Papers - SAKODA Tatsuya
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Suppression of Transformer Inrush Current by Applying Lower Frequency and Lower Voltage Reviewed
Nishijima Kanta, Khaing May Thin, Sakoda Tatsuya, Kamesawa Tomoyuki, Yamamura Syunichiro, Nishimura Toi, Terashi Kohei
IEEJ Transactions on Power and Energy 145 ( 8 ) 496 - 501 2025.8
Authorship：Corresponding author Language：Japanese Publishing type：Research paper (scientific journal) Publisher：The Institute of Electrical Engineers of Japan
Inrush current phenomenon of a three-phase transformer is caused by mismatching polarities and absolute values of the initial magnetic flux induced by residual flux remnants in each phase core before the transformer is energized. Because the residual flux is the final value of magnetic flux transients that occur immediately after disconnection, the inrush current can be suppressed by considering the residual flux and controlling the timing of a circuit breaker. However, when a transformer is first energized in the field, the residual flux is unknown.In this study, we apply lower frequency and lower voltage to a transformer system simulating a three-phase transformer and then integrates the transformer voltage by time to digitally calculate and capture the true residual flux. Next, when the transformers are energized, a phase angle control is applied to the timing of a switch to effectively suppress inrush current. As a result, the usefulness of the lower frequency and lower voltage application to suppress inrush current is indicated.
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Evaluation of the Degradation Degree of 20-Year-aged Insulator by Using Surface Discharge Reviewed
Khin M.O., Nonoka H., Khaing M.T., Sakoda T., Nakayama Y., Niwa K.
2025 IEEE 5th International Conference on Electrical Materials and Power Equipment Icempe 2025 2025.8
Authorship：Corresponding author Language：English Publishing type：Research paper (international conference proceedings) Publisher：2025 IEEE 5th International Conference on Electrical Materials and Power Equipment Icempe 2025
Polymer insulator wherein silicone rubber (SiR) is used as housing material are exposed to various environmental stresses such as ultraviolet (UV) radiation, high temperatures, and pollution, which can lead to degradation over time. Assessing the degree of this degradation is crucial for the long-term maintenance of polymer insulators. This study proposes a novel technique to evaluate the degradation degree of SiR. To effectively expose PDs to specific locations on the SiR surface, we generated surface discharges (SDs). We compared hydrophobicity recovery times between an unused SiR specimen and aged SiR specimens. Our approach involves analyzing the hydrophobicity recovery time after SD treatment. SD treatment temporarily reduces the surface hydrophobicity of SiR, and by measuring the time required for hydrophobicity to recover, we can assess its degradation. Experiments were conducted on SiR specimens extracted from unused, and 20 -year-aged polymer insulators. The SD treatment was applied for 10s. Following each treatment, the contact angle was measured at 5 -minute intervals. Then, we characterized the physical or chemical damage on SiR specimens after the SD treatment using a scanning electron microscope (SEM), and an attenuated reflection-Fourier transform infrared spectroscopy (ATR FTIR). These findings indicate that the contact angle recovery time after SD treatment serves as a valuable parameter for assessing the degradation degree of SiR insulators.
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Linn Htet S.Z., Khaing M.T., Sakoda T., Harada Y., Shibata T., Miyazaki K.
2025 IEEE 5th International Conference on Electrical Materials and Power Equipment Icempe 2025 2025.8
Authorship：Corresponding author Language：English Publishing type：Research paper (international conference proceedings) Publisher：2025 IEEE 5th International Conference on Electrical Materials and Power Equipment Icempe 2025
Partial discharge (PD) activity significantly affects the reliability of insulation systems, with void positioning in epoxy resin playing a critical role in molded transformers. This study investigates the influence of PD in epoxy resin containing two artificial voids (1 mm in diameter) arranged in horizontal and vertical alignments, focusing on PD characteristics, electrical treeing behavior, and breakdown mechanisms. PD activity and electrical tree were monitored using a high-frequency current transformer (HF-CT) sensor and a transient earth voltage (TEV) sensor. The results indicate that void alignment strongly influences PD inception, discharge intensity, and propagation patterns. FFT (Fast Fourier Transform) spectrum analysis revealed distinct differences between the two configurations in vertically and horizontally aligned voids. Furthermore, electrical treeing was observed in the horizontally aligned voids, leading to breakdown. However, in vertically aligned voids, electrical treeing was difficult to detect, and a breakdown occurred suddenly. These findings highlight that void alignment significantly influences PD behavior and breakdown progression, providing valuable insights for improving insulation design and diagnostic techniques in high-voltage applications, contributing to the development of more reliable insulation systems.
