Faculty Profiles - KOJIMA Hiroki

Papers - KOJIMA Hiroki

Division display >> ／ All the affair displays 1 - 119 of about 119

Improvement of AC Conditioning Effect in Vacuum by High Frequency Voltage Reviewed International journal Open Access

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

IEEJ Transactions on Electrical and Electronic Engineering Vol. 19 ( 11 ) page： 1781 - 1787 2024.11

Language：English Publishing type：Research paper (scientific journal) Publisher：John Wiley & Sons

To improve the dielectric strength in vacuum, spark conditioning through repetitive breakdowns by AC or impulse voltage is an effective method. AC conditioning at commercial power frequencies has been widely used due to its cost-effectiveness, but the breakdown voltage after AC conditioning is generally lower compared to impulse conditioning. We have confirmed that multiple breakdowns occur successively in a half cycle of AC voltage application. It is considered that the multiple breakdowns are harmful for conditioning and triggered due to the previous breakdown in the same half cycle. In this paper, we discuss dielectric strength improvement by high frequency AC conditioning and how the multiple breakdowns are induced. We experimentally confirmed that the AC conditioning effect is improved by increasing the frequency of AC voltage for different applied AC conditioning voltages in a sphere-plane electrode system. We focused on the breakdown position from the light emission images during AC conditioning to investigate whether the multiple breakdowns were induced or affected by the previous breakdown. We found that the multiple breakdowns occurred close to the previous breakdown position. The multiple breakdowns would be induced by the collision of metal particles due to the previous breakdown with the counter electrode. These breakdowns do not contribute to removing protrusion and would lead to damage to the electrode surface. The increasing frequency of AC conditioning would suppress the multiple breakdowns and improve the dielectric strength in vacuum. © 2024 The Author(s). IEEJ Transactions on Electrical and Electronic Engineering published by Institute of Electrical Engineers of Japan and Wiley Periodicals LLC.

DOI： 10.1002/tee.24139

Other Link： https://onlinelibrary.wiley.com/doi/full/10.1002/tee.24139

Effective High Frequency AC Conditioning in Vacuum Reviewed International journal

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

IEEE Transactions on Dielectrics and Electrical Insulation 2024

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

To improve the dielectric strength in vacuum, spark conditioning through repetitive breakdowns by ac voltage application is an effective method. High frequency ac conditioning is expected for the higher dielectric strength after conditioning, but the optimized frequency and other conditions for the effective ac conditioning have not been clarified. In this paper, applied ac voltage frequency and electrode material were investigated as parameters for the dielectric strength improvement by ac conditioning. High frequency ac conditioning was conducted for the stainless steel (SUS) electrodes and compared it with that for the Cu electrodes. Experimental results revealed that the high frequency ac conditioning improved the dielectric strength by 13% at 550 Hz for the Cu electrodes, but did not almost improve for the SUS electrodes. For the electrode materials with high electrical conductivity, such as Cu, harmful effects of the unnecessary or multiple breakdowns during conventional or low frequency ac conditioning would be relatively large. Thus, the high frequency ac conditioning can be effective for the electrode materials with high electrical conductivity due to the suppression of the multiple breakdowns. Secondly, impulse conditioning with shot-by-shot polarity reversal, i.e. no unnecessary or multiple breakdowns, was carried out in order to simulate the same condition as the optimized frequency of ac conditioning. As a result, the impulse breakdown voltage after impulse conditioning was similar to that after 550 Hz ac conditioning. This result suggests that high frequency ac conditioning can be sufficiently effective by increasing the ac voltage frequency from 60 Hz to 550 Hz.

DOI： 10.1109/TDEI.2024.3501993

Partial Discharge Progress and Breakdown Characteristics of Oil-impregnated Paper Insulation System for Capacitor Voltage Transformers Reviewed International journal

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

IEEE Transactions on Dielectrics and Electrical Insulation 2024

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

Capacitor voltage transformers (CVTs) with aluminum foil/oil-impregnated papers composite insulation system have been operated over 30 years in substations. An insulation diagnosis of CVT is expected to be based on detecting partial discharge (PD). However, PD characteristics from PD inception to breakdown (BD) have not been clarified yet. In an insulation diagnosis of CVT, it is necessary to diagnose a risk level of BD based on PD characteristics. In this paper, we investigated the PD characteristics using electrical and optical measurement on the fundamental insulation model of CVT and discussed the PD progress characteristics leading to BD. Experimental results revealed that PD, which was generated from the aluminum foil electrode periphery, progressed from the creeping direction to the penetrating direction of the oil-impregnated papers, and then BD occurred. When PD penetrated the oil-impregnated paper, PD charge increased, and hydrogen-based gas was generated. This PD progress can be an important factor to establish insulation diagnosis based on PD measurement of CVT.

DOI： 10.1109/TDEI.2024.3501994

Breakdown Charge Dependence of Counter Electrode Material Adhesion and Dielectric Strength after Spark Conditioning in Vacuum Reviewed International journal

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

IEEE Transactions on Dielectrics and Electrical Insulation Vol. 30 ( 6 ) page： 2592 - 2598 2023.12

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 discuss the breakdown (BD) charge dependence of counter electrode material adhesion and dielectric strength after spark conditioning. We have applied a negative impulse voltage to a rod-plane electrode configuration with a cathode and an anode made of stainless steel (SUS) that is defined as SUS304 by JIS G 4303 or oxygen free high conductivity copper (Cu). The electrode system of cathode SUS and anode Cu is called SUS-Cu electrode system, and the electrode system of cathode Cu and anode SUS is called Cu-SUS electrode system. We have changed BD charge (QBD) in conditioning by changing the limiting resistance in the experimental circuit. As a result, in both cases of SUS-Cu and Cu-SUS electrode systems, the larger the QBD, the smaller the amount of the anode material adhered to the cathode surface. When QBD is large, the dielectric strength increases in SUS-Cu electrode system and decreases in Cu-SUS electrode system. From these results, when QBD is large, we consider that the conditioning characteristics are less likely to be dominated by the anode material, i.e. Cu in the case of SUS-Cu electrode system and SUS in the case of Cu-SUS electrode system. In other words, the decisive parameters such as QBD to maximize the dielectric strength may depend on the combination of electrode materials.

DOI： 10.1109/TDEI.2023.3318887

Other Link： http://hdl.handle.net/2237/0002010894

Expected Lifetime Evaluation Method and Partial Discharge Progress Characteristics in Capacitor Voltage Transformer Reviewed

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

IEEJ Transactions on Power and Energy Vol. 143 ( 10 ) page： 559 - 566 2023.10

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

Aged CVTs have been often used in substations, but the expected lifetime of CVTs is unclear, so that it is difficult to determine the appropriate renewal time. Therefore, in this paper, we focus on oil-impregnated paper capacitors inside CVTs and propose a concept to evaluate their expected lifetime based on the partial discharge (PD) progress characteristics. By using a fundamental oil-impregnated paper capacitor model, the PD progress characteristics were measured and discussed from electrical and optical aspects.

DOI： 10.1541/ieejpes.143.559

Electric Field Grading and Discharge Inception Voltage Improvement on HVDC GIS/GIL Spacer with Permittivity and Conductivity Graded Materials (ε/σ-FGM) Reviewed International coauthorship International journal

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

IEEE Transactions on Dielectrics and Electrical Insulation Vol. 29 ( 5 ) page： 1811 - 1817 2022.10

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

Functionally graded materials (FGM) application with graded permittivity and conductivity is promising as an effective technique for electric field relaxation in SF<sub>6</sub> gas around HVDC GIS/GIL spacers. In order to approach the practical application of FGM to HVDC GIS/GIL spacers, this paper investigates the electric field reduction effect given by permittivity and conductivity graded materials (ε/σ-FGM) based on actual measured permittivity (ε) characteristics of SrTiO<sub>3</sub>-filled epoxy composites and conductivity (σ) characteristics of SiC-filled epoxy composites. In addition, theoretical discharge inception voltage (TDIV) of ε/σ-FGM spacer is calculated under standard lightning impulse (LI) voltage based on the Volume-Time theory. The results show that ε/σ-FGM spacer with grading to lower permittivity (GLP) (ε<sub>r</sub> from 12.7 to 4) containing 0 to 26.9 vol% SrTiO<sub>3</sub>-filled epoxy composite and U-shaped graded conductivity containing 5 to 10 vol% SiC-filled epoxy composite is effective for electric field relaxation under DC steady state (DC-SS) and LI voltage where both resistive and capacitive fields present. It is attributed to the higher ε and σ of FGM spacer near the HV conductor/spacer interface. TDIV under LI voltage is also estimated to be 26% higher at 0.5 MPa-abs, compared to the conventional spacer without ε/σ-grading.

DOI： 10.1109/TDEI.2022.3194489

Other Link： http://hdl.handle.net/2237/0002004749

Melting and Generation of Micro-Protrusions on Cathode Surface during Spark Conditioning Process in Vacuum Reviewed International journal

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

IEEE Transactions on Dielectrics and Electrical Insulation Vol. 29 ( 3 ) page： 1160 - 1166 2022.6

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

Spark conditioning is an effective method to increase the dielectric strength in vacuum by melting and removing micro-protrusions on the cathode surface. We aim to optimize the conditioning effect in vacuum for the application of vacuum circuit breakers (VCBs) to higher voltage levels. To obtain high breakdown (BD) voltage, the control of BD charge in the conditioning process is required. When a BD occurs, we have suggested that micro-protrusions can be generated on the cathode surface by particles from the anode. For the suitable control of BD charge, the meltability of generated micro-protrusions and their size are important. In this paper, we focus on the generation of micro-protrusions on the cathode surface in conditioning process. First, we conduct spark conditioning by repetitive impulse voltages of rod-plane electrodes made of OFHC Cu in a vacuum chamber with different BD charge. After that, we observe the surface of rod cathode with a digital microscope in order to count the number of visible protrusions and measure their radii. By considering the meltability of micro-protrusions, we quantitatively reveal the existence of residual protrusions that cannot be completely melted by BD. Next, we estimate the maximum size of generated micro-protrusions by BD charge. In consideration of the collision of ions into the anode accelerated by the anode sheath voltage, we estimate the meltable amount of anode material by the BD charge. We confirm that the radii of experimentally measured protrusions are close to the estimated ones. Moreover, we consider the difference in dielectric strength due to the difference in the initial surface condition of the electrodes. We clarify that the dielectric strength is improved by applying acid wash and heat treatment to the electrodes before conditioning, which can be optimized by the BD charge and the initial surface condition of the electrode.

