Masanori Kohtoh

Aging effect on electrical characteristics of insulating oil in field transformer

To ensure long-term reliability of transformers, it is important to identify the degradation characteristics of insulating oil in long-term operations, which are the dominant factors of the transformer dielectric strengths. Analysis on aged field insulating oil was conducted to identify how insulating oil in transformers changes and deteriorates with increasing age and what are the impacts on the electrical characteristics such as breakdown voltages. Insulating oil samples were collected from a total of 98 transformers, varying significantly with six manufacturers, manufacture years of 1956 to 1999, and voltage classes of 66 kV to 500 kV. Electrical, physical and chemical characteristics were obtained, and the impact of increasing age and the relationships among characteristics were evaluated. The characteristics found to deteriorate with increasing age were volume resistivities, dielectric loss tangents, interfacial tensions, and total acid values. These characteristics can be used as effective indexes for trend monitoring to identify aging statuses and detect abnormalities at an early stage. The physical characteristics (kinetic viscosities, densities, and flash points) showed different tendencies depending on the oil type and age. Since the physical characteristics depend on the compositions of insulating oil, the possible causes for this result are that the compositions of insulating oil were different between oil types and that the compositions varied in accordance with increasing age. Furthermore, the correlations between characteristics were evaluated. As a result, a correlation was found between volume resistivities and total acid values, and good correlations were found between interfacial tensions and volume resistivities and total acid values. Interfacial tensions, which change with a higher rate than total acid values, can be possibly used as an effective index for insulating oil degradation.

Transformer insulating oil characteristic changes observed using accelerated degradation in consideration of field transformer conditions

To ensure soundness of highly aged transformers in their operations, it is important to identify how aging influences insulating oil in these transformers. The characteristics of insulating oil are roughly classified into electrical, physical, and chemical characteristics. In the past research on insulating oil in transformers, some of the characteristics showed a downward trend with increasing age. Among the electrical characteristics, the volume resistivity and the dielectric loss tangent showed a downward trend with increasing age, which is likely to reflect degradation that accompanies aging. The analysis of insulating oil components found some components that increased with age, which may contribute to degrading electrical characteristics of insulating oil. This study on field transformers gave results obtained through comparison of different samples in relation to the number of years in which they were in the field. Detailed evaluation of aging requires study on aging patterns using the same samples. In this paper, heat-accelerated degradation test was conducted on the same samples, and the patterns of changes in characteristics were studied. In addition, the influences of interactions between characteristics and water and between characteristics and pressboard were examined. The study using accelerated degradation test was conducted on one brand of mineral oil (α1). The obtained characteristics were the breakdown voltage, volume resistivity, interfacial tension, total acid value, and water content, i.e., electrical characteristics and characteristics considered to be correlated with electrical characteristics were extracted. As a result of examination, characteristic deterioration was evident for the water content, breakdown voltage, volume resistivity, total acid value, and interfacial tension in descending order of deterioration. The characteristics showed evident deterioration in the initial stage of accelerated degradation test and, thereafter, showed a saturation tendency with increasing age.

Preparation and preliminary characteristic evaluation of epoxy/alumina nanocomposites

Epoxy/alumina nanocomposites were newly prepared by dispersing 3, 5, and 7 weight % nano-scale boehmite alumina particles in a bisphenol-A epoxy resin. Dispersion of alumina particles in the nanocomposite specimen was observed by scanning electron microscopy (SEM). Flexural properties, dynamic viscoelasticity, permittivity, partial discharge (PD) resistance, and electrical breakdown time were investigated for the nanocomposite specimens in comparison with those of an epoxy resin without nanofillers. The following results were obtained. The nanocomposite specimens keep high transparency similar to the pure resin. Alumina particles with a size below about 50 nm are homogeneously dispersed in the epoxy matrix. Due to dispersion of the nanofillers in the specimens, flexural properties, PD resistance, and electrical breakdown time are improved. The nanocomposites exhibit almost no change in glass transition temperature and permittivity.

