LN2 under the transient bubble disturbance at the quench of RSFCL coils. We have so far used a fundamental model such as sphere-plane electrode system with quasi-uniform electric field. However, when RSFCL will be introduced in a future electric power network, a stack of pancake coils with non-uniform electric field will be adopted for the practical development of RSFCL. In this paper, we designed and fabricated a pancake coil model for RSFCL, and investigated the intrinsic and dynamic BD characteristics of the pancake coil models in LN2. Experimental results are discussed in terms of the volume effect of intrinsic and dynamic BD strength under non-uniform electric field.

DOI： 10.1109/TASC.2019.2909200

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

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

IEEJ Transactions on Fundamentals and Materials Vol. 139 ( 1 ) page： 30 - 37 2019.1

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

<p>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 in the environment of uncontrolled humidity, which is peculiar under surge-wave voltage. On the other hand in the environment of controlled humidity, the PDIV decreased monotonically as the temperature increased. We found that the PDIV increase was attributed to the effect of the relative humidity decrease corresponding to the temperature increase. Furthermore, the PDIV was also influenced by the water amount in insulation film. The PDIV during the temperature increase was lower than that during the temperature decrease with the hysteresis. The lower PDIV at the higher water amount is due to the release of water into the air gap as well as the decrease in the first ionization energy of insulation film, leading to the increase in the generation probability of an initial electron.</p>

DOI： 10.1541/ieejfms.139.30

Simulation on Discharge Inception Voltage Improvement of GIS Spacer with Permittivity Graded Materials (epsilon-FGM) Using Flexible Mixture Casting Method Reviewed International journal

N. Hayakawa, Y. Miyaji, H. Kojima, K. Kato

IEEE Transactions on Dielectrics and Electrical Insulation Vol. 25 ( 4 ) page： 1318 - 1323 2018.8

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

For electric field grading in GIS spacers, the application of functionally graded materials (FGM) with spatial distribution of dielectric permittivity (ε) can be one effective solution. In this paper, we propose the flexible mixture casting (FMC) method as a novel and practical technique to fabricate ε-FGM for GIS spacers. We verified that the relative permittivity (ε r ) can be controlled from ε r = 4 to ε r = 30 by varying the loading ratio of high permittivity filler particles under the FMC method. Toward the practical application of ε-FGM, we verified the relaxation effect of maximum electric field strength and the improvement effect of discharge inception voltage in SF 6 gas in a simplified GIS spacer model using ε-FGM under the FMC method. Simulation results revealed that the maximum electric field strength can be effectively reduced and the discharge inception voltage can be significantly improved compared with using a constant and uniform permittivity.

DOI： 10.1109/TDEI.2018.007236

Volume Effect of Dynamic Breakdown Strength in LN₂ for Insulation Design of Resistive Superconducting Fault Current Limiters Reviewed International journal

N. Hayakawa, K. Ishida, M. Mimbu, H. Kojima, S. Isojima, M. Kuwata

IEEE Transactions on Applied Superconductivity Vol. 28 ( 4 ) page： 5600804 2018.6

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

Toward the practical and rational insulation design of a resistive superconducting fault current limiter, it is necessary to consider the electrical breakdown (BD) due to the transient bubble generation in liquid nitrogen (LN2) at the quench of the HTS materials. We have been investigating the intrinsic BD characteristics of LN2 without bubbles and the dynamic BD characteristics of LN2 under transient bubble disturbance, respectively. In this paper, we have investigated the dynamic BD characteristics of LN2 for different electrode diameters and gap lengths, and have discussed in terms of the volume effect of dynamic BD strength.

DOI： 10.1109/TASC.2018.2799308

Fault Current Limitation Coordination in Electric Power Grid With Superconducting Fault Current Limiters Reviewed International journal

N. Hayakawa, Y. Maeno, H. Kojima

IEEE Transactions on Applied Superconductivity Vol. 28 ( 4 ) page： 5602304 2018.6

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

This paper proposes the concept of “fault current limitation coordination” in a future electric power grid with multiple superconducting fault current limiters (SFCL). Not only the fault current limitation of each SFCL on a transmission line, but also the transient stability of the grid with SFCLs should be taken into account, because the disturbance is brought about in the grid by the sudden appearance and disappearance of a large impedance of SFCL at the fault current limitation and recovery after the fault clearance, respectively. In this paper, both of the fault current limitation of SFCLs and the internal phase angle oscillation of generators were calculated by PSCAD/EMTDC to evaluate the operational feasibility or coordination of SFCLs in a 275/77-kV transmission system model.