DOI： 10.1109/TDEI.2022.3173493

Other Link： http://hdl.handle.net/2237/0002004226

Volume Effect of Dynamic Breakdown Strength in Liquid Nitrogen under Transient Bubble Disturbance for Resistive Superconducting Fault Current Limiters Reviewed International journal

N. Hayakawa, Y. Mori, H. Kojima

IEEE Transactions on Dielectrics and Electrical Insulation Vol. 29 ( 1 ) page： 264 - 269 2022.2

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

Liquid nitrogen (LN 2 ) is indispensable as the cooling and electrical insulating materials for high-temperature superconducting (HTS) power apparatus. Especially for the resistive superconducting fault current limiters (RSFCLs), a transient bubble disturbance in LN 2 is generated due to the quench of HTS materials, which may induce an electrical breakdown in LN 2 even under the rated voltage of RSFCL. We have been investigating not only the intrinsic breakdown characteristics of LN 2 under the steady-state operation, but also the dynamic breakdown characteristics of LN 2 under the fault current limiting operation of RSFCL. In this article, we investigate the heat flux and pressure dependence of the quench-induced dynamic breakdown characteristics of LN 2 by using an actual HTS conductor and a pancake coil model for RSFCL. In addition, the dynamic breakdown strength is discussed and systematized in terms of the volume effect of breakdown strength in LN 2 . A flowchart to estimate the intrinsic and dynamic breakdown strength in LN 2 is proposed toward the reliable and rational insulation design of RSFCL.

DOI： 10.1109/TDEI.2022.3146564

Other Link： http://hdl.handle.net/2237/0002003092

Breakdown Characteristics of Cone-type ε-FGM Spacer for Gas Insulated Switchgears Invited Reviewed

Y. Miyazaki, H. Kojima, H. Masui, H. Mitsudome, H. Yanase, K. Okamoto, K. Watanabe, K. Kato, N. Hayakawa

IEEJ Transactions on Fundamentals and Materials Vol. 141 ( 10 ) page： 546 - 551 2021.10

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

<p>Gas insulated switchgears (GIS) are required to be downsized for the cost reduction, global warming mitigation and energy saving. For the downsizing of GIS, a novel electric field grading technique is highly expected. In order to solve this problem, we have been investigating the electric field control and relaxation technique by “functionally graded materials (FGM)” with spatial distribution of permittivity (ε-FGM) and/or conductivity (σ-FGM). In our previous works, post-type and truncated cone-type ε-FGM spacers were fabricated by “flexible mixture casting method” (FMC method). Compared with the conventional spacers (Uniform spacer) with a uniform permittivity distribution, the electric field grading effects of those ε-FGM spacers were simulated based on Volume-Time theory and verified by the measurement of their breakdown voltages.</p><p>In this paper, toward the application of ε-FGM to the actual GIS spacer, i.e. 30% reduction of spacer diameter or 50% reduction of cross section, we fabricated the 1/4-sized cone-type ε-FGM spacer. By grading the mixing ratio of SrTiO<sub>3</sub> (ε<sub>r</sub>=332) and SiO<sub>2</sub> (ε<sub>r</sub>=4) fillers in epoxy resin, a cone-type spacer with the diameter of 60 mm was fabricated by FMC method. The breakdown voltage of the ε-FGM spacer was measured in a tank filled with SF<sub>6</sub> gas at 0.5 MPa-abs under the negative standard lightning impulse voltage by the step-up method. The maximum breakdown voltage of ε-FGM spacer was verified to be higher by 23% than the Uniform spacer with the same configuration and size of the ε-FGM spacer, which was also consistent with the simulated value.</p>

DOI： 10.1541/ieejfms.141.546

Alternative Test Method of Internal Arcing for Replacing SF₆ by Air in SF₆-Insulated Power Equipment Reviewed International journal

M. Kotari, M. Iwata, H. Kojima, N. Hayakawa

Engineering Research Express Vol. 3 ( 3 ) page： 035005 2021.9

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

To develop an alternative test method of internal arcing for replacing sulfur hexafluoride (SF6) by air in SF6-insulated power equipment, it is important for the public safety to clarify the thermal effect of hot SF6 and air from the equipment on the surroundings. In this study, an analytical model for evaluating the incident heat energy on a metal plate by exhausted hot gas was developed and validated. SF6-air mixtures with a range of gas thermodynamic and transport properties up to 30,000Kwere modelled using computational fluid dynamics (CFD) to simulate the increase in pressure. The analytical results are investigated using the experimental results on SF6-air mixtures. Subsequently, a method of setting the test conditions for the internal arc testing in SF6-insulated power equipment, is proposed based on the analytical model. Using the mixture model, assuming an alternative arcing test for replacing SF6 by air in the SF6-insulated power equipment, the method of setting a current or the duration of current is demonstrated and verified. The results indicate that the current and duration of current can be set such that the incident heat energy on the metal plate by exhausted hot gas are approximately equalized. This study could establish an alternative test method of internal arc in SF6-insulated power equipment.

DOI： 10.1088/2631-8695/ac0fc2

Dielectric Characteristics of Liquid Nitrogen/Synthetic Paper Composite Insulation System for Extra High Voltage HTS Cable Reviewed International journal

N. Hayakawa, Y. Yamakami, T. Masuda, H. Kojima

IEEE Transactions on Applied Superconductivity Vol. 31 ( 5 ) page： 7700204 2021.8

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

For extra-high-voltage high-temperature superconducting (HTS) cables, not only high electrical insulation performance but also low dielectric loss is required. Therefore, we focus on a porous synthetic paper with lower dielectric loss, e.g., Tyvek polyethylene nonwoven fabric, than that of polypropylene laminated paper (PPLP), which has been used for a conventional HTS cable insulation system. In this paper, we compare partial discharge inception characteristics for different liquid nitrogen/synthetic paper composite insulation systems. The partial discharge inception electric field strength in the composite insulation system depends on the electric field strength in the porous area of synthetic paper.

DOI： 10.1109/TASC.2021.3058204

Other Link： http://hdl.handle.net/2237/0002001607

Time Transition of Activated Conductivity Distribution in Air and Charge Accumulation in Air-solid Composite Insulation Systems under DC Partial Discharge Reviewed International journal

R. Nakane, H. Kojima, N. Hayakawa

IEEE Transactions on Dielectrics and Electrical Insulation Vol. 28 ( 3 ) page： 822 - 828 2021.6

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

In this study, charge behavior in gas and on solid insulator due to DC partial discharge (DC-PD) is investigated by using needle-air-PMMA-plane electrode configuration under negative DC voltage. The time evolution of surface potential distributions on solid insulator is measured and compared with the simulation results taking into account the charge behavior in air, such as generation, recombination and motion of positive and negative ions. Especially, a novel transport equation model considering the source of charge carrier due to DC-PD is built. Consequently, in air-solid composite insulation systems under negative DC voltage application, the time transition of activated conductivity distribution in air by DC-PD contributing to the charge accumulation on PMMA surface is evaluated quantitatively. The activated conductivity in air is distributed in the range from 1.0 × 10^-9 to 3.0 × 10^-9 S/m under DC-PD, which is approximately 10 10 times higher than that in non-activated conductivity space by natural ionization. In addition, the activated conductivity distribution in air dominantly contributes to the charge accumulation process from initial state (DC-on) to DC steady-state. The activated conductivity is the crucial parameter for DC electrical insulation in case that charge behavior is activated in gas medium under DC-PD.

DOI： 10.1109/TDEI.2021.009363

Other Link： http://hdl.handle.net/2237/0002001903

Development of Insulation System to Prevent Local Carbonization inside Stator Core of a Product Inverter-fed Motor Reviewed

Y. Yoshitake, H. Kojima, Y. Kasai, S. Maruyama, H. Kojima, N. Hayakawa

IEEJ Transactions on Fundamentals and Materials Vol. 141 ( 4 ) page： 260 - 268 2021.4

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

<p>A new phenomenon, called “local carbonization” inside a stator core was detected as a unique precursor to fragility of the insulation sheets of inverter-fed motors, and the conditions for generation of this carbonization were investigated. A separated stator of a product inverter-fed motor was manufactured, and an evaluation system for simulating a steep surge voltage was constructed. Electrically and mechanically, the results of a parameter survey of insulation system were used to compare the constructed separated stator and element model, and the comparison revealed a correlation between carbonization of the product separated stator and element model. From the consideration of carbonization mechanism, leakage current was revealed to be the dominant design factor. Through quantifying their threshold of carbonization occurrence, insulation system to prevent the carbonization in steeper inverter surge was developed by parameter survey using element model and validation test by separated stator.</p>

DOI： 10.1541/ieejfms.141.260

Electric Field Simulation of Permittivity and Conductivity Graded Materials (ε/σ-FGM) for HVDC GIS Spacers Reviewed International coauthorship International journal

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

IEEE Transactions on Dielectrics and Electrical Insulation Vol. 28 ( 2 ) page： 736 - 744 2021.4

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

This paper discusses the application of the novel technology - functionally graded material (FGM), which combines both spatial permittivity (ε) and conductivity (σ) distributions with the aim to control the electric field around DC-GIS spacer. Some distribution types of ε and/or σ in the spacer bulk, such as U-type and graded to higher conductivity (GHC) are investigated through electric field simulation, in comparison to the uniform spacer with constant ε and σ distributions. The electric field relaxation effect of each distribution type of ε/σ-FGM under DC steady state, DC polarity reversal, DC-on, and lightning impulse voltage superimposed on DC steady state condition are obtained. The results show that ε/σ-FGM with U-type permittivity and GHC-type conductivity distribution of which low σ is applied near the high voltage side of the DC-GIS spacer is the most effective in reducing the maximum electric field under all stated conditions.

DOI： 10.1109/TDEI.2020.009343

Other Link： http://hdl.handle.net/2237/0002001695

Reshaping of Micro-morphology Feature in Electrode Surface under Conditioning by Lightning Impulse Voltage in Vacuum Gaps Reviewed International coauthorship International journal

H. Ma, G. Li, Z. Liu, Y. Geng, J. Wang, H. Kojima

IEEE Transactions on Dielectrics and Electrical Insulation Vol. 27 ( 6 ) page： 2056 - 2063 2020.12

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

The objective of this paper is to determine the reshaping of the micro-morphology feature in both the cathode and anode electrode surfaces after conditioning by lightning impulse voltage in vacuum gaps. Materials of the experimental rod-plane electrodes are Cu and stainless steel. The micro-morphology feature in the electrode surface is recorded by a 3D laser microscopy. Experimental results indicate that conditioning by the lightning impulse voltage reshaped the micro-morphology feature in both the cathode and anode electrode surfaces. The BD characteristics are closely related to the micro-morphology feature and roughness in the cathode surface after conditioning. Moreover, the conditioning process takes different effects in the cathode and anode surfaces. For cathode electrode, the main difference of the micro-morphology feature is in the distributing characteristics of the erosion regions. For anode electrode, there aree different patterns of the anodic conditioning mechanism of the micro-morphology feature. In the molten pattern, conditioning melted the anode surface and removed the micro-protrusions effectively. In the crater pattern, with the increasing BD voltage and energy, the material below the anode surface is heating, which is the cause of formation of the many craters in anode surface.

