Oil-paper Composite Insulation Research Articles

Performance Assessment of Cellulose Paper Impregnated in Nanofluid for Power Transformer Insulation Application: A Review

Insulation cellulose paper is a basic measure for a power transformer’s remaining useful life, and its advantageous low cost, electrical, and mechanical properties have made it an extensive insulation system when impregnated in a dielectric liquid. Cellulose paper deteriorates as a result of ageing due to some chemical reactions like pyrolysis (heat), hydrolysis (moisture), and oxidation (oxygen) that affects its degree of polymerization. The condition analysis of cellulose paper has been a major concern since the collection of paper samples from an operational power transformer is almost impossible. However, some chemicals generated during cellulose paper deterioration, which were dissolved in dielectric liquid, have been used alternatively for this purpose as they show a direct correlation with the paper’s degree of polymerization. Furthermore, online and non-destructive measurement of the degree of polymerization by optical sensors has been proposed recently but is yet to be available in the market and is yet generally acceptable. In mitigating the magnitude of paper deterioration, some ageing assessments have been proposed. Furthermore, researchers have successfully enhanced the insulating performance of oil-impregnated insulation paper by the addition of various types of nanoparticles. This study reviews the ageing assessment of oil-paper composite insulation and the effect of nanoparticles on tensile strength and electrical properties of oil-impregnated paper insulation. It includes not only significant tutorial elements but also some analyses, which open the door for further research on the topic.

Effects of Oil Flow on Space Charge Behaviors in Oil–Paper Composite Insulation

Although the circulating flow of insulating oil can enhance heat dissipation in converter transformers, it can also influence the space charge distribution in oil–paper composite insulation. In this article, the oil–paper composite insulation between flat electrodes was used as the experimental model. Based on the pulsed electroacoustic (PEA) method, we investigated the effect of uncharged flowing oil on the space charge distribution in composite insulation under a dc electric field and polarity reversal. Under a dc electric field, as the oil flow velocity increased, the amount of interface charge between the oil and paper first increased and then decreased. During the polarity reversal process, the field distortion was aggravated at slower oil flows. These experimental results were explained in terms of the effect of oil flow on ionized charges in the oil.

Molecular insights into the liquid-solid behaviour of hydrocarbon and ester insulating liquids with cellulose polymer insulation

Environmentally friendly and biodegradable insulating liquids have broad application prospects in oil-immersed power transformers. The liquid–solid interface structure of insulating oil and cellulose polymers is a critical factor that determines the performance of oil-paper composite insulation widely used in oil-immersed power transformers. In this study, the molecular details about the dynamic behaviour of hydrocarbons and ester insulating liquids on the surface of crystalline cellulose are offered. The microscopic differences between the two insulating liquid and cellulose interfaces are compared based on the density distribution, dynamic structural properties, hydrogen bonds and interaction energy analysis of the liquid–solid interface. The width of the liquid–solid interface formed by the two insulating liquids and cellulose surfaces is different. At the same time, the deflection of different cellulose surface chain lengths and the torsion angle of hydroxymethyl groups are significantly different. These phenomena are mainly attributed to the interaction between the liquid–solid interface. The interaction between the surface of the mineral oil and cellulose is based on the van der Waals effect. Natural ester double-bonded O atoms and H atoms on the cellulose surface form more hydrogen bonds or have a greater electrostatic effect. Furthermore, the adsorption energy of mineral oil on the four cellulose surfaces is basically the same, and the average value is approximately 556 kcal/mol. The (100) and (1 –1 0) cellulose surfaces have a strong adsorption capacity for natural esters, while the (110) cellulose surface has the weakest adsorption capacity for natural esters. This study provides a theoretical basis for the more scientific application of natural esters and the development of new environmentally friendly insulating materials.

Enhanced voltage distribution and breakdown strength performances of oil‐paper composite insulation by adding TiO 2 nanoparticles

Oil–paper composite insulation (OPCI) is used as the main insulation material in the converter transformer. However, the dielectric properties of insulating oil and oil-impregnated paper are markedly mismatched under direct current (DC) voltage, resulting in an unbalanced distribution of voltage, which is considered to be a major cause of large insulation margin but frequent failures. Therefore, it is necessary to adjust the voltage distribution in OPCI to enhance its breakdown strength. In this work, surface-treated TiO2 nanoparticles were employed to modify OPCI. The polarisation current tests revealed that by changing the additive amount of nanoparticles, the polarisation intensities of both oil and oil-impregnated paper could be adjusted. Based on this, a model taking relaxation polarisations into full consideration was established to analyse the voltage distribution in OPCI and predict the optimal concentration. The prepared OPCI with the concentration of 0.02 g/L has desired balanced voltage distribution due to the influence of nanoparticles on interface charge, and therefore high breakdown strengths under both DC voltage and polarity reversal voltage, which can be increased by 22.6% at most. This work puts forward a novel idea to improve insulation property of OPCI and provides a reference for broader application of nanomaterial in power equipment.

Polarization and depolarization characteristics of different structure oiled paper composite insulation under high applied voltage

Transformers are an important part of the power transmission and distribution process of power systems. Designing and manufacturing a stable transformer is essential for the stable operation of the entire power system. Accordingly it is critical for the stable operation of the entire power system to design and manufacture a stable transformer. In order to improve the composite insulation performance of the oil-paper in power transformer, studying the polarization characteristics of oil-paper composite insulation is essential. In this paper, the polarization characteristics of high external applied voltage and different oil-paper composite insulation structures are studied. In the three-electrode system, the cardboard has obvious polarization characteristics at the two poles, and the polarization characteristics are most obvious when the cardboard is at the high pressure pole. The initial value and the attenuation function of the polarization and depolarization current do not match the laws determined by the RC circuit. When the voltage is at a higher level, the difference between the initial values of the polarization and depolarization currents at different positions of the cardboard at the same voltage level is greater, and the polarity effects are more obvious. the above rules still exist when the distance between the electrodes is changed.