DOI： 10.1109/TASC.2018.2818279

Fabrication of GIS Spacer Model with ε-FGM (Functionally Graded Materials) and Theoretical Estimation of its Breakdown Voltage in SF₆ Gas Reviewed

Y. Miyaji, H. Kojima, K. Kato, N. Hayakawa

IEEJ Transactions on Fundamentals and Materials Vol. 138 ( 4 ) page： 155 - 162 2018.4

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

<p>For the relaxation and optimization of electric field distribution around Gas insulated switchgear (GIS) spacers, the application of functionally graded materials (FGM) with spatial distribution of dielectric permittivity (ε) can be one of the effective solutions. In this paper, we fabricated the coaxial disk-type ε-FGM using SrTiO<sub>3</sub>/SiO<sub>2</sub> particles by the centrifugal force method. We also proposed the coaxial cone-type ε-FGM by the flexible mixture casting method and evaluated its electric field reduction effect and breakdown voltage in SF<sub>6</sub> gas by the volume-time theory.</p>

DOI： 10.1541/ieejfms.138.155

Improvement Mechanism of AC Breakdown Voltage of Insulating Oil with Dispersed Nanoparticles Invited International journal

H. Kojima, T. Kondo, M. Hanai, N. Hayakawa

Electronics and Communications in Japan Vol. 100 ( 12 ) page： 69 - 75 2017.12

Authorship：Lead author,　Corresponding author Language：English Publishing type：Research paper (scientific journal) Publisher：Wiley Periodicals, Inc.

We investigated the fundamental insulation characteristics of insulating oil with dispersed nanoparticles. The AC breakdown voltage of the insulating oil increased by mixing the dispersed hydrophilic nanoparticles, because the hydrophilic nanoparticles absorbed moisture in the oil, which decreased the number of weak points in the oil for breakdown. In contrast, the hydrophobic nanoparticles did not contribute to the increase of AC breakdown voltage, and decreased the AC breakdown voltage. On the other hand, in the case of overdosing of nanoparticles, the aggregation of nanoparticles caused the decrease in the AC breakdown voltage of the oil, which depended upon the volume concentration of nanoparticles regardless of the diameter of nanoparticles. The aggregation of nanoparticles was led by dielectrophoresis phenomenon, which brought the gap length dependence of the AC breakdown voltage of the insulating oil with the dispersed nanoparticles, because the gradient of electric field strength at the gap was smaller in the case of shorter gap length.

DOI： 10.1002/ecj.12016

Discharge Pattern Discrimination in Vacuum Interrupter with Non-contact Measurement of Shield Potential Reviewed International journal

F. Kong, H. Kojima, N. Hayakawa, T. Kimura, M. Tsukima

IEEE Transactions on Dielectrics and Electrical Insulation Vol. 24 ( 5 ) page： 3265 - 3269 2017.10

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

Vacuum interrupter has a composite insulation system. When an internal insulation fails, the discharge has complex and various patterns. Previously, based on the discharge development characteristics, we proposed a discrimination method for the discharge pattern in vacuum interrupter. We measured the applied voltage, anode current, and shield potential waveforms, respectively. From the analysis of the measured waveforms, we can estimate the discharge patterns and discharge path lengths. In this paper, a non-contact shield potential measurement was proposed when the shield potential cannot be measured directly. Furthermore, verification tests with this measurement method were conducted and the effectiveness of the discrimination method was confirmed.

DOI： 10.1109/TDEI.2017.006512

Optimum Breakdown Charge for Maximum Dielectric Strength in Spark Conditioning in Vacuum under a Non-uniform Electric Field Reviewed International journal

H. Kojima, T. Takahashi, N. Hayakawa, K. Hasegawa, M. Sakaki

IEEE Transactions on Dielectrics and Electrical Insulation Vol. 24 ( 4 ) page： 2660 - 2665 2017.8

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

Spark conditioning is an effective method of increasing the dielectric strength in vacuum. In this paper, we determined the breakdown (BD) charge dependence for spark conditioning in vacuum under various non-uniform electric fields and with different anode materials. We discuss the BD mechanism in terms of BD arising from anode heating or cathode heating. Experimental results revealed that for anode heating BD, the longer the gap distance, the larger the BD charge needed to obtain maximum dielectric strength. In contrast, for cathode heating BD, the BD charge for maximum dielectric strength was smaller than that for anode heating BD. From these results, we estimated the optimal BD charge to maximize the 50% BD electric field strength, i.e. the dielectric strength, for spark conditioning in vacuum.