DOI： 10.1109/TDEI.2020.008888

Partial Discharge Inception Voltage under Inverter Surge Voltage Considering Interactive Effects of Environmental Factors Reviewed

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

IEEJ Transactions on Fundamentals and Materials Vol. 140 ( 7 ) page： 349 - 356 2020.7

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

<p>Effects of environmental factors on partial discharge inception voltage (PDIV) under inverter surge voltage was discussed with multiple regression analysis in this study. As the environmental factors, humidity, temperature, and pressure were taken into account. These factors can change in high-voltage motors used for actual vehicles. The PDIV under a surge voltage showed different changes from that under sinusoidal voltage due to variation of initial electron generation probability depending on environmental change. Our results revealed that humidity has a significant effect on the PDIV under surge voltage, and there are interactive effects between humidity and temperature, and humidity and pressure. We discussed the effect of humidity and these interactions in terms of the generation of initial electrons from the air and verified this by measuring the density of negative ions in the air.</p>

DOI： 10.1541/ieejfms.140.349

Fault Current Limitation Coordination of Multiple SFCLs in IEEE 14-Bus Test System Reviewed International journal

N. Hayakawa, Y. Mori, H. Kojima

Journal of Physics: Conference Series Vol. 1559 ( 1 ) page： 012138 2020.6

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

We have proposed the concept of "fault current limitation coordination" of multiple superconducting fault current limiters (SFCL) to be introduced in a future electric power grid. Using a simplified test system, the transients on fault current and internal phase angle of generators were analyzed by PSCAD/EMTDC and the modeling of resistive SFCL with a variable resistance as a function of both current and temperature at the recovery under load. In this paper, we extended the analyses to the fault current limitation coordination of multiple SFCLs in IEEE 14-bus test system. The effective coordination strategy on the introduction and operation of multiple SFCLs, e.g. introduction lines of SFCLs, operation parameters such as operating current, critical current and prospective fault current of each SFCL, was discussed and optimized for an arbitrary fault location in the test system.

DOI： 10.1088/1742-6596/1559/1/012138

Quench-induced Dynamic Breakdown Characteristics of HTS Pancake Coil Model for Resistive SFCL Reviewed International journal

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

Journal of Physics: Conference Series Vol. 1559 ( 1 ) page： 012100 2020.6

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

Quench-induced dynamic breakdown (BD) characteristics of LN2 under transient bubble disturbance at the quench of HTS tapes are crucial to the reliable and rational insulation design of resistive superconducting fault current limiters (RSFCL). We have been investigating the dynamic BD characteristics using a nichrome pancake coil model for RSFCL using bifilar nichrome tapes as a heater. In this paper, we fabricated an HTS pancake coil model for RSFCL using GdBCO tapes and discussed the dynamic BD characteristics as a function of heat flux generated from the actually quenched HTS tapes.

DOI： 10.1088/1742-6596/1559/1/012100

Elemental Consideration of Carbonization Phenomena in Inverter Fed Motor Reviewed

Y. Yoshitake, H. Kojima, Y. Kasai, S. Maruyama, H. Kojima, N. Hayakawa

IEEJ Transactions on Fundamentals and Materials Vol. 140 ( 1 ) page： 40 - 47 2020.1

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

<p>“Carbonization” were detected as the precursor phenomena on the insulation burning of rotating machine, and we have studied the generation condition for higher <i>dV/dt</i> of the inverter surge. We built the experimental setup of elemental model, which could simulate the steeper surge from SiC device inverter and construct the stator insulation system inside the motor core to control the insulation sheets configuration. By revealing the quantitative conditions of the carbonization generation, we verified that the voltage to suppress the carbonization generation depended on the location of the mold release layer. Therefore, we developed the guideline of insulation design to prevent the carbonization for the steeper inverter surge.</p>

DOI： 10.1541/ieejfms.140.40

Comparison of Permittivity Gradient in Resin Injected and Simulated Cone-Shaped Spacers Reviewed International journal

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

IEEE Transactions on Dielectrics and Electrical Insulation Vol. 26 ( 5 ) page： 1678 - 1685 2019.10

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

Functionally graded materials are at the cutting edge of material technology. They will enable high efficiency and downsizing of apparatus leading to energy conservation in the power sector. Spacers with graded relative permittivity enable a decrease in the diameter of gas-insulated switchgears. Distribution of epoxy-based composite materials (resins) with different permittivities dominate the permittivity gradient in spacers. This paper compares resin injected spacers with simulation. High-permittivity resins, mixtures of high/low-permittivity resins at various mixing ratios, and low-permittivity resins were continuously injected to form spacers. In these spacers, both experiment and simulation showed similar tendencies of resin distribution and demonstrated permittivity gradients. Moreover, the high-permittivity resins were localized on the outlet sides rather than the inlet sides in both experimental and in simulation, and the simulation showed a stronger localization behavior than in the experiment.

DOI： 10.1109/TDEI.2019.008240

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

Discharge Characteristics of Composite Insulation System with Floating Electrode and Solid Insulator 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： 1219 - 1225 2016.4

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

For the development of vacuum interrupters for their higher voltage application in power transmission systems, it is necessary to clarify the discharge characteristics of composite insulation systems with a shield (floating electrode) and a solid insulator in vacuum. In this paper, we focus on the composite discharge patterns via the shield and the solid insulator. For cathode-shield-insulator-anode (c-s-i-a) and cathode-insulator-shield-anode (c-i-s-a) electrode configurations, the discharge can be classified into two independent processes: breakdown in vacuum gap and surface flashover on the solid insulator. Furthermore, we have found that the breakdown development time increases with the increase in the gap length and the surface flashover development time depends on the voltage peak after flashover inception, which are consistent with discharge development characteristics for individual gap breakdown and surface flashover, respectively. These results are significant to understanding and discriminating the composite discharge patterns and discharge path in vacuum interrupters.

DOI： 10.1109/TDEI.2015.005588

Pressure Dependence and Size Effect of LN₂ Breakdown Characteristics Under Transient Thermal Stress Reviewed International journal

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

IEEE Transactions on Applied Superconductivity Vol. 26 ( 3 ) page： 7700604 2016.4

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

Electrical insulation of liquid nitrogen (LN2) for a high-temperature superconducting (HTS) power apparatus can be categorized into two breakdown (BD) modes. One is the intrinsic or conventional BD of LN2. The other is the BD of LN2 with transient bubbles, such as a quench. In this paper, the BD characteristics of LN2 with transient bubbles were investigated for different LN2 pressures and electrode diameters. Experimental results revealed that the BD strength of LN2 with transient bubbles was lower than the intrinsic BD strength of LN2. The difference between the intrinsic BD strength and the BD strength with transient bubbles in LN2 became smaller with the increase in the LN2 pressure and electrode diameter. Furthermore, the BD characteristics of LN2 with continuous bubbles were investigated and compared with the BD characteristics of LN2 with transient bubbles. As a result, the insulation design of an HTS power apparatus in consideration of their quench condition will be rationalized than the design based on the continuous thermal stress.

DOI： 10.1109/TASC.2016.2532469

Effective Secondary Electron Emission Coefficient under Transient Surface Charging on Insulator in Surface Flashover Development in Vacuum Reviewed

Y. Nakano, H. Kojima, K. Tsuchiya, N. Hayakawa

IEEJ Transactions on Fundamentals and Materials Vol. 136 ( 3 ) page： 128 - 134 2016.3

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

Based on the mechanism of surface flashover development in vacuum, we have investigated that the surface flashover can be suppressed by the increase in electric field perpendicular to the insulator surface. In this study, we clarified the suppression conditions of surface flashover in terms of the transient surface charge and the effective secondary electron emission coefficient along the insulator surface.

DOI： 10.1541/ieejfms.136.128

Reduction Method of Fluctuation in Partial Discharge Inception Voltage for Inverter-fed Rectangular Wire Motors Reviewed

T. Wakimoto, H. Kojima, N. Hayakawa

IEEJ Transactions on Fundamentals and Materials Vol. 136 ( 3 ) page： 121 - 127 2016.3

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

We investigated the decisive factors which affect partial discharge inception voltage (PDIV) fluctuation between the coil wires under a sinusoidal voltage by using a rectangular wire motor. As the 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 pre-discharge. 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, pre-discharge condition and measuring frequency.

DOI： 10.1541/ieejfms.136.121

Fabrication and Simulation of Permittivity Graded Materials for Electric Field Grading of Gas Insulated Power Apparatus Reviewed International journal

N. Hayakawa, J. Ishiguro, H. Kojima, K. Kato, H. Okubo

IEEE Transactions on Dielectrics and Electrical Insulation Vol. 23 ( 1 ) page： 547 - 554 2016.2

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

An innovation for the higher performance and reliability of electric power apparatus can be brought about by a novel technique on electrical insulating materials. The application of functionally graded materials (FGM) with spatial distribution of dielectric permittivity in solid insulator is an effective technique to contribute to the above innovation, because FGM can control the electric field distribution with a simple shape. In this paper, we investigated the fabrication and simulation techniques of FGM with various spatial distributions of dielectric permittivity using centrifugal force. Firstly, we developed a simulation technique to estimate the dielectric permittivity distribution of FGM by considering the particle movement in a viscous fluid under centrifugal force. Secondly, based on the simulation technique, we fabricated FGM samples with various relative permittivity distribution and confirmed the validity of the simulation technique. Consequently, the fabrication results provided a good agreement with the simulation results.

DOI： 10.1109/TDEI.2015.005237

Classification of Impulse Breakdown Mechanisms under Non-uniform Electric Field in Air Reviewed International journal

H. Kojima, K. Hotta, T. Kitamura, N. Hayakawa, A. Otake, K. Kobayashi, T. Kato, T. Rokunohe, H. Okubo

IEEE Transactions on Dielectrics and Electrical Insulation Vol. 23 ( 1 ) page： 194 - 201 2016.2

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

To clarify the breakdown mechanism and the conditions leading to the breakdown under non-uniform electric field in air, we studied the details of breakdown characteristics and mechanism in air by using an ultra-high speed measurement system. We clarified that there were two types of breakdown mechanism in air. The one is the channel heating breakdown, and the other is the leader-Type channel development breakdown. These breakdown mechanisms are dominated by three conditions: Arrival of secondary streamer from high-voltage anode to opposite grounded cathode, channel heating, and leader-Type channel initiation. We studied these conditions quantitatively, and revealed the conditions leading to the breakdown, and the breakdown mechanisms in air were classified. Finally, we clarified the factors, e.g. critical instantaneous power for the secondary streamer arrival, dominating these conditions based on the breakdown mechanism under non-uniform electric field in air.

