Faculty Profiles - KOJIMA Hiroki

Papers - KOJIMA Hiroki

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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

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