Vegetable Oil-paper Insulation Research Articles

Comparative analysis of methanol generation mechanism in different oil-cellulose insulation based on ReaxFF MD

Currently, methanol is used as an aging indicator to evaluate the aging condition of transformer insulating paper. Nevertheless, the generation mechanism of methanol in vegetable oil-paper insulation is still unclear, which is not conducive to further research on methanol analysis. In this study, three types of oil-paper insulation models of palm fatty acid ester, FR3 natural ester, and mineral oil are established by reaction molecular dynamics (ReaxFF MD) simulation. Firstly, the pyrolysis products of the three oil-paper insulation systems are described. Then the methanol generation pathways are analyzed using the elemental tracing method. There are two identical elementary reactions for the production of methanol in the three types of oil-paper insulation systems. One is that the methyl produced by pyrolysis combines with the hydroxyl group separated from the cellulose to generate methanol, and the other is that the hydroxymethyl separated from the cellulose combines with free hydrogen ion to form methanol. Based on experimental and simulation results, it is demonstrated that vegetable oil produced more methanol than mineral oil as aging progressed. In addition, it is further proved that methanol is co-generated from insulating oil and cellulose-insulating paper during the pyrolysis process. In other words, the methanol produced by the oil-paper insulation system is much higher than that of a single paper/oil insulation. Furthermore, the mineral oil-paper system produced more methanol than the other two vegetable oil-paper systems are observed. Therefore, this study provides a theoretical basis for the employment of methanol in the condition assessment of transformers with different insulating oils.

Micro-Mechanism Influence of Copper on Thermal Decomposition of Vegetable Oil-Paper Insulation Based on ReaxFF-MD

The ecological environment is increasingly damaged due to the impact of mineral oil extraction and leakage. As a kind of sustainable resource, a vegetable oil transformer has bright application prospects in future. However, the stability of vegetable oil has always been a problem to be solved. In addition, the metallic copper inside the transformer has a certain catalytic effect on the deterioration of vegetable oil. In this article, a copper–oil–paper insulation model is established. Based on the molecular dynamics simulation method, the catalytic mechanism of copper on the deterioration of oil-paper insulation is revealed from the microscopic level. Then, the effects of temperature, contact area, and oxygen on the catalytic effect of copper are discussed. The results show that copper could accelerate the decomposition of oil-paper insulation by attracting H atoms and O atoms in oil-paper. The increase in temperature accelerates the progress of the catalytic reaction and aggravates the deterioration of oil-paper insulation. The oxygen concentration has an important influence on the catalytic reaction of copper. As the oxygen concentration increases, the catalytic effect of copper is weakened. The abovementioned research could provide some theoretical reference for further exploration of effective oil-paper insulation deterioration protection technology.

Investigating a New Approach for Moisture Assessment of Transformer Insulation System

With the wide application of vegetable insulating oil in power transformers, it is essential to investigate the influence of moisture on the frequency domain dielectric spectrum (FDS) analysis in order to propose an accurate moisture assessment method for vegetable oil-paper insulation system. In this regard, various vegetable oil-paper samples are prepared in a laboratory environment. FDS curves for all samples are obtained at different moisture contents and operating temperatures. A new method to estimate the moisture content in insulation paper is proposed based on the low-frequency characteristic quantity. Also, the effect of operating temperature and water migration between the oil and paper insulation on the FDS analysis is investigated. Based on this analysis, a compensation factor is introduced to eliminate the influence of temperature on paper moisture assessment. Furthermore, an improved normalization method for the temperature effect on the FDS of vegetable oil-paper insulation with different moisture contents is proposed. The proposed moisture assessment and temperature normalization methods are validated thorough their application on an operating power transformer.

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.

Investigation on thermal aging characteristics of vegetable oil-paper insulation with flowing dry air

In order to investigate the thermal aging characteristics of oil-paper insulation in open-type transformers, a thermally accelerated aging experiment with flowing dry air is performed. The vegetable oil, NOMEX paper, pressboard, copper and silicon steel are sealed into the aging vessels and further aged, for 216 hours, by supply of flowing dry air at 170 °C. Several important parameters associated with the aging are observed and evaluated during the aging process. It is shown that the degree of polymerization (DP) of pressboard gradually decreases and fits well into the second-order kinematic model. Little deterioration of the NOMEX paper under 170 ° is confirmed by the tensile strength which stabilizes at a certain level after aging. In addition, the logarithm of the furfural concentration in oil is found to have a good linear relationship with DP of pressboard. Interestingly, when the aging time is about 150 hours and DP is 329, the aging process reaches an inflection point at which the DP approaches the leveling off degree of polymerization (LODP) value, the moisture content and the furfural content of oil begin to fall down while the acid number and the dissipation factor of oil increase with a faster rate. However, the viscosity of the fluid, one of the important parameters of transformer operation, does not change significantly. The obtained results demonstrate that exposure to air leads to serious oxidation of oil and rapid increase of acid and dissipation factor, and this way oxygen should be avoided for the open-type transformer filled with vegetable oil.

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.

A new mathematical model of moisture equilibrium in mineral and vegetable oil-paper insulation

Moisture equilibrium characteristics of oil-paper insulation systems are important for insulation condition evaluation of oil-filled transformers. This paper presents moisture equilibrium curves of mineral and vegetable oil-paper insulation systems obtained by both direct and indirect methods. A new mathematical model is proposed to determine moisture content in oil-paper insulation systems. Experiments on moisture equilibrium in oil-paper insulation systems are conducted. The results estimated through the mathematical model agree with experimental results. The moisture equilibrium curves between vegetable and mineral oil-paper insulation systems are presented and compared. The moisture content in the vegetable oil-impregnated paper is significantly smaller than that in the mineral oil-impregnated paper, when both types of insulating oils have the same moisture content.