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Evaluation of Degradation Degree on Silicone Rubber by Using Surface Discharge Reviewed
May Thin Khaing, Nonoka Hirose, Takuma Miyake, Tatsuya Sakoda, Shigehiko Goto, Tomikazu Anjiki, Tomoki Chiba
IEEJ TRANSACTIONS ON ELECTRICAL AND ELECTRONIC ENGINEERING ( 20 ) 998 - 1005 2025.7
Authorship：Corresponding author Language：English Publishing type：Research paper (scientific journal)
DOI： 10.1002/tee.24256
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EVALUATION OF DEGRADATION DEGREE OF ARTIFICIALLY DEGRADED SILICONE RUBBER SURFACE Reviewed
Nakayama Y., Niwa K., Khaing M.T., Sakoda T.
Iet Conference Proceedings 2025 ( 14 ) 1067 - 1071 2025.6
Authorship：Corresponding author Language：English Publishing type：Research paper (international conference proceedings) Publisher：Iet Conference Proceedings
Polymer materials such as silicone rubber (SiR) have been used as housing material for insulators in distribution lines, however because polymer insulators are more expensive than porcelain insulators in Japan, their application is still limited. If their price can be reduced, there is potential for the installation of polymer insulators to accelerate. One way to reduce the cost is to improve the materials. Finding optimal alternative materials, requires grasping the extent to which such materials resist degradation during long-term use. In this study, we first propose a method for making degraded surfaces that resemble the surface characteristics of aged insulators. Additionally, we propose a method for evaluating the degree of surface degradation at which surface discharge (SD) is generated for a short time on the SiR surface. These two methods can contribute to the introduction of new materials for polymer insulators.
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Watanabe K., Sakoda T., Takahashi A., Negishi N., Miyazaki K.
Electrical Engineering in Japan English Translation of Denki Gakkai Ronbunshi 218 ( 2 ) 2025.6
Authorship：Corresponding author Language：English Publishing type：Research paper (scientific journal) Publisher：Electrical Engineering in Japan English Translation of Denki Gakkai Ronbunshi
To establish a novel section locating technique for a water-tree degraded 22 kV cross-linked polyethylene (XLPE) cable, that is, included in another undegraded cables, we measured a reflected injection pulse at the impedance mismatch, which is dependent on water trees. The injection pulse was propagated into cables to be measured, and the measured arrival times of reflected pulses containing residual charges due to water trees were varied depending on the locations of water-tree degraded cables. The section locating was achieved by using the arrival time of the reflected pulse.
DOI： 10.1002/eej.23506
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May Thin Khaing, Watanabe Koki, Sakoda Tatsuya, Takayama Toru, Miyazaki Kazunori
IEEE Transactions on Dielectrics and Electrical Insulation 32 ( 5 ) 2915 - 2921 2025.2
Authorship：Corresponding author Language：English Publishing type：Research paper (scientific journal)
A residual charge method is useful for measuring water tree in 22-kV crosslinked polyethylene (XLPE) cable systems. However, the conventional residual charge method cannot locate a water-tree degraded cable section because the time-resolved signal response from a water-tree degradation section cannot be obtained. In this study, we inject a sine wave pulse with a frequency of 1 MHz into measuring object cables when ac voltage is applied to the cables to release trapped charges due to the water-tree. We examine to locate a water-tree degraded cable section in 22-kV XLPE cable systems, including water-tree degraded cables and a Y-branch cable joint. The time-resolved residual charges induced by the voltage pulse induced by an injection coil (IJ coil) can locate a water-tree degraded cable section.