DOI： 10.1109/TDEI.2015.005352

Analysis of Conditioning and Damaging Process in Vacuum based on Breakdown Probability Distribution Reviewed International journal

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

IEEE Transactions on Dielectrics and Electrical Insulation Vol. 23 ( 1 ) page： 43 - 48 2016.2

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

For the enhancement of insulation strength in vacuum, spark conditioning is an important and effective method. In order to enhance the insulation performance of vacuum circuit breakers, we have to clarify the conditioning effect and improve a conditioning procedure. In this paper, we discuss the conditioning process of Cu and stainless steel rod electrodes based on breakdown (BD) probability distribution. We applied a negative impulse voltage to a rod-plane electrode configuration. We adopted two procedures for the voltage application, i.e. up-down and 3 times methods. For the Cu rod with the up-down method, BD voltage increased by the conditioning effect, however, the minimum BD voltage did not increase or even decreased due to the damaging effect by BD. A moderate conditioning procedure, e.g. 3 times method, could increase the minimum BD voltage as well as the stabilization of insulation strength. The Weibull analysis of the development of a BD probability distribution has enabled us to discriminate the conditioning and damaging effects. Based on these results, we can discuss a suitable conditioning procedure.

DOI： 10.1109/TDEI.2015.005248

Control and Suppression of Impulse Surface Flashover in Vacuum Based on Discharge Development Mechanisms Reviewed International journal

Y. Nakano, H. Kojima, S. Iitsuka, K. Tsuchiya, N. Hayakawa

IEEE Transactions on Dielectrics and Electrical Insulation Vol. 23 ( 1 ) page： 35 - 42 2016.2

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

Surface discharge on solid insulator in vacuum is one of the significant issues for the development of vacuum circuit breakers (VCBs) and vacuum interrupters (VIs). For the development of higher voltage VCBs and VIs, control and suppression techniques of the surface flashover are required. The aim of present study is to suppress the surface flashover, based on physical mechanisms of flashover development. Impulse surface flashover in vacuum, starting with explosive electron emission (EEE) from cathode, develops with the formation of transient charge on the insulator as well as the development of secondary electron emission avalanche (SEEA) along the surface toward anode. To reduce the EEE current from the cathode and the SEEA development, we changed the electrode geometry such as thickness of alumina ceramics and tip radius of cathode rod, respectively, and installed a plane electrode as the back electrode of the cathode rod. As the results, EEE current was reduced by the field relaxation effect of transient charging due to EEE, and SEEA was also suppressed by the perpendicular electric field of the plane electrode. When the flashover was suppressed by using the cathode back electrode, transient charge amount was smaller on alumina dielectrics at the end of the plane electrode area. From the above results, it would be an effective technique for the control and suppression of surface flashover in vacuum to enlarge the perpendicular electric field for the insulator.

DOI： 10.1109/TDEI.2015.005307

Transient Charging Characteristics in Surface Flashover Development Process due to Explosive Electron Emission in Vacuum Invited International journal

Y. Nakano, H. Kojima, K. Tsuchiya, N. Hayakawa

Electrical Engineering in Japan Vol. 193 ( 2 ) page： 10 - 17 2015.11

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

For the development of higher voltage vacuum circuit breakers and vacuum interrupters, surface discharge in a vacuum is one of the most significant issues. We measured the charge activity on alumina dielectrics in a vacuum through ultrahigh-speed optical and electrical measurements. We found that surface flashover in a vacuum starts with an explosive electron emission (EEE) with higher current density, which consists of electron emission from the cathode, and results in surface flashover development. The transient charge on the alumina dielectrics formed by electrons emitted by EEE influences the distortion of electric field distribution around the cathode rod and then the subsequent flashover development. In particular, in the case of a shorter gap between the cathode rod and the alumina dielectrics, the electron emission may be suppressed by the relaxation effect of the electric field due to transient charge accumulation.

DOI： 10.1002/eej.22739

Size Effect on Breakdown Strength in Sub-cooled Liquid Nitrogen for Superconducting Power Apparatus Reviewed International journal

N. Hayakawa, S. Nishimachi, H. Kojima, H. Okubo

IEEE Transactions on Dielectrics and Electrical Insulation Vol. 22 ( 5 ) page： 2565 - 2571 2015.10

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

Toward the practical and rational insulation design of high temperature superconducting power apparatus, the ac breakdown (BD) characteristics and mechanisms of liquid nitrogen (LN2) were systematically investigated and discussed for different pressures at P = 0.1-0.3 MPa and temperatures at T = 65-77 K under quasiuniform electric field with the field utilization factor ζ= 0.986-0.674. We obtained the BD strength (EB) in sub-cooled LN2 (SLN2) and analyzed them in terms of size effect, taking account of the stressed liquid volume (SLV) with the electric field distribution in SLN2. Together with the other researchers' data on BD characteristics of LN2, we extended the evaluation by applying the concept of α% SLV (SLV with the electric field strength higher than the decisive electric field factor, α of the maximum electric field strength). As the result, we derived that EB with the 50 % probability decreased linearly with the increase in α% SLV on a universal line, EB = 78.4 × (α% SLV)-1/8.15, which was irrespective of the pressure and temperature of LN2.

DOI： 10.1109/TDEI.2015.005089

Impulse Surface Flashover Development Associated with Transient Charging by Explosive Electron Emission in Vacuum Reviewed International journal

Y. Nakano, H. Kojima, K. Tsuchiya, N. Hayakawa

IEEE Transactions on Dielectrics and Electrical Insulation Vol. 22 ( 4 ) page： 2390 - 2397 2015.8

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

Surface discharge on solid insulator in vacuum, which is one of the most significant issues under high electric field, prevents the development of higher voltage vacuum circuit breakers and vacuum interrupters. To clarify the mechanism of the surface discharge in vacuum, charge activities on solid insulators in the development process of the surface discharge should be investigated. By using electrostatic probes behind the alumina dielectrics and measuring fast-framing images of the surface discharge, in the present paper, we clarified that the impulse surface flashover starts in developing with explosive electron emission (EEE) on cathode, which is specific phenomenon on the electron emission site exploding due to extreme field emission current density. Experimental results revealed the characteristics of transient charging by EEE in the surface flash over development as follows: (1) the probe current due to EEE increases with the EEE inception electric field on the cathode, and also depends on the work function of the cathode material, (2) the transient charge distribution on the alumina dielectrics is determined by the EEE spot on the cathode, (3) EEE occurs several times due to the erosion and the regeneration of electron emission sites. (4) the successive EEE makes the transient charge distribution smooth on the alumina dielectrics.

DOI： 10.1109/TDEI.2015.004948

Fault Current Limitation Coordination in Power Transmission System With Superconducting Fault Current Limiting Cables (SFCLC) Reviewed International journal

H. Kojima, T. Osawa, N. Hayakawa

IEEE Transactions on Applied Superconductivity Vol. 25 ( 3 ) page： 5401904 2015.6

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

We investigate a Superconducting Fault Current Limiting Cable (SFCLC), which is an HTS cable with fault current limiting function. From the viewpoint of the fault current limitation coordination in a power transmission system, it is necessary to investigate the interaction of current limiting functions between multiple SFCLCs introduced in the system. We constructed a numerical model to analyze electrical and thermal behaviors of SFCLC based on resistance generation characteristics of superconductors. In the model power system, two SFCLCs were connected in parallel and two fault points were assumed. We calculated the current limiting characteristics of each SFCLC in the model system, and evaluated the current limitation effect and temperature rise of SFCLCs on each fault point. Depending on the fault point, there are two types of substantial connection of SFCLCs, namely in series and in parallel during the fault. In the substantially parallel connection, each SFCLC operates as the case of single SFCLC introduction. In the substantially series connection, the current limiting effect is determined by SFCLC with the smaller critical current. These results imply that the efficiency and reliability of a power transmission system will be improved by the introduction of multiple SFCLCs, as well as the single SFCLC.

DOI： 10.1109/TASC.2015.2392251

Quench-Induced Dynamic Breakdown Characteristics of LN₂ and LN₂/PPLP Composite Electrical Insulation System for Superconducting Power Apparatus Reviewed International journal

N. Hayakawa, T. Matsuoka, S. Nishimachi, H. Kojima

IEEE Transactions on Applied Superconductivity Vol. 25 ( 3 ) page： 7700304 2015.6

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

In HTS power apparatus, quench due to the fault current generates dynamic electrical/thermal combined stress. This stress may induce the electric breakdown (BD) under the operating voltage. We investigated the quench-induced dynamic BD characteristics of LN2. In addition we investigated the quench-induced dynamic partial discharge characteristics of LN2/polypropylene laminated paper composite electrical insulation system for HTS cables. The criteria of the combined electrical/thermal stress for the dynamic BD were obtained for both test samples, and the reduction rate of the dynamic BD criteria against the static BD criteria without thermal stress was discussed.

DOI： 10.1109/TASC.2014.2361405

Power Flow Control and Maintenance Strategy of Transformers in Transmission and Distribution Systems Reviewed

H. Kojima, K. Fukaya, N. Hayakawa, M. Hanai, H. Okubo

IEEJ Transactions on Power and Energy Vol. 135 ( 5 ) page： 316 - 321 2015.5

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

Transmission and distribution (T&D) systems for electric power are assembled with a large amount of aged apparatus, the deterioration of which may cause a decrease in reliability. In order to maintain high efficiency and high quality in T&D systems, it is important to minimize both present performance costs and future performance costs. Typical present performance costs are T&D losses, and typical future performance costs are failure losses and apparatus maintenance for overall T&D system optimization. We have to consider concurrently two concepts that are an asset management system and a smart grid. This integrated concept, which we have proposed as Intelligent Grid Management System (IGMS), achieves optimum control that includes the present power flow and the future maintenance of the T&D system. In this paper, the effect of temperature rises in transformer windings is formulated and integrated into an objective function to determine transformers with power flow control. The derived power flow control optimizes the load sharing of each transformer by keeping a balance between the T&D loss and degradation in the transformers. The degradation of aged transformers is prevented by reducing their load factor, or the aged transformer is replaced with a new one that accepts a relatively high load factor.

