Faculty Profiles - KOJIMA Hiroki

Papers - KOJIMA Hiroki

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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

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