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Evaluation of Degradation Degree on Silicone Rubber by Using Surface Discharge Reviewed
Khaing M.T., Hirose N., Miyake T., Sakoda T., Goto S., Anjiki T., Chiba T.
IEEJ Transactions on Electrical and Electronic Engineering 20 ( 7 ) 1 - 8 2025.1
Authorship：Corresponding author Language：English Publishing type：Research paper (scientific journal) Publisher：IEEJ Transactions on Electrical and Electronic Engineering
Silicone rubber (SiR) insulators in outdoor may be degraded owing to multiple environmental stresses such as ultraviolet (UV) rays, high temperature and pollution adhesion. Evaluation of the degradation degree of SiR can contribute to the long-term maintenance of SiR insulators. We proposed a novel technique to grasp the degradation degree of SiR. The surface discharge (SD) on the SiR surface temporarily decreased the surface hydrophobicity, and then the hydrophobicity recovery time was evaluated. The SD treatments were performed on SiR specimens cut out from unused, 10 and 20-year-aged SiR insulators, and the treatment time was from 5 to 25 s. After each SD treatment, the contact angle was evaluated at intervals of 10 min. As a result, it was found that the contact angle recovery time increases with the increase of SD treatment time. Also, the contact angle recovery times on 10 and 20-year-aged SiR specimens were longer than that on an unused SiR specimen, i.e. the contact angle recovery time after the SD treatment is useful for investigating the degradation degree. © 2025 Institute of Electrical Engineers of Japan and Wiley Periodicals LLC.
DOI： 10.1002/tee.24256
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Watanabe Koki, Sakoda Tatsuya, Takahashi Akira, Negishi Naoyuki, Miyazaki Kazunori
IEEJ Transactions on Fundamentals and Materials 145 ( 1 ) 23 - 29 2025.1
Language：Japanese Publishing type：Research paper (scientific journal) Publisher：The Institute of Electrical Engineers of Japan
To establish a novel section locating technique for a water-tree degraded 22 kV cross-linked polyethylene (XLPE) cable that is included in another undegraded cables, we measured a reflected injection pulse at the impedance mismatch which is dependent on water trees. The injection pulse was propagated into cables to be measured, and the measured arrival times of reflected pulses containing residual charges due to water trees were varied depending on the locations of water-tree degraded cables. The section locating was achieved by using arrival time of the reflected pulse.
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Partial Discharge and Electrical Treeing Phenomena Within Two Spherical Voids in Epoxy Resin Reviewed
Htet S.Z.L., Kondo T., Khaing M.T., Sakoda T., Harada Y., Takei K., Ikeda M., Miyazaki K.
2024 10th International Conference on Condition Monitoring and Diagnosis, CMD 2024 358 - 361 2024.10
Authorship：Corresponding author Language：English Publishing type：Research paper (scientific journal) Publisher：2024 10th International Conference on Condition Monitoring and Diagnosis, CMD 2024
Partial discharge (PD) and electrical treeing phenomena in an epoxy resin electrode which had two spherical voids and the insulation paper were investigated. The alignment of the two voids was vertical against the applied electric field direction, and experiments with not only two voids of the same size (diameter of 1 mm) but also two voids of different size (diameters of 1 mm and 1.5 mm) were carried out. AC high voltage was applied until the breakdown occurred in the epoxy resin electrode. PD and electrical treeing phenomena were detected by using a high frequency current transformer (HF-CT) sensor, a transient earth voltage (TEV) sensor and an ultra-high frequency (UHF) sensor. We here focused on variations of the number of PDs per unit time and frequency components of detected signals with the elapsed time from the voltage application. As a result, we investigated a unique electrical tree development characteristic which differed in the size of two voids. Although the electrical tree development was temporarily decelerated by the insulation paper, the breakdown suddenly occurred from a point of high electric field in the insulation paper. Additionally, we investigated the temporal variations of the number of PDs which differed in the combination of the void size.
