Tomohiro Kawashima

Dielectric permittivity characteristic of mesoporous-alumina/epoxy composite

This paper presents an attempt to derive the dielectric permittivity characteristics of polymer composite filled with the metal oxide particle which has mesoporous structure. Experiments were carried out on the epoxy composites filled with alumina microparticles which have the mesoporous structure (mesoporous-alumina/epoxy composites) with different particle content. Measurement of the specific gravity of mesoporous-alumina/epoxy composites indicated that the porosity of mesoporous-alumina particle in the epoxy matrix was higher than that of nonporous-alumina particle. Furthermore, we evaluated relative permittivity of mesoporous-alumina/epoxy composites by measuring the capacitance of its specimens. As the results, we verified that the permittivity of mesoporous-alumina/epoxy composites was lower than that of nonporous-alumina/epoxy composites due to the particle porosity.

Dielectric breakdown mechanism of polypropylene laminated paper in liquid nitrogen

Breakdown characteristics of PPLP (Polypropylene laminated paper) impregnated with LN2 for its application to high-temperature superconducting cable are presented. In this paper, we investigated the breakdown characteristics of PPLP, which was the three-laminated structure of kraft paper (KP), polypropylene film (PP film) and KP, in LN2 by comparison with those of the papers with different laminated structure, e.g., single-layer of KP, single-layer of PP film and the 2 layer laminated paper of KP and PP. The breakdown characteristics under the application of impulse voltage, ac voltage and dc voltage were obtained, and the breakdown mechanisms were discussed. Experimental results revealed that the breakdown strength of PPLP under the application of impulse voltage and dc voltage was higher than that of the single-layer material. This could be attributed to the dispersion effect of the streamer by KP layer of PPLP. In order to obtain the unified explanation for the breakdown mechanism with the different voltage polarity and the different laminated structure, we suggested the injection model of the negative charge into the KP layer of PPLP and the less-injection model of the charge into the PP.

Specific gravity and dielectric permittivity characteristics of mesoporous-alumina/epoxy composite

Introduction of metal oxide nanoparticles to polymer composite material is known to have unique dielectric behavior and significant advantage in the electrical insulation performance of electrical power apparatus. This paper presents an attempt to derive the dielectric characteristics of polymer composite filled with the metal oxide particle which has mesoporous structure. Experiments were carried out in the epoxy composites filled with alumina microparticles which have the mesoporous structure (mesoporous-alumina/epoxy composites) with different particle content. The measurement of the specific gravity of mesoporous-alumina/epoxy composites confirmed the existence of pore inside the mesoporous particles even in the epoxy matrix. Furthermore, we evaluated relative permittivity of mesoporous-alumina/epoxy composites by measuring the capacitance of the specimens. As the results, we verified that the permittivity of mesoporous-alumina/epoxy composites was lower than that of nonporous-alumina/epoxy composites.

Influence of DC prestress on impulse breakdown strength of insulating paper-liquid nitrogen composite system

Breakdown characteristics of PPLP (Polypropylene laminated paper)® impregnated in liquid nitrogen (LN2) are presented. PPLP has three-laminated structure of kraft paper (KP), polypropylene film (PP film) and KP and has been used as an electrical insulating material in high-temperature superconducting cables. In this report, we investigated the influence of DC prestress on impulse breakdown strength of KP, PP film and PPLP in LN2. We performed DC prestress test in which DC voltage of 10 kV was applied as prestress and then impulse breakdown voltage was superimposed for the breakdown test. By the application of DC prestress of homo polarity with impulse voltage, the impulse breakdown strength of KP and PP film was found to be increased. On the other hand, by the application of DC prestress of hetero polarity, the breakdown strength of KP was found to be increased, which was unique to paper material. We proposed the negative charge injection model into KP to explain this result. Furthermore, the breakdown strength of PPLP was almost the same under the application of DC prestress of 10 kV. The influence of DC prestress on impulse breakdown strength appeared to be different depending on the specimen, and the possible model at this moment was discussed.

Influence of prestressing on the breakdown of insulating paper-liquid nitrogen composite system

We have been studying a cryogenic electrical insulation system with polypropylene laminated paper (PPLP ® ) for high-temperature superconducting cable. PPLP ® , which has the three laminated structure of a kraft paper (KP), polypropylene (PP) film and a KP, has been reported to have excellent insulation performance in liquid nitrogen (LN 2 ). However, questions still remain in breakdown mechanism of PPLP ® due to its laminated structure. In this paper, it is investigated the influence of DC prestressing on the impulse breakdown characteristic of KP and PP film to understand basic breakdown characteristic of PPLP ® in LN 2 . Furthermore, DC prestressing effects on impulse breakdown strength (Fb) of KP and PP film was investigated, and the charge injection model was discussed. As the result, the Fb of KP and PP film under hetero polarity with DC prestressing field application did not decreased compared with normal impulse Fb, which is different from the conventional DC prestressing effects. Furthermore, it is particularly interesting that the Fb of KP increased under the positive DC prestressing field with negative impulse field in spite of hetero polarity DC prestressing field application. This result could be explained based on the charge injection model, in which negative charge may be easily injected inside KP through cellulose fiber interface.