DOI： 10.1541/ieejpes.135.316

Transient Charging Characteristics in Surface Flashover Development Process due to Explosive Electron Emission in Vacuum Reviewed

Y. Nakano, H. Kojima, K. Tsuchiya, N. Hayakawa

IEEJ Transactions on Fundamentals and Materials Vol. 134 ( 11 ) page： 578 - 584 2014.11

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

For the development of higher voltage vacuum circuit breakers (VCB) and vacuum interrupters (VI), surface discharge phenomenon in vacuum is one of the most significant issues. We measured the charge activity on alumina dielectrics in vacuum through ultra-high speed optical and electrical measurements. We found out that the surface flashover in vacuum starts with an explosive electron emission (EEE) with the higher current density, which is the electron emission from the cathode, and results in the surface flashover development. Transient charge on the alumina dielectrics formed by electrons emitted by EEE influences the distortion of electric field distribution around the cathode rod and then the subsequent flashover development. Especially, in the case of the shorter gap between the cathode rod and the alumina dielectrics, the electron emission could be suppressed by the relaxation effect of the electric field due to the transient charge accumulation.

DOI： 10.1541/ieejfms.134.578

Other Link： https://cir.nii.ac.jp/crid/1520853833628404608

Influence of Longitudinal Temperature Distribution on Current Limiting Function of Superconducting Fault Current Limiting Cable (SFCLC) Reviewed International journal

H. Kojima, T. Osawa, N. Hayakawa, M. Hanai, H. Okubo

Journal of Physics: Conference Series Vol. 507 ( PART 3 ) page： 032026 2014.5

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

We have proposed a Superconducting Fault Current Limiting Cable (SFCLC), which is an HTS cable with fault current limiting function. SFCLC is expected to limit the fault current and also immediately recover the cable function after the fault clearance. In the SFCLC operation, a longitudinal temperature distribution will exist due to heat penetration, AC loss, dielectric loss and the performance of cryocooling system, which will influence its current limitation characteristics. In this paper, we investigate the influence of the longitudinal temperature distribution on current limiting function and temperature rise after the current limitation of SFCLC. We suggested the effective measures of parameter control, i.e. decreasing the critical current (Ic@77K), n value at flux flow region (n1-0), increasing the coefficient of longitudinal temperature gradient (α), inflow temperature (Tin) to achieve both the higher current limiting function and the lower temperature rise. © Published under licence by IOP Publishing Ltd.

DOI： 10.1088/1742-6596/507/3/032026

Technique for Discriminating the Type of PD in SF₆ Gas Using the UHF Method and the PD Current with a Metallic Particle Reviewed International journal

M. Hanai, H. Kojima, N. Hayakawa, R. Mizuno, H. Okubo

IEEE Transactions on Dielectrics and Electrical Insulation Vol. 21 ( 1 ) page： 88 - 95 2014.2

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

The relationship between the PD current pulse waveform at the PD source and the radiated electromagnetic (EM) waveform is investigated using a needle electrode for a metallic particle in SF6 gas at 0.2MPa. A leader discharge with a larger PD charge magnitude and a longer propagation length than for a streamer discharge can be an indicator of breakdown in the SF6 gas. Simultaneous and synchronous measurement of the PD current, EM signals and light emission in the SF6 gas revealed that it is possible to discriminate between a streamer discharge and a leader discharge in both time and frequency components. Compared with a streamer discharge, a leader discharge is characterized by a longer rise/fall time of around 0.9 ns in the PD current pulse waveform, and by a lower frequency component of around 250 MHz in the EM frequency spectrum. A satisfactory correlation was obtained between the actual measured EM wave spectrum and the spectrum derived using a spectrum analyzer from the product of the actual PD current and the transfer function from the needle electrode to the UHF sensor that receives the EM wave. This new technique enables streamer and leader types of PD in SF6 gas to be discriminated between by measuring the EM wave with a UHF detection system. © 1994-2012 IEEE.

DOI： 10.1109/TDEI.2013.004021

Pre-discharge and Flashover Characteristics of Impulse Surface Discharge in Vacuum Reviewed International journal

Y. Nakano, H. Kojima, N. Hayakawa, K. Tsuchiya, H. Okubo

IEEE Transactions on Dielectrics and Electrical Insulation Vol. 21 ( 1 ) page： 403 - 410 2014.2

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

Surface discharge on solid insulators is one of the significant issues in the development of vacuum circuit breakers (VCB) and vacuum interrupters (VI). We measured the charge activity on alumina dielectrics in vacuum through ultra-high speed and high sensitivity optical measurements. We identified the existence of charge activity with a small light emission on alumina dielectrics before the surface flashover development process, and defined this as a 'pre-discharge' and measured its characteristics. We classified the charge activities into three processes: (1) pre-discharge, (2) surface flashover initiation, and (3) surface flashover expansion, in the development of surface discharge in vacuum. Utilizing this classification, we found that the surface predischarge in vacuum is generated by the electron emission from the cathode, and the subsequent flashover initiates with a steep increase in current and develops with the distortion of the electric field due to transiently-formed charging on the alumina dielectrics in vacuum. © 2013 IEEE.

DOI： 10.1109/TDEI.2013.004136

Development of a Partial Discharge Inception Calculation Model of a Nonuniform Air Gap Considering the Effect of Humidity Invited International journal

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

Electrical Engineering in Japan Vol. 186 ( 3 ) page： 1 - 9 2014.2

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

Insulation design employing discharge simulation based on physics phenomena is used in designing electrical instruments to facilitate their miniaturization and obtain high insulation reliability. We developed a partial discharge inception calculation model for a nonuniform air gap based on the theories of electron avalanche and streamer phenomena. The calculated and experimental PDIVs (Partial Discharge Inception Voltages) were compared under conditions of constant humidity, and we confirmed that these agreed well with each other to within about 10%. Furthermore, we incorporated a relationship between negative ions and humidity into the model and confirmed that the calculated and experimental PDIVs agreed to within about 15%. Therefore, this model was judged to be an effective method for predicting changes in the PDIV when the humidity changes. © 2013 Wiley Periodicals, Inc.

DOI： 10.1002/eej.22351

Integration of Asset Management and Smart Grid with Intelligent Grid Management System Reviewed International journal

M. Hanai, H. Kojima, N. Hayakawa, K. Shinoda, H. Okubo

IEEE Transactions on Dielectrics and Electrical Insulation Vol. 20 ( 6 ) page： 2195 - 2202 2013.12

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

Electric power transmission and distribution (T and D) systems are composed of a great deal of aged apparatus, which may cause a decrease in reliability owing to their deterioration. In order to maintain high efficiency and high quality in T and D systems, the authors have proposed an "intelligent grid management system" (IGMS), which determines the optimum maintenance strategy and optimum power flow control based on condition monitoring and diagnostic results of the operating power apparatus. This means that the IGMS essentially includes both concepts of an asset management system and a smart grid. Further, the IGMS optimizes power flow routes and maintenance plans based on the failure risk, T and D loss, overload operation, life estimation of the power apparatus, customer outage, and other metrics. The impact of individual apparatus failure affects the entire T and D system's performance, causing blackouts and secondary failures. Reduction in reliability of whole system is highly dependent on ageing of the materials. The IGMS evaluates all of the events occurring in the T & D system as the cost. Additionally, the risks are evaluated in the cost according to the impact of the failure rate estimated by the condition monitoring diagnosis results of the power apparatus. Insulation system determines the transition in reliability and maintenance cost of the system. In this paper, the IGMS is applied to T and D system models including aged apparatus, such as transformers and circuit breakers, and suitable power flow routes and maintenance strategies are derived. Consequently, by the effective application of the IGMS, the system reliability can achieve an optimum state, and the total cost can be minimized. © 2013 IEEE.

DOI： 10.1109/TDEI.2013.6678870

Feasibility Study on Superconducting Power Flow Controller and Fault Current Limiter (SPFCL) Reviewed International journal

N. Hayakawa, X. Mao, H. Kojima, M. Hanai, H. Okubo, N. Kashima, S. Nagaya, S. Sugimoto

IEEE Transactions on Applied Superconductivity Vol. 23 ( 3 ) page： 5600504 2013.6

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

A superconducting power flow controller and fault current limiter is proposed in this paper, which can not only control power flow in the steady operating state, but also limit fault current in the fault condition. We verified that the superconducting power flow controller and fault current limiter will contribute to the enhancement and coordination of both static stability in the steady operating condition and transient stability in the fault condition. © 2002-2011 IEEE.

DOI： 10.1109/TASC.2012.2230677

Physical Mechanisms of Partial Discharges at Nitrogen Filled Delamination in Epoxy Cast Resin Power Apparatus Reviewed International coauthorship International journal

D.-E. A. Mansour, H. Kojima, N. Hayakawa, M. Hanai, H. Okubo

IEEE Transactions on Dielectrics and Electrical Insulation Vol. 20 ( 2 ) page： 454 - 461 2013.4

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

For proper insulation design of epoxy cast resin power apparatus such as molded transformers, it is important to consider the criticality of defects that might exist into the dielectric material. One of the most important defects is the delamination defect at the interface between resin and conductor. Accordingly, this paper aims to investigate partial discharge (PD) characteristics and to clarify their mechanisms for micro gap delamination defect of epoxy cast resin power apparatus. An experimental model simulating delamination defect was built. Nitrogen gas at atmospheric pressure was used as a typical filled gas inside the delamination of epoxy cast resin power apparatus. Simultaneous measurements of PD light emission intensity, PD light emission image and PD current pulse were carried out. PD inception voltage was measured for different delamination depths. PD generation characteristics were recorded. It was found that multiple PD events were detected only by PD light emission images and could not be detected by their current pulses as a result of very small magnitudes of PD current pulses. Time transition of PD activity for 10 minutes were obtained and analyzed using the captured light emission images. Based on the obtained results and comparing with SF6 gas filled delamination, physical mechanisms behind PD activity could be discussed.

DOI： 10.1109/TDEI.2013.6508747

Identification of Metallic Particle Defect in GIS Based on Partial Discharge Mechanism Reviewed

H. Kojima, T. Suzuki, N. Hayakawa, M. Hanai, H. Okubo

IEEJ Transactions on Power and Energy Vol. 133 ( 1 ) page： 114 - 120 2013.1

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

A metallic particle defect seriously degrades an insulation performance of gas insulated switchgear (GIS). Therefore, the estimation of risk of the metallic particle is important to maintain the reliability of GIS. In this paper, based on a partial discharge (PD) mechanism, we suggest a novel method to estimate the metallic particle defect using PD properties, which are the minimum instantaneous voltage of positive PD extinction, the instantaneous voltage of negative PD inception, and the negative PD pulse number from PD generation characteristics of applied voltage phase.

DOI： 10.1541/ieejpes.133.114

Current Limitation and Recovery Function for Superconducting Fault Current Limiting Transformer (SFCLT) Reviewed International journal

J. Himbele, H. Kojima, N. Hayakawa, M. Hanai, H. Okubo

Physics Procedia Vol. 36 page： 841 - 844 2012.9

Language：English Publishing type：Research paper (scientific journal) Publisher：Elsevier B.V.

