Takaaki Kano

Creeping discharge developing on vegetable-based oil / pressboard interface under AC voltage

The behavior of creeping discharges developing on the pressboard surface was investigated experimentally in palm fatty acid ester oil, natural rapeseed oil and commercial mineral oil under variable ac voltages with 60 Hz, respectively. The oilimpregnated pressboard was used as the solid dielectric and it was immersed completely into the same oil. The tungsten needle electrode was installed in the one side of pressboard surface without the counter electrode as a high voltage electrode and a grounded fine copper rod was adhered tightly in the other side of pressboard as a back side electrode. The discharge shape, streamer extension, streamer velocity and tracking at the oil/pressboard interface were observed as a function of the time and voltage by using a still camera equipped with a night viewer. Simultaneously, the discharge current and dissipated energy were measured. The gas components dissolved in oils by the creeping discharge were also examined by the dissolved gas-in-oil analysis (DGA). In this paper, it is presented that there are distinctive events due to the occurrence of creeping discharges.

Study on the oxidative stability of palm fatty acid ester (PFAE) as an insulating oil for transformers

Today, esters obtained from plant oils are attracting attention as insulating oils from the viewpoints of renewable resources and low environment impact in the event of leakage. We have developed a palm fatty acid ester (PFAE) synthesized from palm/coconut oil for use as an environmentally friendly insulating oil. PFAE is expected to be an alternative to mineral oil because of its high fluidity and high dielectric characteristics. On the other hand, because insulating oil for transformers is used over several decades, high oxidative stability is required. In our study, we investigate oxidative stability with regard to the electro-chemical characteristics of PFAE compared to another vegetable oil. As a result, the breakdown voltage and total acid value of PFAE showed less change than the other oil. We discuss differences in the oxidation stability of PFAE and it superiority over the other vegetable oil.

Partial discharge properties of ester oils having different molecular structures

Partial discharge (PD) properties and dissolved gas analysis (DGA) are investigated on three kinds of fatty acid alkyl esters synthesized from palm oil, fatty acid methyl esters synthesized from soybean oil and from rice bran oil with low viscosity to be adopted in transformers. For comparison, the properties of rapeseed oil of vegetable oil having high viscosity are also investigated. The results revealed that the number of PD and average PD charge depended on the molecular structure of the ester oils. It is indicated that the number of PD of large molecular ester oils and ester oils containing double bonds at hydrocarbon part is relatively few. PD in the ester oils with high viscosity were triggered by bubble. DGA showed that there were no remarkable acetylene gases by PD in the ester oils, though hydrogen gas was generated. CO and CO2 were also generated in the ester oils, especially remarkably generated by PD in ester oils containing double bonds at hydrocarbon part.

Comparative study of partial discharge characteristics and dissolved gas analysis on palm-based oil as insulating material

Partial discharge (PD) properties and dissolved gas analysis (DGA) are investigated on two kinds of fatty acid alkyl esters synthesized from palm oil with additive, namely Pastell 2H-08 and Palm Fatty Acid Ester (PFAE) to be adopted in power transformers. For comparison, the properties of mineral oil which is commercially used transformer oil were also investigated in the research works. The results revealed that PD repetition rate number of the ester oils with additive increase remarkably at above 15 kVrms of voltage application than that of mineral oil. PD charge of ester oils with additive tends to saturate at about 10,000 pC, while PD charge of mineral oil seems to keep increasing with increasing the applied voltage at above 20 kVrms. The amounts of generated hydrocarbon gases in ester oils with the additive are higher than that of mineral oil. It is found that combustible gas pattern of the ester oils with the additive are different from that of conventional mineral oil. Duval analysis of ester oils with additive and mineral oil are classified as high energy electrical discharge, while for mineral oil is classified as low energy electrical discharge.

Consideration on the relationship between dielectric breakdown voltage and water content in fatty acid esters

Recently, fatty acid esters synthesized from the natural oil obtained from plants have attracted much attention as an insulating oil from the viewpoint of environmental problems, fluidity, and greater resistance to dielectric breakdown than conventional mineral oil. In the present study, to clarify the reason for the superior moisture tolerance of fatty acid esters, the inter- and intramolecular movements of fatty acid esters and the situation of dissolved water in the esters are investigated using nuclear magnetic resonance (NMR) and infrared (IR) spectroscopic measurements and density functional theory (DFT) calculations; it became clear that water molecules in fatty acid esters are trapped by the ester group of fatty acid esters. This is why fatty acid esters have a higher moisture tolerance against electrical breakdown than mineral oil.

Attempts to measure ion mobilities in EHD liquids by the time-of-flight method

Two different kinds of attempts were made to measure ion mobilities in EHD liquids by using the time-of-flight technique. The first attempt was a reverse polarity method which was similar to that used by L. Yang et al. [5]. The stainless steel coaxial cylindrical electrodes with the effective length of 55 mm and the electrode spacing of 1.5 mm were utilized. The purified two sample liquids; hydrofluoroether (HFE7600) and 2, 3-dihydrodecafluoropentane (HFC43-10) with very low dark current (less than 20 pA at 150 V) were used. In this attempt, it is deduced that the mobility value is of the order of 1×10-8 m2/Vs. In the second attempt, the plate-mesh-plate electrode configuration was employed. One plate was used to apply high voltage pulses and the other plate worked as a counter electrode. The center mesh electrode was kept at low dc voltages. This triode electrode design allows us to monitor the amount of charge injected from the charge injection space to the charge detection space through the mesh electrode during and after the high voltage pulse. Two sample liquids; HFE 7600 and HFC43-10 were used without any purification. Ion drift currents were found to have very long tails. If the tails are included into the drift times, the mobility of ions can be as low as approximately 1×10-9 m2/Vs in both liquids.

Comparative study of the insulation paper aging in palm fatty acid ester (PFAE) and mineral oil

For this study, we conducted accelerating experiments to investigate the aging of paper in PFAE and mineral oil. After the aging test, we evaluated paper tensile strength, water content of oils, and acid value of oils. As a result of the paper aging test, Kraft paper was found to degrade slower in PFAE than in mineral oil in a nitrogen atmosphere. Besides, we examined the relationships between the water contents of PFAE and paper. This examination revealed that water migrated readily from paper to PFAE. In addition, we measured the infrared spectroscopy of PFAE having various water contents. Consequently, it is suggested that water molecules are trapped by the ester group. These results indicate that water migrates readily from paper to PFAE.