Mineral Oil Paper Research Articles

Thermal Index Estimation of Thermally Upgraded Kraft Paper in Mineral Oil and Natural Ester Insulating Liquids Under Accelerated Aging Conditions

The thermal index for thermally upgraded Kraft paper was determined for a series of five mineral insulating liquids and two natural esters using IEEE Standard C57.100 Annex A.4.2 and compared with the values in IEEE Standard C57.91.

Fracture toughness as an alternative approach to quantify the ageing of insulation paper in oil

Oil-immersed transformers use paper and oil as insulation system which degrades slowly during the operation of these machines. Cellulose materials are used generally as insulation solid in power transformers. The degree of polymerization (DP), defined as number of repeating β-glucose residues in the cellulose molecule, is a critical property of cellulosic insulation material used in transformers, since it provides information about paper ageing and its mechanical strength. The fast-developing electric power industry demanding superior performance of electrical insulation materials has led to the development of new materials, as well as different drying techniques performed during transformer manufacturing and service when required. Both developments have caused some practical difficulties in the DP measurement. Moreover, the increasing interest in synthetic dielectric materials replacing cellulose materials requires measuring alternative properties to the DP to quantify the degradation of insulation solids over time. In this sense, this paper proposes the possibility of analyzing paper degradation through fracture toughness. This approach is different from the study of mechanical properties such as tensile strength or strain because it provides a tool for solving most practical problems in engineering mechanics, such as safety and life expectancy estimation of cracked structures and components which cannot to be considered through the traditional assessment of the mechanical resistance of the material. An accelerated thermal ageing of Kraft paper in mineral oil was carried out at 130 °C during different periods of time, to obtain information on the kinetics of the ageing degradation of the paper. Double-edged notched specimens were tested in tension to study their fracture toughness. The evolution of the load–displacement curves obtained for different ageing times at the ageing temperature of 130 °C was utilized to the determination of the stress intensity factor. Furthermore, different kinetic models based on this stress intensity factor were applied to relate its evolution over time as a function of the temperature. Finally, the correlation between the DP and stress intensity factor, which depends on the fiber angle, was also defined.Graphic abstract

Study of Thermallay accelerated aging of Transformer Insulation Paper in Mineral Oil and Gas-to-Liquid Dielectrics using Scanning Electron Microscopy (SEM), Thermogravimetric Analysis (TGA) and X-ray Diffraction (XRD)

Study of Thermallay accelerated aging of Transformer Insulation Paper in Mineral Oil and Gas-to-Liquid Dielectrics using Scanning Electron Microscopy (SEM), Thermogravimetric Analysis (TGA) and X-ray Diffraction (XRD)

Quantification of Kraft paper ageing in mineral oil impregnated insulation systems through mechanical characterization

Power transformers use Kraft paper, thermally upgraded Kraft and other polymeric papers (ex. Nomex) as the main solid insulation between the winding conductors. Dielectric oil used in transformers as an insulating and cooling fluid typically has an operating temperature range of 60–90 °C. These service temperatures can cause slow degradation of both the oil and the insulating paper winding, with a loss of mechanical and dielectric properties. In this sense, this work analyzes paper degradation through Young’s Modulus, yield stress, rupture strength and strain under ultimate strength. An accelerated thermal ageing of the paper in mineral oil was carried out at temperatures of 110, 130 and 150 °C over different periods of time, in order to obtain information on the kinetics of the ageing degradation of the paper. The evolution of the mechanical properties and micro mechanisms of paper failure are analysed as a function of temperature and ageing time. Finally, the results obtained are compared with the traditional method of degradation analysis, based on the degree of polymerisation measurement.

Fourier Transform Infrared (FTIR) Spectroscopy Analysis of Transformer Paper in Mineral Oil-Paper Composite Insulation under Accelerated Thermal Aging

Mineral oil is the most popular insulating liquid for high voltage transformers due to its function as a cooling liquid and an electrical insulator. Kraft paper has been widely used as transformer solid insulation for a long time already. The degradation process of transformer paper due to thermal aging in mineral oil can change the physical and chemical structure of the cellulose paper. Fourier transform infrared (FTIR) spectroscopy analysis was used to identify changes in the chemical structure of transformer paper aged in mineral oil. FTIR results show that the intensity of the peak absorbance of the O–H functional group decreased with aging but the intensity of the peak absorbance of the C–H and C=O functional groups increased with aging. Changes in the chemical structure of the cellulose paper during thermal aging in mineral oil can be analyzed by an oxidation process of the cellulose paper and the reaction process between the carboxylic acids in the mineral oil and the hydroxyl groups on the cellulose. The correlation between the functional groups and the average number of chain scissions of transformer paper gives initial information that the transformer paper performance can be identified by using a spectroscopic technique as a non-destructive diagnostic technique.

Thermal aging effects on the moisture equilibrium curves of mineral and mixed oil-paper insulation systems

Moisture equilibrium curves are well accepted methods for moisture estimation of insulation paper in field transformers. The thermal aging effects on moisture partition diagrams of mineral oil-paper and a new aging-resistant mixed oil-paper insulation system were studied. The purpose was to assess the moisture content accurately, particularly in transformers used for long-term operations. Thermal aging experiment was conducted at 130 °C for 80 days, and moisture equilibrium curves at five different aging degrees and three different temperatures were constructed. Results showed that under the same oil moisture content, the moisture content in paper decreased as the aging condition aggravated. The mass fraction of moisture content in oil and paper was calculated at each aging degree. Comparative results revealed that the moisture absorption capacity of insulation paper had an age-related decreasing tendency, whereas the moisture absorption capacity of insulating oil had an age-related increasing tendency. Moisture partition diagrams of mineral and aging-resistant mixed oil-paper samples were compared. Results showed that with the same oil moisture concentration, the moisture of insulation paper impregnated in mixed oil was remarkably lower than that impregnated in mineral oil.