DOI： 10.1109/TDEI.2017.006462

Discharge Pattern Discrimination for Composite Insulation System in Vacuum Invited International journal

F. Kong, H. Kojima, T. Kimura, M. Tsukima, N. Hayakawa

Electrical Engineering in Japan Vol. 200 ( 3 ) page： 3 - 12 2017.8

Language：English Publishing type：Research paper (scientific journal) Publisher：Wiley Periodicals, Inc.

In order to develop higher-voltage vacuum circuit breakers (VCBs), the investigation of discharge phenomenon in vacuum is necessary. In vacuum interrupters (VIs) with composite insulation systems, the discharge has complex and various patterns at the internal insulation failures. In this paper, based on the discharge development characteristics, we propose a discrimination method for the discharge pattern in vacuum. We measured the applied voltage, anode current, and shield potential waveforms, respectively. From the feature of the measured waveforms, we classified the discharge patterns. Furthermore, based on the relation between discharge development time and discharge path length, we estimated the discharge patterns and discharge path lengths. In addition, we conducted a verification experiment of the discrimination method using a VI model. Experimental results revealed that the estimated discharge patterns and discharge path lengths were consistent with the observed discharge ones and the effectiveness of the discrimination method was confirmed.

DOI： 10.1002/eej.22982

Breakdown Characteristics and Mechanisms of Liquid Nitrogen under Transient Thermal Stress for Superconducting Fault Current Limiters Reviewed International journal

N. Hayakawa, T. Matsuoka, H. Kojima, S. Isojima, M. Kuwata

IEEE Transactions on Applied Superconductivity Vol. 27 ( 4 ) page： 7700305 - 7700305 2017.6

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

We have been investigating the electrical breakdown (BD) characteristics of liquid nitrogen (LN2) with a transient thermal stresses to simulate the quench conditions of resistive-type superconducting fault current limiters (SFCL). We call such BD of LN2 with the transient thermal stress as “dynamic BD,” and discriminate it from that with a continuous thermal stress. In this paper, we investigated the dynamic BD characteristics of LN2 for different LN2 pressures, electrode sizes, and terms of transient thermal stress input. In addition, we obtained the BD characteristics of LN2 with the continuous thermal stress, and compared them with the dynamic BD characteristics. Furthermore, we analyzed the statistical scattering of the time from the thermal stress input to the dynamic BD occurrence and discussed the dynamic BD mechanisms of LN2. We also proposed a flowchart of rational and reliable electrical insulation design of resistive-type SFCL based on the dynamic BD characteristics of LN2.

DOI： 10.1109/TASC.2017.2651115

Electrical Insulation Characteristics and Mechanisms of Low Dielectric Loss Materials for High-Voltage HTS Cables Reviewed International journal

N. Hayakawa, K. Ishida, T. Matsuoka, H. Kojima

IEEE Transactions on Applied Superconductivity Vol. 27 ( 4 ) page： 4800904 - 4800904 2017.6

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

Toward the practical development of high-voltage high-temperature superconducting (HTS) cables, high electrical insulation characteristics, and low dielectric loss become important. Dielectric loss drastically increases with the increase of operation voltage of HTS cables. Therefore, it is highly expected to find a novel insulating material with low dielectric loss for the high-voltage HTS cables. We have focused on Tyvek®, polyethylene (PE) nonwoven fabric, and Tyvek®/PE, a syntactic paper bonding PE-sheet on Tyvek® by heat, as low dielectric loss materials. In this paper, we investigated fundamental partial discharge inception characteristics and their mechanisms of Tyvek® immersed in liquid nitrogen (LN2).In addition, we compared the electrical insulation characteristics of Tyvek®, Tyvek®/PE, and polypropylene laminated paper (PPLP®), and discussed their mechanisms.