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Diagnosis of Degradation Degree on SiR Surface Used for Insulator by Surface Discharge Reviewed
Khaing M.T., Miyake T., Sakoda T., Goto S., Anjiki T., Chiba T., Kasuga Y.
2024 10th International Conference on Condition Monitoring and Diagnosis, CMD 2024 386 - 389 2024.10
Authorship：Corresponding author Publishing type：Research paper (scientific journal) Publisher：2024 10th International Conference on Condition Monitoring and Diagnosis, CMD 2024
Silicone rubber (SiR) insulators have been mostly preferred for high-voltage transmission and distribution lines because of their superior hydrophobicity characteristic. Hydrophobicity of SiR insulators may be, however, degraded by multiple stresses and may eventually lead to dry band arcing. Hence, a diagnostic technique of degradation degree on SiR surface will be useful for the long-term maintenance of SiR insulators. In this study, the artificially degraded layers were first made on two SiR surfaces by red clay adhesion, UV irradiation and surface heating. To diagnose the degradation degree on the SiR surface, an unused SiR surface, two artificially degraded SiR surfaces (Sample A (UV, Heating) and Sample B (Heating, UV)) and two aged SiR surfaces (10-year-aged and 20-year-aged) were exposed to a surface discharge (SD) source. The applied voltage and treatment time of SD against the experimental SiR surfaces were 8 kVp-p and 10 s, respectively. After the SD treatment, the hydrophobicity recovery time of each experimental SiR surface was evaluated with the contact angle of a water droplet on the SiR surface. Then, monitoring of water-repellent maintenance (i.e., the contact angle of water droplets on the SiR surface remaining in stable condition) on each experimental SiR surface was also conducted. It was found that the hydrophobicity recovery times and water-repellent change times (duration of contact angle of water droplets on the SiR surface in stable condition) of Sample A (UV, Heating) and Sample B (Heating, UV) were similar to those of 10-year-aged and 20-year-aged SiR surfaces, respectively. Additionally, water-repellent change times of Sample B (Heating, UV) and 20-year-aged SiR surfaces were shorter compared with those of Sample A (UV, Heating) and 10-year-aged SiR surfaces.
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Characterization of Surface Discharge Used for Measuring Degradation Degree of Polymer Insulator Reviewed
Khaing M.T., Miyake T., Sakoda T.
Proceedings of the IEEE International Conference on Properties and Applications of Dielectric Materials 254 - 257 2024.8
Language：English Publishing type：Research paper (scientific journal) Publisher：Proceedings of the IEEE International Conference on Properties and Applications of Dielectric Materials
Polymer insulators with silicone rubber (SiR) insulation have unique properties and are installed all over the world. However, due to pollution deposits on the surface, the characteristics of polymer insulators become degraded. We have already investigated that the degradation degree of the polymer insulator can be evaluated by a 20-cycle salt-fog aging test or a surface discharge (SD) treatment. However, the decrease mechanism of the contact angle by such discharge treatment wasn't interpreted in detail. In this study, to characterize SD generated on the surfaces of SiR, we first measured the SD current. The observed maximum SD current did not significantly differ from that obtained during the 20-cycle-salt-fog aging test. In addition, the SD consisted of filamentary discharges. In measurements of optical emissions, emissions with wavelengths of less than 291 nm which can cut C-H bond of SiR weren't observed. Therefore, it is thought that the direct energy transfer from energetic particles with higher energy than the binding energy of C-H changes chemical characteristics and decreases the contact angle on the SiR surface.
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Partial Discharge Behavior Prior to Breakdown in Epoxy Resin Reviewed
Htet S.Z.L., Kondo T., Miyake T., Sakoda T., Nishimura T.