Analysis results for insulating oil components in field transformers

To identify the aging characteristics of oil-immersed transformer oil in long-term operations, it is important to survey various characteristics of insulating oil of field transformers. The insulating oil characteristics defined in various standards can be roughly classified into electrical, physical, and chemical characteristics. In a past study, the authors identified through field survey what kind of characteristics receive significant impacts as a result of insulating oil aging and evaluated the characteristics as possible candidates for indexes of aging. This paper examines the elements that influence the characteristics of transformer insulating oil due to aging. The electrical characteristics (breakdown voltages, volume resistivities, and dielectric loss tangents) of insulating oil are affected by such elements as water contents and electrostatic charges and possibly by trace components in the insulating oil. The components detected in field transformer oil are methyl acetate, 2-methylfuran, phenol, methyl formate, furan, methanol, ethanol, acetone, isopropyl alcohol, and methyl ethyl ketone. Five of these components, i.e., methanol, ethanol, acetone, isopropyl alcohol, and methyl ethyl ketone, are alcohols and ketones. These five components are generated by age-related oxidation. The five components, except for methanol, was found to have age-related increasing tendencies. Methyl acetate, 2-methylfuran, phenol, methyl formate, and furan are considered to be components mainly generated by the reason of insulating paper aging. Whereas 2-methylfuran and furan were found to have age-related increasing tendencies, other components were found to have none. Analysis of insulating oil components is effective in diagnosing the aging status of insulating oil and the presence or absence of abnormalities. This paper summarizes the result of survey on the relationships between the concentrations of components and the ages of insulating oil based on the analysis results of the oil components.

Investigation of electrostatic charging mechanism in aged oil-immersed transformers

Electrostatic charging mechanism in aged oil-immersed transformers is investigated. Sulfonium ions, which are generated by sulfoxide compounds and hydrogen ions, are identified as the prime compounds that increase electrostatic charging tendency (ECT) of mineral insulating oil. Sulfoxide compounds are generated by the oxidation of sulfide compounds that are contained in new oil. Hydrogen ions are considered to be supplied by organic acids that are generated due to the oxidation of hydrocarbons. Negative ions with low mass are generated by the dissociation of organic acids during the generation of sulfonium ions. Because these negative ions are hydrophilic, the oil is charged positively. Water is also generated by the deterioration of insulating oils and has the same effect. On the other hand, negative charging of oil is detected with the addition of sulfonic acids, which reflects the generation of positive ions with low mass and hydrophility.

Surface breakdown characteristics of silicone oil for electric power apparatus

This paper describes the surface breakdown characteristics of silicone oil which has the possibility of application to innovative switchgears and transformers as an insulating medium. At the first step, we have experimentally studied the impulse breakdown characteristics of the configuration with a triple-junction where a solid insulator is in contact with the electrode. The test configuration consists of solid material (Nomexreg or pressboard) and liquid insulation oil (silicone or mineral oil). We have discussed the experimental results based on the maximal electric field at the triple-junction. As the second step, we have studied the configuration which may improve the surface breakdown characteristics by lowering the electric field on the insulator surface

Dielectric properties of biodegradable polylactic acid and starch ester

In order to examine the applicability of biodegradable polymers to the fields of electrical insulation, several dielectric properties of two typical biodegradable polymers, polylactic acid (PLA) and starch ester (SE), are examined. A fairly larger amount of space charge is accumulated in both polymers in comparison to low-density polyethylene (LDPE). This seems partly due to the presence of hydroxyl and carbonyl groups in these polymers. Permittivity and conductivity are higher in SE than PLA that has the values close to those of LDPE. The dielectric breakdown strength is lower in SE and is higher in PLA than LDPE. As for the resistance to photodegradation by ultraviolet photons, SE is stronger than PLA, although the two are much inferior to LDPE.