DOI： 10.1109/TASC.2016.2645796

Calculation Model for Predicting Partial-discharge Inception Voltage in a Non-uniform Air Gap while Considering the Effect of Humidity Reviewed International journal

T. Rokunohe, T. Kato, H. Kojima, N. Hayakawa, H. Okubo

IEEE Transactions on Dielectrics and Electrical Insulation Vol. 24 ( 2 ) page： 1123 - 1130 2017.4

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

A calculation model for predicting the inception of partial discharge (PD) in a non-uniform air gap on the basis of theories of electron avalanche and streamer phenomena is proposed. "Partial discharge inception voltages" (PDIVs), under an assumed absolute humidity of less than 20g/cm3, calculated using the proposed model were compared with experimentally measured ones and found to be almost equivalent. Furthermore, the relationship between negative ions and humidity was incorporated into the calculation model, and it was confirmed that the calculated and measured PDIVs tend to be the roughly the same under a wide humidity range under various gap length between electrodes. The proposed partial-discharge inception model based on theories mentioned above was therefore judged to be effective for predicting changes in PDIV when humidity changes.

DOI： 10.1109/TDEI.2017.005920

Improvement Mechanism of AC Breakdown Voltage of Insulating Oil with Dispersed Nanoparticles Reviewed

H. Kojima, T. Kondo, M. Hanai, N. Hayakawa

IEEJ Transactions on Fundamentals and Materials Vol. 137 ( 2 ) page： 101 - 106 2017.2

Authorship：Lead author,　Corresponding author Language：Japanese Publishing type：Research paper (scientific journal) Publisher：The Institute of Electrical Engineers of Japan

<p>We investigated the fundamental insulation characteristics of insulating oil with dispersed nanoparticles. The AC breakdown voltage of the insulating oil increased by mixing the dispersed hydrophilic nanoparticles, because the hydrophilic nanoparticles absorbed moisture in the oil, which decreased the number of weak points in the oil for breakdown. In contrast, the hydrophobic nanoparticles did not contribute to the increase of AC breakdown voltage, and decreased the AC breakdown voltage. On the other hand, in the case of overdosing of nanoparticles, the aggregation of nanoparticles caused the decrease in the AC breakdown voltage of the oil, which depended upon the volume concentration of nanoparticles regardless of the diameter of nanoparticles. The aggregation of nanoparticles was led by dielectrophoresis phenomenon, which brought the gap length dependence of the AC breakdown voltage of the insulating oil with the dispersed nanoparticles, because the gradient of electric field strength at the gap was smaller in the case of shorter gap length.</p>

DOI： 10.1541/ieejfms.137.101

Reduction Method of Fluctuation in Partial Discharge Inception Voltage for Inverter-Fed Rectangular Wire Motors Invited International journal

T. Wakimoto, H. Kojima, N. Hayakawa

Electrical Engineering in Japan Vol. 198 ( 2 ) page： 3 - 10 2017.1

Language：English Publishing type：Research paper (scientific journal) Publisher：Wiley Periodicals, Inc.

We investigated the decisive factors that affect partial discharge inception voltage (PDIV) fluctuation between the coil wires under a sinusoidal voltage by using a rectangular wire motor. As a result, we found that PDIV under low humidity fluctuates by the surface charge, and PDIV under high humidity varies transitionally with the surface conductivity activated by predischarge. From these analyses, we proposed the measurement method of PDIV with a low fluctuation and transition for the rectangular wire motors, which will be optimized by environmental humidity, predischarge condition, and measuring frequency.

DOI： 10.1002/eej.22908

Measurement and Evaluation of Partial Discharge Inception Voltage for Enameled Rectangular Wires under AC Voltage Reviewed International journal

T. Wakimoto, H. Kojima, N. Hayakawa

IEEE Transactions on Dielectrics and Electrical Insulation Vol. 23 ( 6 ) page： 3566 - 3574 2016.12

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

Recently, the market for EVs (Electric vehicles) and HEVs (Hybrid electric vehicles) has been expanding. In these vehicles, rectangular wires have started to be used for the traction motor coil to increase the motor performance. As a result, the insulation techniques become important because the applied voltage between the wires is increasing due to a change in the motor structure and the increase in the driving voltage. In order to safeguard the motor insulation, partial discharge (PD) between coil wires must be controlled, and to do this, the coil wires should be coated with an insulator of a suitable thickness. For this purpose, the PD measurement technique is important in design and inspection on the manufacturing lines. However, the PD measurement between wires is unstable, and is especially affected by the measurement method and environmental factors. In this paper, we clarify the factors as well as the mechanism affecting the PD inception voltage (PDIV) between enameled rectangular wires under AC voltage. The effects of the ambient humidity, the pre-discharge conditions and the measuring frequency on the PDIV were analyzed. As a result, it was confirmed that when there was pre-discharge in a high humidity environment, the PDIV displayed fluctuating characteristics in which it momentarily decreased and then increased. We observed the discharges and analyzed the electric field distribution. The results showed that in high humidity environment, pre-discharge causes moisture contained in the air to adhere to the coating surfaces, resulting in an increase in the surface conductivity, which influences the electric field strength in the air gap between coil wires. These analyses indicated that the PDIV can be measured stably and reliably at high frequencies in a high humidity environment.