EFFECT OF THERMAL AGEING ON THE OIL-PAPER INSULATION

Insulation system is one of the most important part of electrical devices. This insulation system is usually composed by solid part and liquid part. Oil impregnated insulation paper is used in transformers for many years. There are a lot of factors which have significant influence on the insulation system during the life time of electrical devices. Temperature is one of the factors which has strong influence on condition of oil paper insulation. This paper is focused on measuring of electrophysical properties of oil paper as relative permittivity, AC breakdown voltage and polarization index of insulation before and after thermal ageing. Vegetable oil paper insulation and mineral oil paper insulation were investigated and analyzed.

Thermal aging characteristics of insulation paper in mineral oil in overloaded operation of transformers

AbstractA sudden capacity increase in demand during the summer peak, or in contingencies such as malfunctioning transformers, may cause an overload in normal transformers. In this paper, on the basis of overloaded transformer operation in distribution substations, thermal aging testing in oil was carried out under various overload patterns, such as short‐term overload and long‐term overload, but with the winding insulation paper's lifespan loss kept constant. From the results, various characteristics, such as the mean degree of polymerization and production of furfural and (CO2 +CO), and their effects on the lifespan loss of the insulation paper, were obtained. © 2012 Wiley Periodicals, Inc. Electr Eng Jpn, 182(2): 1–8, 2013; Published online in Wiley Online Library (wileyonlinelibrary.com). DOI 10.1002/eej.21292

Characteristics of moisture diffusion in vegetable oil-paper insulation

Vegetable insulating oils are fire-resistant and environment-friendly liquid dielectrics. The current paper presents an experiment system to simulate moisture diffusion in the insulation system consisting of vegetable insulating oil and insulation paper. The results show the coefficient of moisture diffusion in vegetable oil-paper insulation. The present paper also discusses a new numerical method of solving the diffusion equation. Using the numerical method and the diffusion coefficient, the moisture contents in the insulation paper can be predicted. A practical and significant equilibrium time constant is proposed. Based on defining the equilibrium time constant, the relationships between equilibrium time constant and temperature, initial moisture content, and thickness of paper/pressboard are simulated. The moisture diffusion characteristics of vegetable oil-paper and that of mineral oilpaper are compared as well. The experiment and analysis results show that the diffusion rate of moisture in vegetable oil-paper insulation is about half of that in mineral oil-paper insulation.

Influence of vegetable oil on the thermal aging of transformer paper and its mechanism

With the development of new insulation materials, vegetable oil-the best substitute for mineral oil-has gradually been widely used in the liquid insulation of transformers. To investigate the influence of vegetable oil on the thermal aging rate of oil paper, transformer paper impregnated with mineral oil and vegetable oil underwent thermally accelerated aging at three different temperatures. Degree Polymerization (DP) of paper was measured to indicate the aging degree of transformer paper. The aging rate of paper in mineral oil and vegetable oil was compared quantitatively, and results showed that vegetable oil retarded paper's degradation rate and extended its useful lifetime. The reasons contributing to such phenomenon were analyzed using X-ray Photoelectron Spectroscopy (XPS) and molecular simulation software. Transformer paper in vegetable oil had larger activation energy. Due to the larger interaction force between and natural ester molecules, molecule was easily bonded with natural ester, weakening the hydrolysis process of cellulose. Cellulose was chemically modified by natural ester during thermal aging process, and the reactive -OH (hydroxyl) groups on the cellulose became esterified with fatty acid esters. The molecule was firmly bounded to the ester groups on glucose produced by esterification. The long-chain fatty acid esterified to cellulose was parallel with cellulose chains and acted as a water barrier to further weaken the hydrolysis process.

Evaluation of Aging for Thermally Upgraded Paper in Mineral Oil

AbstractThe life of a transformer is limited by the degradation of its solid insulation. Winding conductors and other solid insulation materials in oil-immersed transformers have been insulated using cellulose products. For many years, manufacturers have met the needs of special applications by designing transformers using thermally upgraded materials to achieve lighter weight, higher power density and increased life. Recently, the effects of thermally upgraded insulation on diagnostic techniques such as gas-in-oil analysis, and indicators of their insulation degradation, have been reviewed. This paper describes evaluations of amine-impregnated thermally upgraded paper insulation's thermal degradation characteristics and decomposition reactions in mineral transformer oil. The thermal resistance of the thermally upgraded paper is evaluated by comparison with Kraft paper insulation. Further, aging degradation mechanisms of decompositional degradation of the thermally upgraded paper due to aging in mineral t...

耐熱絶縁紙の油中加熱劣化特性と劣化メカニズムの推定

耐熱絶縁紙の油中加熱劣化特性と劣化メカニズムの推定

Vegetable oils, an alternative to mineral oil for power transformers- experimental study of paper aging in vegetable oil versus mineral oil

This article compares the thermal aging of insulating paper in mineral oil with that of paper in vegetable oil over the temperature range of 70 to 190°C and shows that the use of vegetable oil as an insulating liquid is a very promising option.