Electrical breakdown of dielectric elastomer and its lamination effect

Dielectric elastomers have potential as materials for artificial muscle actuators due to their excellent properties such as contractility. To understand the breakdown mechanism, breakdown tests of the dielectric elastomers were performed under different applied field waveforms (dc, ac, or impulse). In addition, the influence of the lamination number on the breakdown strength was investigated. Regardless of the applied field waveform, the breakdown strength decreases as the temperature increases. For a given temperature, the breakdown strength has the following order: impulse breakdown strength > dc breakdown strength > ac breakdown strength. Heating due to the dielectric loss under ac voltage application is considered to reduce the ac breakdown strength displayed at the RMS value, suggesting that a thermal process is responsible for breakdown. Under all applied waveforms, the breakdown strength increases as the lamination number increases.

DC prestressing effect on breakdown characteristic of insulating paper in liquid nitrogen and insulating oil

Currently, high temperature superconducting (HTS) cables are developed mainly as AC cables with PPLP®-liquid nitrogen composite insulating system. On the other hand, solar and wind power are expected as alternative energy of fossil fuels in the future. DC transmission using superconducting technology having low transmission loss and environmental burden is most suitable for such new energies. However, DC HTS cable may raise the problem that charge injection may degrade the insulating performance, which is the same as the problem in existing DC oil immersion cables. Moreover, the influence of charge injection on breakdown characteristics of PPLP® in liquid nitrogen (LN2) is not clarified yet. In this paper, DC prestressing effects on impulse breakdown strength (Fb) of PPLP® was investigated, and the charge injection model in LN2 was discussed from the conventional knowledge to charge injection in DDB oil. As a result, the opposite polarity relations in impulse Fb and the prestressing effects in LN2 appeared compared with the results in DDB oil. The DC prestress effect on impulse Fb in DDB oil can be explained based on the charge injection model in LN2 with the opposite polarity charge injection. Therefore, the difference of breakdown characteristics of PPLP® in DDB oil and in LN2 are influenced by the polarity of the charge that is easy to inject into KP layer and can explain using the charge injection model developed for the breakdown characteristics in LN2.

Partial discharge inception voltage under surge voltage application influenced by surface charge on insulating sheet

To understand the discharge characteristic under surge voltage application, the partial discharge inception voltage (PDIV) was investigated by using sphere-plane electrode under surge voltage application. Polyimide sheet with thickness of 0.125 mm was placed on the plane electrode and the air gap between the spherical electrode and the polyimide (PI) sheet was set to 0.1 mm. As the result, PDIV under surge voltage application was higher than that under DC voltage application. In addition, in both surge voltage application and DC voltage application, the negative PDIV was lower than positive one. This is because that the appearance probability of the initial electron is changed by voltage waveform and voltage polarity. In case of PI with surface charge, PDIV was changed according to surface potential value and PDIV without surface charge was almost the same as PDIV subtracted the surface potential value. From this result, the measurement scatter of PDIV might be small by considering the surface charge condition.

Study on charge dynamics in water-tree XLPE insulation for deterioration diagnosis

Water tree is one of the most significant mode of degradation of XLPE insulation cables which leads to the failure. We have successfully proposed pulse method to detect and locate water tree degradation by applying bias (poling) and probing pulse voltage. Based on the proposed method, the degraded water-tree XLPE cable was detected. We studied on the mechanism of residual charge related to water tree degradation by using the XLPE film sample contained water tree. The decay process of charge under application of pulse voltage was explained by Pool-Frenkel emission process. The result of the numerical simulation was demonstrated. Assuming trapping and de-trapping process, the differences in pulse response was explained depending on the degree of water tree degradation.

Study on short time charge behavior in pulsed residual charge method for water tree diagnostics of XLPE cables

Water tree generated in insulation parts of the XLPE (Cross Linked Polyethylene) cable is concerned as one of the causes of deterioration or breakdown. For the stable supply of electric power, the facilitated and accurate diagnosis technique is important to measure the deterioration of power cable. In our previous study, we proposed a residual charge measurement method using pulse voltage (charge larder method) and suggested that this method makes it possible the degradation position to measure more accurately than conventional methods. In this method, the deterioration signal is detected when the charge trapped at the tip of water tree is depolarized by the short time pulse voltage. However, quantitative evaluations have not been studied yet. In this paper, we investigated the residual charge behavior around the water tree under pulse voltage application and performed the numerical analysis using Maxwell-Wagner model.