Superconducting Fault Current Limiting Transformer (SFCLT) is expected to have both superconducting transformer function and superconducting fault current limiter function. We have so far developed the 2 MVA-class SFCLT based on YBCO coated conductors, and experimentally evaluated its current limitation and recovery characteristics. In this paper, we constructed a simulation model for current limitation and recovery characteristics of SFCLT for different HTS coil characteristics and design, and derived the optimized design for the effective current limitation and extended recovery criteria under the rated voltage of the 2 MVA-class SFCLT.

DOI： 10.1016/j.phpro.2012.06.137

Recent Progress in Electrical Insulation Techniques for HTS Power Apparatus Reviewed International journal

N. Hayakawa, H. Kojima, M. Hanai, H. Okubo

Physics Procedia Vol. 36 page： 1305 - 1308 2012.9

Language：English Publishing type：Research paper (scientific journal) Publisher：Elsevier B.V.

This paper describes the electrical insulation techniques at cryogenic temperatures, i.e. Cryodielectrics, for HTS power apparatus, e.g. HTS power transmission cables, transformers, fault current limiters and SMES. Breakdown and partial discharge characteristics are discussed for different electrical insulation configurations of LN2, sub-cooled LN2, solid, vacuum and their composite insulation systems. Dynamic and static insulation performances with and without taking account of quench in HTS materials are also introduced.

DOI： 10.1016/j.phpro.2012.06.295

Superconducting Fault Current Limiting Cable (SFCLC) with Current Limitation and Recovery Function Reviewed International journal

H. Kojima, F. Kato, N. Hayakawa, M. Hanai, H. Okubo

Physics Procedia Vol. 36 page： 1296 - 1300 2012.9

Authorship：Lead author,　Corresponding author Language：English Publishing type：Research paper (scientific journal) Publisher：Elsevier B.V.

We have proposed a Superconducting Fault Current Limiting Cable (SFCLC), which is an HTS cable with fault current limitation function. SFCLC will transmit the bulk power with low loss in steady state, and also limit the fault current during the fault condition, and recover into the superconducting state after the fault clearance. Using a numerical model, we investigated the current limitation and recovery characteristics of SFCLC for different voltage and current levels, and cable lengths. We verified the feasible conditions and performance of SFCLC with the current limitation and recovery function.

DOI： 10.1016/j.phpro.2012.06.293

Feasibility Study on a High-Temperature Superconducting Fault-Current-Limiting Cable (SFCLC) Using Flux-Flow Resistance Reviewed International journal

H. Kojima, F. Kato, N. Hayakawa, M. Hanai, H. Okubo

IEEE Transactions on Applied Superconductivity Vol. 22 ( 2 ) page： 4800105 - 4800105 2012.4

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

We propose a high-temperature superconducting fault-current-limiting cable (SFCLC), which is a high-temperature superconducting cable with a fault-current-limiting function. The SFCLC is expected to achieve fault-current limitation by flux-flow resistance of a long cable length. Particularly, in this paper, we focus on E-I-T characteristics of a YBCO-coated conductor at the flux-flow region. Based on the obtained E-I-T characteristics, we develop a numerical model and investigate current-limiting characteristics of the SFCLC for different cable lengths and fault conditions into a model power system.

DOI： 10.1109/TASC.2011.2173339

Development of the Partial Discharge Inception Calculation Model in a Non-uniform Air Gap Considering the Effect of Humidity Reviewed

N. Yanagita, T. Kato, T. Rokunohe, T. Iwata, H. Kojima, M. Hayakawa, H. Okubo

IEEJ Transactions on Fundamentals and Materials Vol. 131 ( 12 ) page： 1017 - 1023 2011.12

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

Insulation design with discharge simulation based on physics phenomena is used in designing electrical instruments to facilitate their miniaturization and get high insulation reliability. We developed a partial discharge inception calculation model in a non-uniform air gap based on theories of electron avalanche and streamer phenomena. The calculated and experimental PDIVs were compared under a constant humidity condition, and we confirmed these agreed well with each other within about 10 %. Furthermore, we built a relationship between negative ions and humidity into the model, and confirmed the calculated and experimental PDIVs (Partial Discharge Inception Voltage) agreed within about 15 %. Therefore, this model was judged as a method for predicting changes in PDIV when the humidity changes.

DOI： 10.1541/ieejfms.131.1017

Conditioning Mechanism of Cu-Cr Electrode Based on Electrode Surface State under Impulse Voltage Application in Vacuum Reviewed International journal

H. Kojima, N. Hayakawa, R. Nishimura, H. Okubo, H. Sato, H. Saito, Y. Noda

IEEE Transactions on Dielectrics and Electrical Insulation Vol. 18 ( 6 ) page： 2108 - 2114 2011.12

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

Because of high insulation performance and environmental-friendliness, vacuum is attracting considerable attention, and vacuum circuit breakers (VCBs) are expected to develop into higher voltage level. In this paper, we investigated the relationship between the surface condition and the conditioning mechanism of Cu-Cr electrode during impulse voltage application in vacuum. The experimental results showed that a layer of anode material was deposited on the cathode surface, owing to the impulse voltage applications. In addition, we investigated the content ratio of cathode and anode material on the surface after the saturation of electrode conditioning and relationship between the breakdown (BD) discharge area and deposited area during electrode conditioning process. We found that under impulse voltage application, the cathode surface characteristics approach the anode material characteristics, but that the content ratio of Cu to Cr differed between the electrodes. This means that anode material could determine the final state of the conditioning process.

DOI： 10.1109/TDEI.2011.6118651

Identification of Insulation Defect by Sequential Measurement of Partial Discharge in GIS Reviewed

M. Yoshida, K. Suzuki, H. Kojima, N. Hayakawa, M. Hanai, H. Okubo

IEEJ Transactions on Power and Energy Vol. 131 ( 11 ) page： 905 - 911 2011.11

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

To maintain the insulation reliability on gas insulated switchgear (GIS), several partial discharge (PD) monitoring techniques have been utilized. To estimate the risk of a defect, it is important to identify the kind of defects and its condition. In this paper, every PD pulse of various defect types was sequentially measured by using high-speed PD measuring system. The sequential PD measurement was proved to be useful to identify the defect condition, because the temporal change of PD characteristics could show the accumulation of the space charge around the defect in GIS.

DOI： 10.1541/ieejpes.131.905

Analysis of Current Limiting and Recovery Characteristics of Superconducting Fault Current Limiting Transformer (SFCLT) with YBCO Coated Conductors Reviewed International journal

N. Hayakawa, T. Kito, H. Kojima, M. Hanai, H. Okubo

IEEE Transactions on Applied Superconductivity Vol. 21 ( 3 ) page： 1422 - 1425 2011.6

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

Superconducting Fault Current Limiting Transformer (SFCLT) is expected to function as superconducting transformer in steady state and superconducting fault current limiter in fault condition. We have designed, fabricated and tested SFCLT with YBCO coated conductors to be operated in liquid nitrogen (LN2) at 77 K and verified its technological feasibility. In this paper, we developed a simulation model to analyze electrical and thermal behaviors of SFCLT by solving the circuit equation and the heat conduction equation. By the use of this simulation model, we could evaluate the transition of resistance and temperature of the YBCO coated conductors during the fault current limitation as well as after the fault clearance, and verified the recovery criteria of SFCLT. © 2011 IEEE.

DOI： 10.1109/TASC.2011.2105234

Current Limiting and Recovery Characteristics of 2 MVA Class Superconducting Fault Current Limiting Transformer (SFCLT) Reviewed International journal

H. Kojima, M. Kotari, T. Kito, N. Hayakawa, M. Hanai, H. Okubo

IEEE Transactions on Applied Superconductivity Vol. 21 ( 3 ) page： 1401 - 1404 2011.6

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

We have been developing Superconducting Fault Current Limiting Transformer (SFCLT) with the function of both superconducting transformer in steady state and superconducting fault current limiter in fault condition. As the Step-5 of SFCLT project, 2 MVA class HTS-SFCLT using YBCO coated conductors with the ratings of 22 kV/6.6 kV has been developed. One of the important concerns for the development of HTS-SFCLT is the current limiting and recovery characteristics as well as the transformer function. Thus, in this paper, current limitation and recovery tests of the HTS-SFCLT are carried out. The current limiting test have revealed that the HTS-SFCLT exhibits the excellent current limiting function as a superconducting fault current limiter, where the prospective short-circuit current IPRO=784 Apeak is reduced to 34% at the first peak and 18% at the 5th cycles. The HTS-SFCLT has been able to recover into superconducting state immediately after the fault clearance. According to the transition of generated resistance of the HTS-SFCLT, the criteria of recovery into superconducting state are quantified for different combinations of load current before the fault and prospective short-circuit current during the fault.

DOI： 10.1109/TASC.2010.2089413

Progress in Development of Superconducting Fault Current Limiting Transformer (SFCLT) Reviewed International journal

N. Hayakawa, H. Kojima, M. Hanai, H. Okubo

IEEE Transactions on Applied Superconductivity Vol. 21 ( 3 ) page： 1397 - 1400 2011.6

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

We have been developing Superconducting Fault Current Limiting Transformer (SFCLT), which has multifunction of both a superconducting transformer in a steady state as well as a superconducting fault current limiter in a fault condition. This paper introduces the progress in our SFCLT project since 1998, from Step-1 to the latest Step-5, with the concept, design, fabrication and test results of SFCLT. In the latest Step-5, we developed a 2 MVA, 22/6.6 kV class SFCLT with YBCO coated conductors and verified the fundamental function as a transformer, effective current limiting function as a fault current limiter, and recovery characteristics after the fault clearance with its operational criterion.

DOI： 10.1109/TASC.2010.2089412

Development Process of Impulse Surface Flashover on Alumina Dielectrics in Vacuum Reviewed International journal

H. Kojima, N. Hayakawa, T. Ishida, H. Okubo, K. Tsuchiya

IEEE Transactions on Dielectrics and Electrical Insulation Vol. 18 ( 3 ) page： 918 - 923 2011.6

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

For higher electrical insulation performance of vacuum circuit breakers (VCB), the surface insulation characteristics in vacuum should be improved. In this paper, we investigated the development mechanism of impulse surface flashover on alumina ceramic insulator in vacuum. We measured the still images and ultra fast framing images of fast developing surface flashover in synchronous with applied impulse voltage and current waveforms. The light emission transition in initiation of surface flashover was also clarified. We found the dark area was formed around the cathode and conducting channel was formed from the anode in developing process of surface flashover in vacuum.