The temperature effect to the compound electric field distribution of oil-paper insulation

This paper adopts a typical double-layer oil-paper insulation structure model, and obtains the mathematical expression of electric field distribution of oil-paper composite insulation. On the basis of the mathematical expression of the electric field distribution of oil-paper combination insulation, the numerical calculation results of the electric field distribution of oiled paper composite insulation are obtained. It is found that the temperature has little effect on the field strength distribution in transformer oil and paperboard under high AC content, and the temperature is low in low AC content. It has a great influence on the distribution of field strength in oil-paper insulation, especially the field strength in transformer oil.

Failure of nano-modified oil impregnated paper under repeated impulse voltage: Effects of TiO2 nanoparticles on space charge characteristics

Since the first application of oil-paper composite insulation in power transformers 100 years ago, researchers are now trying to improve the performance of transformer oil by suspending nanoparticles in the dielectric. However, the performance of paper impregnated with nano-modified oil remains unclear. We previously found nano-oil impregnated paper had a good capability to sustain repeated impulses. The amount of space charges accumulated in the nano-oil impregnated paper is much smaller than that in fresh oil impregnated paper. We considered this behavior to be the main reason for the better electrical performance of the nano-oil-impregnated paper. In this work, based on those achieved results, we focus on the modification mechanism of nano-oil impregnated paper. We analyzed the effects of TiO2 nanoparticles on the relative permittivity and their relationship with the charge injection process. We also discuss the influence of TiO2 nanoparticles on charge trap states and the space charge behavior. The dielectric characteristics and trap energy of both nano-oil-impregnated paper and fresh-oil-impregnated paper are determined in this work.

A Study of Electrical Aging of the Turn-to-Turn Oil-Paper Insulation in Transformers with a Step-Stress Method

This study investigates the electrical aging characteristics of oil-paper composite insulation. In this paper, a coils model and test system for oil-paper transformer insulation are designed. The electrical aging tests of the model coils at room temperature and operating temperature are performed using the step-stress method. A new method for estimating the unknown parameters in the inverse power exponent model is proposed, and the inverse power model life formula expressed by voltage and electric field strength is obtained. Considering the electrical aging process at the operating temperature, the allowable field strength of the inter-turn insulation of oil-immersed transformers can be improved. The new parameter estimation method proposed in this paper can be used to calculate the unknown parameters of the step-stress solid insulation aging model. This method has a clear physical meaning and can be solved easily. When the stressed voltage is below a certain threshold value, the life of the transformer coil turn-to-turn insulation grease oiled paper model is longer at operating temperatures compared with that at room temperature; when the voltage is higher than the threshold, it presents the opposite tendency.

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.

Effect of temperature on frequency dependent dielectric parameters of oil-paper insulation under non-sinusoidal excitation

Frequency domain spectroscopy is a potential tool for non-invasive condition assessment of oil-paper insulation in power equipment. On the other hand, it is well known that the results of frequency domain spectroscopy are affected predominantly by temperature. Moreover, the voltage used for frequency domain spectroscopy is sinusoidal is nature. But the actual insulation system at site is stressed by voltage that may deviate significantly from sinusoid during operation. Considering these two aforesaid facts, in this paper, the response of oil-paper insulation has been studied at different temperatures using both sinusoidal as well as non-sinusoidal excitation. An experimental sample has been prepared in the laboratory that emulates the composite insulation of oil immersed type power transformer at site. The results of frequency response using sinusoidal voltage waveform at different temperature show good agreement with the conventional results that validates the scheme of the experimental setup. Some more information is obtained from the response of non-sinusoidal excitation that could help to get a better interpretation of the actual physical condition of oil-paper composite insulation.

Dielectric frequency response of oil-paper composite insulation with transient moisture distribution

Because the temperature and load of transformer are variable, the moisture is usually inhomogeneous in oil-paper, especially for the traction transformer. In order to research the dielectric relaxation behavior of the oil-paper composite insulation with an inhomogeneous moisture distribution, it was proposed that a theoretical model for dielectric medium with an arbitrary distribution of conductivity and dielectric constant. The dielectric frequency response of oil-paper with transient moisture distribution was simulated by using the proposed model. The results show that transient moisture distribution caused the concentration polarization(CP), which enhanced the low-frequency relaxation. The model is validated against experimental data.

Dielectric frequency response of oil-paper composite insulation modified by nanoparticles

The research on dielectric frequency response of oil-paper composite insulation modified by nanoparticles has been carried out in order to seek possible way to improve insulation properties thereof. Before and after nanoparticles being added, the dielectric frequency responses of oil-paper composite insulation with different moisture contents at different temperatures have been measured within the frequency from 0.1 mHz to 1 MHz. The Cole-Cole relaxation model has been used to analyzing the measured data. The results show that the variation rules of insulation condition can be indicated by the parameters of relaxation model. By adding nanoparticles, a new low frequency relaxation appeared.

Implementation of an Integrated, Portable Transformer Condition Monitoring Instrument in the Classroom and On-Site

The development of integrated, portable, transformer condition monitoring (TCM) equipment for classroom demonstrations as well as for student exercises conducted in the field is discussed. Demonstrations include experimentation with real-world transformers to illustrate concepts such as polarization and depolarization current through oil-paper composite insulation. The developed equipment has also been used to understand and illustrate the phenomenon of recovery voltage. Finally, the portability and robustness of the equipment enables students to collect data from transformers installed on-site for the purpose of validating the nature of curves obtained in real-world environments.