Suppression of increase in electrostatic charging tendency of insulating oil by aging used for power transformer insulation

Additives to insulating oil and oil treatment by absorbents are investigated by thermal aging tests with the aim of suppressing flow electrification in power transformer insulation. 1, 2, 3-benzotriazol (BTA) is found to be effective for suppressing the increase of electrostatic charging tendency (ECT) in oils that contain either sulfide or sulfoxide compounds. Sulfide compounds are contained in new oil while sulfoxide compounds are generated by the oxidation of sulfide compounds. However, 2, 6-ditertiary-butyl paracresol (DBPC) is only effective for oils that contain sulfide compounds. The addition of sulfoxide compounds significantly increases the ECT of DBPC-added oil by the thermal aging tests. DBPC can suppress the conversion of sulfide to sulfoxide by blocking the generation of peroxide. However, DBPC has negligible effect on suppressing the increase in ECT by sulfonium ions, which are generated by the oxidation of sulfoxide. Negatively charged BTA ions, which are generated by the dissociation of hydrogen ions from BTA molecules, can mitigate the increase in ECT by sulfonium ions, which charge insulating oil positively. The effect of BTA is found to be lost when the concentration in oil decreases due to the adsorption on solid insulators. Oil treatment by absorbents such as clay, activated carbon and silica gel is found to remarkably decrease the ECT of deteriorated oil. However, additional thermal aging test increases the ECT to the same level as before the oil treatment. Although the additives and the oil treatment by absorbents are effective for suppressing and decreasing the ECT of insulating oils, their long-term effect is not assured. Thus periodical measurements of the ECT of insulating oils are crucial for the sake of preventive maintenance of aged transformers.

Influence of Diverse Compounds on Electrostatic Charging Tendency of Mineral Insulating Oil used for Power Transformer Insulation

Some cases reported recently indicate flow electrification that becomes obvious due to aging deterioration of insulating oil and cellulose insulation in aged oil-immersed transformers. It is supposed that the inclusion of minor components influences the aging deterioration of mineral insulating oil, but details including the aging mechanism have not been clarified. This paper examines the influence of various compounds on the electrostatic charging tendency (ECT) of insulating oil. Mineral insulating oil mainly consists of hydrocarbon compounds, but it also contains extremely small amounts of sulfur compounds, nitrogen compounds and oxygen compounds. Sample oil was prepared by adding various compounds to synthetic oil suffering from small aging changes of the ECT. After heating the sample oil, the ECT was measured, and the influence of various compounds was examined. Aging changes of the ECTs were small in pure hydrocarbon compounds. On the other hand, increases by aging were detected in the ECTs of the sample oils to which impurities such as sulfur compounds and nitrogen compounds were added. In particular, outstanding increases were detected in the ECTs of sample oil to which sulfide compounds and sulfoxide compounds classified as sulfur compounds were added. It has been estimated that sulfur compounds influence the increase by aging of the ECT of mineral insulating oil. It has been also estimated that the increase by aging of the ECT of mineral insulating oil starts from sulfide compounds contained in new oil and that the ECT increase process triggered by sulfide compounds progresses by the production of sulfoxide compounds and compounds of high ECT.

Effect of glass transition on conduction current in biodegradable poly-L-lactic acid [electrical insulation]

The temperature dependence of electrical conduction current in additive-free poly-L-lactic acid was measured. Although the current obeys the Arrhenius formula, a hump appears under some conditions, depending on whether the temperature is being ascended or descended, and also on whether the sample was poled or short-circuited prior to the measurements. That polar groups consisting mainly of carbonyl groups in the sample changed their directions was confirmed by Fourier-transform infrared spectroscopy. The permittivity increases around 74/spl deg/C. From these results, it is confirmed that orientational polarization or depolarization occurs around 74/spl deg/C by the glass transition, and that its resultant current and the conduction current overlap with each other.