DOI： 10.1109/TDEI.2011.5931081

Evaluation of UHF Method for Partial Discharge Measurement by Simultaneous Observation of UHF Signal and Current Pulse Waveforms Reviewed International journal

M. Yoshida, H. Kojima, N. Hayakawa, F. Endo, H. Okubo

IEEE Transactions on Dielectrics and Electrical Insulation Vol. 18 ( 2 ) page： 425 - 431 2011.4

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

The partial discharge - current pulse waveform analyzing (PD-CPWA) system can measure partial discharge (PD) current pulses continuously in high speed, and is being used for the study of PD mechanisms in SF6 gas in a laboratory. On the other hand, the ultra high frequency (UHF) method is becoming popular for the on-site monitoring of gas-insulated switchgears (GIS). The former can detect PD with high accuracy and make it possible to analyze and understand discharge physics in detail. The latter is suitable to detect PD in actual power equipment, even in noisy site condition. Clarification of coincidence of PD characteristics measured with both methods is very important for further application of the UHF method. In this paper, PD current pulses generated from artificial defects in SF6 gas were synchronously measured with both methods. Measured data were analyzed and compared in many PD characteristics. As a result, many characteristics coincided well between them. However, the scatter of PD and the limited equipment performance of the UHF method may cause some detection failures. An appropriate and advanced use of the UHF method was pointed out. © 2011 IEEE.

DOI： 10.1109/TDEI.2011.5739446

Mechanism of Small Current Generation under Impulse Voltage Applications in Vacuum Reviewed

K. Aoki, H. Yasukawa, H. Kojima, M. Homma, T. Shioiri, H. Okubo

IEEJ Transactions on Fundamentals and Materials Vol. 130 ( 9 ) page： 781 - 786 2010.9

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

Small discharge not to accompany breakdown can occur under high electric field in vacuum, however the mechanism is not well clarified. We have found that the current of small discharge decreases with repeated voltage applications, and leads to electrode conditioning effect of raising withstand voltage. The inception of the current is delayed with the decrease of current, and the inception time and waveform change by gap length. On the other hand, under low vacuum condition, the current increases and reaches saturation with repeated voltage applications. From these discussions, we concluded that the generating process of small current depended on the adsorption and absorption gas of electrodes.

DOI： 10.1541/ieejfms.130.781

Partial Discharge Activities under AC/Impulse Superimposed Voltage in LN₂/Polypropylene Laminated Paper Insulation System for HTS Cables Reviewed International journal

N. Hayakawa, R. Yamaguchi, Y. Ukai, H. Kojima, F. Endo, H. Okubo

Journal of Physics: Conference Series Vol. 234 ( PART 3 ) page： 032020 2010.7

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

This paper discusses electrical insulation characteristics of high temperature superconducting (HTS) cables for their reliable and rational insulation design. We focused on a severe and practical operational condition of HTS cables, i.e. under commercial ac / lightning impulse superimposed voltage. Experimental results for a HTS cable insulation model composed of liquid nitrogen (LN2) and polypropylene (PP) laminated paper revealed that ac / impulse superimposed voltage could trigger partial discharge (PD) by the impulse voltage invasion and cause consecutive PD under the subsequent ac voltage. The PD generation characteristics were summarized for different ac / impulse superimposed voltages, and a criterion for consecutive PD generation was identified. © 2010 IOP Publishing Ltd.

DOI： 10.1088/1742-6596/234/3/032020

Development of 2MVA Class Superconducting Fault Current Limiting Transformer (SFCLT) with YBCO Coated Conductors Reviewed International journal

M. Kotari, H. Kojima, N. Hayakawa, F. Endo, H. Okubo

Journal of Physics: Conference Series Vol. 234 page： 032070 2010.7

Partial Discharges and Associated Mechanisms for Micro Gap Delamination at Epoxy Spacer in GIS Reviewed International journal

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

IEEE Transactions on Dielectrics and Electrical Insulation Vol. 17 ( 3 ) page： 855 - 861 2010.6

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

For accurate detection and diagnosis of Partial Discharges (PDs) in Gas Insulated Switchgears (GISs), there is a need for better understanding of the physical mechanisms for the PD activity. Accordingly, we have been investigating the PD characteristics and associated mechanisms for electrode/epoxy delamination of GIS spacers as one of the severest defects in GIS. The gas pressure inside delamination usually changes from subatmospheric pressure, just after delamination initiates, to filled SF6 gas pressure in the GIS tank, after SF6 gas infiltrates into the delamination area. Therefore, in this paper, the PD activity at different gas pressures, from 0.02 to 0.4 MPa, is acquired. Delamination gap length at the level of 50 μm is considered, simulating possible size of actual delamination in a GIS spacer. The relative PD generation rate is analyzed regarding the phase characteristics at different pressures and then discussed from the viewpoint of PD mechanism in order to assess the possibility of delamination diagnosis in GIS spacers. Experimental results show that, PD parameters and phase characteristics change significantly with infiltrating SF6 gas from the GIS tank into the delamination. The obtained results give a characterized feature for the delamination defect and can be useful for delamination diagnosis.

DOI： 10.1109/TDEI.2010.5492259

Charge Accumulation Effects on Time Transition of Partial Discharge Activity at GIS Spacer Defects Reviewed International journal

D.-E.A. Mansour, K. Nishizawa, H. Kojima, N. Hayakawa, F. Endo, H. Okubo

IEEE Transactions on Dielectrics and Electrical Insulation Vol. 17 ( 1 ) page： 247 - 255 2010.2

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

The partial discharge (PD) measurements are considered as the most important tool for condition monitoring of Gas Insulated Switchgears (GISs). However, if spacer surface is involved in PD activity, charge accumulation process greatly affects the time transition of PD characteristics. This paper investigates this effect considering the defect types of delamination at the electrode/spacer interface and metallic particles adhered to the spacer surface as the most serious defects that can lead to major failure. Two different electrode configurations were built to simulate the two defect types. The PD characteristics were measured and analyzed from immediately after voltage application up to several minutes where steady state characteristics have been obtained. As a result, it was found that the generation rate of PD pulses changed considerably with time for both defect types. This tendency of PD characteristics has been discussed considering the effect of charge accumulation. Comparing these effects for negative and positive PD enabled to clarify the similarities and differences in the charge accumulation effect for both defect types which can contribute to the defect type identification in GIS. It was found that the effects of charge accumulation on the electric field strength for the delamination defect case as well as on the ionization volume for the metallic particle case were the responsible parameters for the different PD tendencies.

DOI： 10.1109/TDEI.2010.5412024

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

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

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

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

Spatiotemporal Behavior of Hydrogen Recycling due to the Rotating Magnetic Field Reviewed International journal

H. Kojima, S. Takamura

Japanese Journal of Applied Physics Vol. 42 ( 9A ) page： 5797 - 5798 2003.9

Authorship：Lead author,　Corresponding author Language：English Publishing type：Research paper (scientific journal) Publisher：The Japan Society of Applied Physics

A rotating helical magnetic field (RHF) moderates the toroidal nonuniformity of plasma particle flux to the wall, and increases effective wall surface area for hydrogen recycling. Employing a one-dimensional numerical modeling, the dynamic response of neutral gas pressure to the external RHF has been analyzed. The fast dynamic behavior is determined by the plasma particle reflection on the wall and the toroidal diffusion of neutrals, while the longer-time dynamic behavior depends more on the absorption of neutrals into the wall.

DOI： 10.1143/JJAP.42.5797

Structural Dynamics of Deep Negative Potential Well Formation in Small Tokamak Device CSTN-IV International journal

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

Europhysics Conference Abstracts Vol. 27A page： P-2.130 2003.9

Effects of Rotating Helical Magnetic Field on Inhomogeneities of Hydrogen Recycling in a Tokamak Discharge. Reviewed International journal

H. Kojima, S. Takamura

Japanese Journal of Applied Physics Vol. 41 ( 11A ) page： 6557 - 6562 2002.11

Authorship：Lead author,　Corresponding author Language：English Publishing type：Research paper (scientific journal) Publisher：The Japan Society of Applied Physics

A rotating helical magnetic field (RHF) makes an effective recycling wall area increase so that the wall pumping may be enhanced more than that in the absence of RHF by rotating a poloidally periodic edge magnetic field structure. On the other hand, a mitigation of the toroidally asymmetric recycling property due to the RHF has been observed. The reason for such a mitigation of toroidal inhomogeneity comes from the fact that the application of the RHF moderates a toroidal nonuniformity of the plasma flux to the wall while the original plasma-wall interaction is nonuniform toroidally due to a shift of the toroidal plasma loop. Such mitigation is discussed, employing a one-dimensional numerical modeling.

DOI： 10.1143/JJAP.41.6557

Characteristics of Local Helical Coil System for Dynamic Ergodic Divertor Invited International journal

Y. Kikuchi, H. Kojima, T. Taniyama, M. Toyoda, Y. Uesugi, S. Takamura

Electrical Engineering in Japan Vol. 139 ( 1 ) page： 16 - 23 2002.4

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

Heat and particle flux control is one of the most important and difficult problems in steady-state fusion devices. The Dynamic Ergodic Divertor (DED) is an advanced concept in which a rotational perturbed magnetic field is applied to make the tokamak edge magnetic structure stochastic so that it may decrease the heat and particle flux onto the local target in the conventional divertor. This rotational perturbed field is created by two sets of local helical coils (LHC) with a phase difference of 90° in the supplied ac currents. In DED experiments, it is important to study the perturbed magnetic field structure generated by these coil currents. In this paper, a new coil configuration is considered, which has an additional series of return windings so as to improve the mode quality of the rotating helical field (RHF). We obtain the mode structure of RHF using a new Fourier analysis. From this result, we confirm that the parasitic mode of RHF and the coil self-inductance are decreased by the new coil configuration. © 2002 Scripta Technica, Electr. Eng. Jpn.

DOI： 10.1002/eej.1142

Toroidal Dependence of Hydrogen Recycling Property with Helical Magnetic Perturbation in Long Time Discharge International journal

H. Kojima, S. Takamura

Europhysics Conference Abstracts Vol. 25A page： 2129-2132 2001.12

Effects of Dynamic Island Formations on Rotating Helical Magnetic Field Penetration into the Tokamak Plasma International journal

Y. Kikuchi, H. Kojima, T. Taniyama, M. Toyoda, Y. Uesugi, S. Takamura

Europhysics Conference Abstracts Vol. 25A page： 277-280 2001.12

Characteristics of Local Helical Coil System for Dynamic Ergodic Divertor Reviewed

Y. Kikuchi, H. Kojima, T. Taniyama, M. Toyoda, Y. Uesugi, S. Takamura

IEEJ Transactions on Fundamentals and Materials Vol. 121 ( 3 ) page： 249 - 255 2001.3

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

Heat and particle flux control is one of the most important and difficult problems in steady state fusion devices. Dynamic Ergodic Divertor (DED) is the advanced concept that a rotational perturbed magnetic field is applied to make tokamak edge magnetic structure stochastic so that it may decrease the heat and particle flux onto the local target in the conventional divertor. This rotational perturbed field is created by two sets of local helical coils (LHC) with the phase difference of 90 degrees in supplied AC currents. For the DED experiment, it is important to know the perturbed magnetic field structure due to this coil currents. We obtain the mode structure of the rotating helical field using a new Fourier analysis so that we may improve the mode quality by employing a new coil configuration which has an addtional series of return windings so as to avoid unnecessary mode and decrease the coil self-inductance.