IEEJ Transactions on Electrical and Electronic Engineering 19 ( 12 ) 1927 - 1935 2024.7
Authorship：Corresponding author Publishing type：Research paper (scientific journal) Publisher：IEEJ Transactions on Electrical and Electronic Engineering
To evaluate the deterioration degree of insulation material by monitoring partial discharges (PDs), it is necessary to fully understand the PD behavior prior to breakdown. We performed measurements of temporal variations in the number and magnitude of PDs in a void of epoxy resin. It was found that the number of PDs with larger magnitude is less before breakdown and that there is a decreased tendency of the number and magnitude of PDs with the elapsed time. The tendency is remarkable as the applied electric field is small. Additionally, the time to breakdown becomes short when the scattering of the number of PDs per unit time decreases. © 2024 Institute of Electrical Engineers of Japan and Wiley Periodicals LLC.
DOI： 10.1002/tee.24160
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Ko T.Z.W., Torihara R., Sakoda T., Hayashi N.
IEEJ Transactions on Electrical and Electronic Engineering 19 ( 6 ) 1105 - 1112 2024.6
Authorship：Corresponding author Language：English Publishing type：Research paper (scientific journal) Publisher：IEEJ Transactions on Electrical and Electronic Engineering
Lightning surges are one of the major reasons of bypass diode (BPD) failure in photovoltaic modules. In this study, standard lightning impulse voltages (SLIVs) were directly applied to Schottky barrier diodes (SKB diodes) and rectifier diodes (PNJ diodes) with different electrical specifications. The critical voltages Vc of SLIV at which the diodes were damaged and the temporal variations of diode current (iD) and voltage (vD) were measured. It was found the iD-vD characteristics can be categorized into a total of eight patterns: four patterns for the forward SLIV application and the four for the reverse SLIV application, regardless of diode types. The measured results imply that the irreversible voltage dip that occurs before the termination of the reverse breakdown contributes significantly to diode damage in the both forward and reverse SLIV applications. SKB diodes exposed to the reverse SLIV are easily damaged at the lowest magnitude of SLIV among all combinations of diode types and SLIV directions. Once a diode has been damaged due to the first SLIV application, the second application of SLIV to the same diode causes remarkable change in the iD-vD characteristic even if the magnitude of the second SLIV is smaller than Vc. © 2024 Institute of Electrical Engineer of Japan and Wiley Periodicals LLC.
DOI： 10.1002/tee.24064
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Short-time Fabrication of Degraded Surface on Silicone Rubber Used for Insulators Reviewed
Khaing M.T., Yoshimura K., Miyake T., Sakoda T., Goto S., Anjiki T., Chiba T., Kasuga Y.
IEEE Transactions on Dielectrics and Electrical Insulation 31 ( 2 ) 1054 - 1061 2024.4
Publishing type：Research paper (scientific journal) Publisher：IEEE Transactions on Dielectrics and Electrical Insulation
Silicone rubber (SiR) insulators are degraded due to multiple stresses. To develop a degradation diagnostic technique, a lot of aged insulators are required. However, collecting a lot of several decades of used SiR insulators is difficult. Therefore, it is desired to fabricate the degraded SiR surface whose characteristics are similar to those of degraded SiR insulators in outdoor. To grasp characteristics of the degraded SiR surface, we first investigated characteristics of aged SiR insulators. Foreign matters on aged SiR specimens made the contact angle larger, and Si-O, Si-CH<sub>3</sub>, C-H, and O-H bonds decreased. Next, to fabricate the artificially degraded SiR surface, we proposed a 24 h – fabrication - cycle which consists of a red clay adhesion process, a UV irradiation process, a surface heating process, a heat radiation process, and a surface cleaning process. We evaluated effects of the UV irradiation and the surface heating, and then it was found that a combination of UV irradiation for breaking molecular structure and surface heating for promoting LMW dispersion played an important role in adhering red clay to SiR. The contact angle and the chemical structure on the artificially degraded SiR surface, which was fabricated by UV irradiation for 6 h and surface heating at 20°C or 40°C, were similar to those of a SiR specimen aged 10 years with the hydrophobicity of HC 2.
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Characteristics of Aged Silicone Rubber Insulators Used in Outdoor for 20 Years Reviewed
Khaing M.T., Yoshimura K., Miyake T., Sakoda T., Kanenari U., Nishihiro Y.