DOI： 10.1109/TDEI.2016.005645

Dependence of Spark Conditioning on Breakdown Charge and Electrode Material under a Non-Uniform Electric Field in Vacuum Reviewed International journal

H. Kojima, T. Takahashi, N. Hayakawa, K. Hasegawa, H. Saito, M. Sakaki

IEEE Transactions on Dielectrics and Electrical Insulation Vol. 23 ( 5 ) page： 3224 - 3230 2016.10

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

Spark conditioning is an effective method for improving dielectric strength in vacuum. In this paper, we discuss the dependence of spark conditioning on breakdown (BD) charge and electrode material in vacuum under various non-uniform electric field and circuit parameters. We applied a negative impulse voltage to a rod-plane electrode configuration with a cathode made of stainless steel (SS), Cu-Cr, or OFHC Cu (Cu), and an anode made of SS or Cu. We changed the gap distance, the tip radius of the rod electrode and the circuit parameters. As a result, the conditioning effect, i.e. improving BD electric field strength by the conditioning, depended on the BD charge (QBD) and reached its maximum value against QBD. The optimal QBD required to achieve the maximum conditioning effect decreased with the cathode melting point. Moreover, when BD caused by cathode heating (CH-BD) occurred readily, with the SS anode, the BD electric field strength at the cathode surface after conditioning depended on QBD, regardless of the gap distance and the tip radius. Thus, for CH-BD, we estimated the withstand voltage that could be achieved by spark conditioning from the electrode configuration and QBD. However, when BD caused by anode heating (AH-BD) occurred easily with the Cu anode, the optimal QBD was larger than that for CH-BD because the cathode temperature was lower for AH-BD than for CH-BD.

DOI： 10.1109/TDEI.2016.7736889

Discharge Pattern Discrimination for Composite Insulation System in Vacuum Reviewed

F. Kong, H. Kojima, T. Kimura, M. Tsukima, N. Hayakawa

IEEJ Transactions on Fundamentals and Materials Vol. 136 ( 9 ) page： 594 - 602 2016.9

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

In order to develop higher-voltage vacuum circuit breakers (VCB), the investigation of discharge phenomenon in vacuum is necessary. In vacuum interrupters (VI) with composite insulation systems, the discharge has complex and various patterns at the internal insulation failures. In this paper, based on the discharge development characteristics, we propose a discrimination method for the discharge pattern in vacuum. We measured the applied voltage, anode current, and shield potential waveforms, respectively. From the feature of the measured waveforms, we classified the discharge patterns. Furthermore, based on the relation between discharge development time and discharge path length, we estimated the discharge patterns and discharge path lengths. In addition, we conducted a verification experiment of the discrimination method using a VI model. Experimental results revealed that the estimated discharge patterns and discharge path lengths were consistent with the observed discharge ones and the effectiveness of the discrimination method was confirmed.

DOI： 10.1541/ieejfms.136.594

Discharge Characteristics for Various Discharge Patterns under Negative Lightning Impulse Voltage in Vacuum Reviewed International journal

F. Kong, Y. Nakano, H. Kojima, N. Hayakawa, T. Kimura, M. Tsukima

IEEE Transactions on Dielectrics and Electrical Insulation Vol. 23 ( 2 ) page： 813 - 816 2016.4

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

In order to develop higher voltage vacuum interrupters (VIs), electrical insulation performances in vacuum should be improved. The discharge in VI has various and complex patterns, such as breakdown in vacuum gap and surface flashover on a solid insulator. In this paper, we investigated three discharge patterns: the discharge in vacuum gap, the discharge in vacuum gap via shield, and the surface flashover on solid insulator, respectively. By measuring shield potential and insulator surface potential, we analyzed the discharge characteristics for various patterns, and clarified the relevant discharge development process. We found that the breakdown development process in vacuum gap depends on the gap length and the surface flashover development process on a solid insulator depends on the applied voltage and surface distance.

DOI： 10.1109/TDEI.2015.005388

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