DOI： 10.1541/ieejfms1990.121.3_249

Penetration Process of the Externally Applied Rotating Helical Field into the Tokamak Plasma International journal

M. Kobayashi, H. Kojima, S. Takamura

Europhysics Conference Abstracts Vol. 24B page： 1404-1407 2001.1

Research Tokamak System with Multimode Discharges Using Inverter Power Supply Invited International journal

H. Kojima, M. Kobayashi, K. Tashiro, M. Takagi, S. Takamura

Electrical Engineering in Japan Vol. 133 ( 4 ) page： 8 - 18 2000.12

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

In the Current Sustaining Tokamak of the Nagoya University (CSTN)-IV research tokamak system, using a compact 40-kHz pulse width modulation (PWM) inverter power supply, which is controlled through the LabVIEW program, we construct a new tokamak discharge system with multimode including a stable alternating current discharge and a high-repetition high-duty one. These discharge modes can be operated continuously for as long as 60 seconds. The continuous discharge of long duration is able to simulate the important physical and chemical processes of long discharges in fusion devices, in which the heat load to the wall and the particle balance in the plasma-wall system are crucial topics in order to realize a long pulse fusion reactor, like ITER. An ergodic divertor is one of the tools used to control the particle balance and the heat load to the wall. In addition, we installed another inverter power supply to generate a rotating magnetic perturbation for a dynamic ergodic divertor (DED) with the appropriate measurement system so that we could carry out experiments on heat and particle control with DED in prolonged operation.

DOI： 10.1002/1520-6416(200012)133:4<8::AID-EEJ2>3.3.CO;2-9

DOI： 10.1002/1520-6416(200012)133:4<8::AID-EEJ2>3.0.CO;2-I

Frequency Dependence of Tokamak Plasma Response to the Externally Applied Rotating Helical Field Reviewed International journal

M. Kobayashi, H. Kojima, K. Zhai, S. Takamura

Physics of Plasmas Vol. 7 ( 8 ) page： 3288 - 3300 2000.8

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

The frequency dependence of the tokamak plasma response to the externally applied rotating helical field was investigated by measuring the radial profile of the perturbation field with small magnetic probes which were inserted in the plasma. The experimental results are discussed, directly comparing with the generally accepted linear theory, and taking into account the EXB drift and diamagnetic effect of the plasma. It was found that the experimental results are in good agreement with the simple linear analysis based on the single helicity approximation in a cylindrical geometry, except for the ergodic region, which comes from the overlapping of several sideband components of the perturbation. It is also found that the radial component of the perturbation field is still amplified inside the resonance surface (r<rs), even when a partial shielding takes place at r ≈rs. © 2000 American Institute of Physics.

DOI： 10.1063/1.874194

Electrostatic Fluctuations in the Current Sustained Torus IV at Nagoya University Reviewed International journal

K. Zhai, M. Kobayashi, K. Tashiro, H. Kojima, M. Takagi, S. Takamura

Physics of Plasmas Vol. 7 ( 6 ) page： 2449 - 2455 2000.6

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

A detailed experimental investigation of plasma fluctuation and its modification due to radial electric field effects, both its shear and curvature, has been carried out on the Current Sustained Torus IV at Nagoya University [Takamura et al., J. Plasma Fusion Res. 74, 38 (1998)] through measurement with an array of Langmuir probes. The observed statistical dispersion relation indicates a drift-wave-type turbulence. An examination of the radial electric field shear and curvature effects on the fluctuations as well as comparisons to theoretical predictions are presented. The decorrelation of turbulence is found to be influenced by the radial electric field shear in our experiment. A modification of the poloidal correlation length and the two-poloidal-point coherency due to the radial electric field has also been identified. Nonlinear analysis of quadratic mode coupling indicates that the radial electric field can affect the fluctuation amplitudes by modifying the coupling between different spectral components of the fluctuations. © 2000 American Institute of Physics.

DOI： 10.1063/1.874084

Interaction of Externally Applied Rotating Helical Field with Tokamak Plasma Reviewed International journal

M. Kobayashi, T. Tuda, K. Tashiro, H. Kojima, K. Zhai, S. Takamura

Nuclear Fusion Vol. 40 ( 2 ) page： 181 - 193 2000.2

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

The penetration process of a rotating magnetic perturbation into a rotating tokamak plasma has been investigated, taking account of the mode structure of the perturbations, and using a small magnetic probe in the small research tokamak CSTN-IV. It was observed that the radial component of the perturbation was amplified in the plasma when magnetic islands are formed, while the poloidal component was attenuated but, deep inside the plasma, amplified. Using a resistive MHD code, such modifications of external perturbation were found to be caused by a redistribution of plasma current due to the formation of magnetic islands. Considering the phase velocity of both the perturbation and the intrinsic plasma rotation, it was also found that the sideband component substantially affects the penetration process with magnetic islands forming. Therefore, in dynamic ergodic divertor operation, it is necessary to put the emphasis not only on the frequency of the perturbation fields but also on their direction, mode structure and the radial profile of the intrinsic plasma rotation at the same time.

DOI： 10.1088/0029-5515/40/2/304

Research Tokamak System with Multi-Mode Discharges Using Inverter Power Supply Reviewed

H. Kojima, M. Kobayashi, K. Tashiro, M. Takagi, S. Takamura

IEEJ Transactions on Fundamentals and Materials Vol. 119 ( 12 ) page： 1436 - 1444 1999.12

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

In Current Sustaining Tokamak in Nagoya university (CSTN)-IV research tokamak system using a compact 40kHz pulse width modulation (PWM) inverter power supply, which is controlled through LabVIEW program, we construct a new tokamak discharge system with multi-mode including a stable alternating current discharge and a high-repetition high-duty one. These discharge modes can be operated continuously for as long as 60sec. The continuous discharge with long duration is able to simulate the important physical and chemical processes of long time discharges in fusion devices, in which the heat load to the wall and the particle balance in the plasma-wall system are crucial topics in order to realize a long pulse fusion reactor, like ITER. Employing ergodic divertor (ED) is one of tools to control the particle balance and the heat load to the wall. In addition, we installed another inverter power supply to generate a rotating magnetic perturbation for dynamic ergodic divertor (DED) with the appropriate measurement system so that we may carry out experiments on heat and particle control with DED at long time operation.

DOI： 10.1541/ieejfms1990.119.12_1436

Experimental Investigation on Penetration of Rotating Magnetic Perturbations into the Tokamak Plasma International journal

M. Kobayashi, T. Tuda, Z. Kan, H. Kojima, K. Tashiro, S. Takamura

Europhysics Conference Abstracts Vol. 23J page： 437-440 1999.7

Two-dimensional Structure of the Detached Recombining Helium Plasma Associated with Molecular Activated Recombination Reviewed International coauthorship International journal

D. Nishijima, N. Ezumi, H. Kojima, N. Ohno, S. Takamura, S. I. Krasheninnikov, A. Yu. Pigarov

Journal of Nuclear Materials Vol. 266 page： 1161 - 1166 1999.3

Language：English Publishing type：Research paper (scientific journal) Publisher：Elsevier B.V.

The structural change of the detached plasmas associated with the molecular activated recombination (MAR) has been studied in a high density helium plasma with helium or hydrogen puff in the linear divertor plasma simulator, NAGDIS-II. The ion particle flux with the hydrogen gas puff starts to decrease in the upstream region close to the plasma source, compared to that with the helium puff. The reduction of the ion particle flux along the magnetic field is found to depend on the plasma density. With the hydrogen gas puff, the attenuation of the ion particle flux is getting smaller as increasing plasma density, which is opposite tendency in pure helium detached plasma where the conventional electron-ion recombination (EIR) is dominating.

DOI： 10.1016/S0022-3115(98)00565-0

Contribution of Molecular Activated Recombination to Hydrogen Plasma Detachment in the Divertor Plasma Simulator NAGDIS-II Reviewed International coauthorship International journal

N. Ezumi, D. Nishijima, H. Kojima, N. Ohno, S. Takamura, S. I. Krasheninnikov, A. Yu. Pigarov

Journal of Nuclear Materials Vol. 266 page： 337 - 342 1999.3

Language：English Publishing type：Research paper (scientific journal) Publisher：Elsevier B.V.

We have performed experiments on detached hydrogen plasmas associated with molecular activated recombination (MAR) in a linear divertor plasma simulator. Reductions of the ion particle flux and heat load to the target plate were clearly observed in hydrogen plasmas with the hydrogen gas puff. Detailed analysis of Balmer series spectra with the Collisional-Radiative Atomic-Molecular data (CRAMD) code shows that MAR mainly appears as a weak dependence of the Balmer series emission intensities on experimental conditions in detached hydrogen plasmas. The plasma conditions necessary to obtain plasma detachment through MAR or EIR (electron ion recombination including the radiative and three-body recombination) in tokamak divertors are also discussed.

DOI： 10.1016/S0022-3115(98)00533-9

High-duty Long-duration Repetitive Tokamak Discharge with Static or Rotating Helical Magnetic Field at the Edge International journal

S. Takamura, M. Kobayashi, K. Tashiro, H. Kojima, K. Zhai, N. Ohno, M. Y. Ye, Y. Uesugi, M. Takagi

Europhysics Conference Abstracts Vol. 22C page： 722-725 1998.6

Investigation of Hydrogen Plasma Detachment in the Divertor Plasma Simulator NAGDIS-II International coauthorship International journal

N. Ezumi, D. Nishijima, H. Kojima, N. Ohno, S. Takamura, S. I. Krasheninnikov, A. Yu. Pigarov

Europhysics Conference Abstracts Vol. 22C page： 7-10 1998.6

Change in Detached Helium Plasma's Structure Associated with Molecular Activated Recombination International journal

D. Nishijima, N. Ezumi, H. Kojima, N. Ohno, S. Takamura

Europhysics Conference Abstracts Vol. 22C page： 114-117 1998.6

　 PREV - NEXT 　

1