Ieej Transactions on Electrical and Electronic Engineering 19 ( 4 ) 454 - 460 2024.4
Publishing type：Research paper (scientific journal) Publisher：Ieej Transactions on Electrical and Electronic Engineering
To collect aged silicone rubber (SiR) insulators with different numbers of years in use, which is for developing a degradation diagnostic system of SiR, is difficult. Therefore, it is desired to make degraded SiR surface which has similar characteristics to the degraded surface in outdoor. To realize the fabrication, we characterized the surfaces of aged SiR specimens cut out from a 22 kV line post insulator used in the coastal area for 20 years. The results showed that to introduce fine particles into the surface is necessary for making degraded surface having the minute surface roughness. Additionally, it was found that to activate the dispersion of LMW with foreign matters is also necessary. © 2024 Institute of Electrical Engineer of Japan and Wiley Periodicals LLC.
DOI： 10.1002/tee.23991
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Khaing M.T., Miyake T., Sakoda T., Kanenari U., Nishihiro Y.
Journal of Electrostatics 128 2024.3
Publishing type：Research paper (scientific journal) Publisher：Journal of Electrostatics
To establish an evaluation technique of the degradation degree of silicone rubber (SiR) surfaces, we first fabricated artificially degraded SiR surfaces. The fabrication process consisted of 5 steps and a 1-cycle test required 24 h. We fabricated the degraded SiR surfaces by a 1-cycle or a 2-cycle test. Next, the artificially degraded SiR surface was exposed to surface discharge (SD). The hydrophobicity recovery time on artificially degraded SiR surface increased with the increase in SD treatment time. It was found that the degradation degree of the SiR surface can be evaluated by hydrophobicity recovery time after SD treatment.
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Characteristics of Aged Silicone Rubber Insulators Used in Outdoor for 20 Years Reviewed
19 ( 4 ) 454 - 460 2024.1
Authorship：Last author Language：English Publishing type：Research paper (scientific journal)
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Degradation Monitoring of Silicone Rubber Used for Insulators Reviewed
May Thin Khaing, Kosei Yoshimura, Takuma Miyake, Tatsuya Sakoda, Shigehiko Goto, Tomikazu Anjiki, Tomoki Chiba, and Yasunori Kasuga
23rd Int. Symposium on High Voltage Engineering (ISH) 866 - 870 2023.8
Authorship：Last author Language：English Publishing type：Research paper (international conference proceedings)
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Formation of Degradation Layer on Silicone Rubber Used for Insulators Reviewed
Khaing M.T., Yoshimura K., Sakoda T., Goto S., Anjiki T., Chiba T., Kasuga Y.
2023 IEEE Electrical Insulation Conference, EIC 2023 2023.6
Language：English Publishing type：Research paper (international conference proceedings) Publisher：2023 IEEE Electrical Insulation Conference, EIC 2023
Silicone rubber (SiR) insulators have been increasingly used for transmission and distribution systems. However, it is concerned that performance and property of SiR insulator in operation are degraded due to multiple stresses. We now aim to develop a diagnosis technique of degradation degree of SiR insulator exposed to the multiple stresses. In order to investigate the degradation characteristics and establish the diagnosis technique, aged specimens are required. However, it is difficult to collect a lot of aged SiR insulators used for more than several decades. Therefore, we tried to fabricate the artificial degradation surface, which is similar to ones obtained from actual fields. To analyze the surface degradation of actually aged SiR insulators, hydrophobicity class (HC) of 22kV-SiR insulators arranged for 10 and 20 years in coastal areas was first evaluated. Next, considering the multiple stresses, the degradation surface was fabricated by red clay, UV ray irradiation and surface heating. Aged SiR specimens and artificially degraded SiR spmecimens were characterized by using a contact angle meter, a scanning electron microscopy (SEM), and an attenuated total reflectance-Fourier transform infrared spectrometer (ATR-FTIR). The hydrophobicity of artificially degraded SiR specimen can be decreased due to red clay adhesion/ surface pollution on its surface as the similar condition as actually aged SiR specimen. It was also found that Si-O bond, Si-CH3 bond and C-H bond can be controlled by UV irradiation and the heating temperature.