Oil-paper Insulation Research Articles

Effect of electric field on bubble generation and dissolution characteristics in oil–paper insulation

AbstractThe interturn paper insulation of oil‐immersed power transformer windings, under the combined influence of electric fields, moisture, and conductor heating, will produce bubbles, which pose a significant threat to the insulation system. However, there is limited research on the characteristics of bubble evolution in oil–paper insulation under the influence of electric fields and the subsequent dissolution process. Based on the continuous observation of bubble size using electron microscopes, experimental and theoretical investigations into the formation and dissolution of bubbles under electric field conditions are presented. The effects of different field strengths on bubble evolution and dissolution characteristics were studied. The results showed that the electric field promoted both the generation and dissolution of bubbles, with a more pronounced effect observed at higher field strengths (below partial discharge (PD) inception electric field, hereafter referred to as PDIE). However, when the field strength exceeded PDIE, the bubbles tended to shrink and gradually increase in size. The changes in bubble volume were related to not only gas diffusion but also oxygen consumption and fault gases generation due to PD. A better understanding of the formation and dissolution characteristics of bubbles under varying field strengths is achieved. Furthermore, it also provides a reference for assessing the risk of bubble generation and conducting bubble‐related fault diagnosis during the overload operation of oil‐immersed power equipment.

Research on The Dielectric Property of Nano-doped Aramid Paper

Oil-paper insulation systems are widely used in high-voltage equipment. Reducing the dielectric constant of insulating paper can improve the dielectric coordination of the oil-paper combination system. At present, a large number of new synthetic insulation materials have been produced, and some new composite insulation papers have low dielectric constant and high electrical strength properties. This article studied the optimal process flow for preparing aramid paper in the laboratory, trial-produced laboratory hand sheets with three types of nanofillers at different doping concentrations, and conducted relative dielectric constant and dielectric loss tests to select Laboratory samples with the best performance; the main research results achieved in the paper are as follows: ①Researched and determined the optimal process flow for making aramid paper in the laboratory, explored the effects of different rolling pressures and rolling temperatures on the dielectric properties of the paper, and determined the optimal rolling temperature and temperature of meta-aramid paper. The pressures are 200℃and 0.3Mpa respectively. ②Meta-aramid paper doped with SiO2, TiO2 and x nanoparticles was prepared, and the relative dielectric properties of the three nanoparticles at the optimal doping amounts (15%, 3% and 4%, respectively) were compared. Constant and dielectric loss, it is determined that the dielectric properties of 4% C3N4 nanoparticle-doped paper are at a relatively excellent level. Compared with pure paper, its relative dielectric constant is not much different, and it can better cooperate with transformer oil. At the same time The dielectric loss is reduced, so nanoparticles of this type and concentration are used as fillers to improve the electrical properties of aramid paper.

Research Progress and Prospect of Condition Assessment Techniques for Oil–Paper Insulation Used in Power Systems: A Review

Oil–paper insulation is the critical insulation element in the modern power system. Under a harsh operating environment, oil–paper insulation will deteriorate gradually, resulting in electrical accidents. Thus, it is important to evaluate and monitor the insulation state of oil–paper insulation. Firstly, this paper introduces the geometric structure and physical components of oil–paper insulation and shows the main reasons and forms of oil–paper insulation’s degradation. Then, this paper reviews the existing condition assessment techniques for oil–paper insulation, such as the dissolved gas ratio analysis, aging kinetic model, cellulose–water adsorption isotherm, oil–paper moisture balance curve, and dielectric response technique. Additionally, the advantages and limitations of the above condition assessment techniques are discussed. In particular, this paper highlights the dielectric response technique and introduces its evaluation principle in detail: (1) collecting the dielectric response data, (2) extracting the feature parameters from the collected dielectric response data, and (3) establishing the condition assessment models based on the extracted feature parameters and the machine learning techniques. Finally, two full potential studies are proposed, which research hotspots’ oil–paper insulation and the electrical–chemical joint evaluation technique. In summary, this paper concludes the principles, advantages and limitation of the existing condition assessment techniques for oil–paper insulation, and we put forward two potential research avenues.

Simultaneous detection of multiple aging characteristic components in oil-paper insulation using sensitive Raman technology and microfluidics

The aging characteristic components of oil-paper insulation reflect the aging status of the power equipment. In this study, we designed a novel microfluidic chip capable of automatic and rapid extraction of aging components from insulating oil. Combined with Raman spectroscopy technology, it enables simultaneous detection of various aging components. By optimizing the microfluidic chip structural and adopting an optical window encapsulation, it eliminates interference from the Polydimethylsiloxane (PDMS). Measurements and analyses were carried out on multiple oil samples containing three aging products (furfural, acetone, and methanol). The results indicate that this novel microfluidic chip facilitates simultaneous detection of multiple components, significantly improving the detection sensitivity of complex oil. The detection limits for furfural, acetone, and methanol in insulating oil are 0.43 mg/L, 1.04 mg/L, and 2.31 mg/L, respectively. This provides a new approach for the online detection of oil-paper insulation equipment.

Dielectric response characteristics and moisture exposure evaluation of oil-paper insulation based on microstrip ring resonator

Moisture ingress significantly accelerates the aging of oil-paper insulated equipment, making accurate assessment of moisture content critical for the operation and maintenance of oil-immersed insulated power equipment. In this paper, a novel method for rapid moisture content evaluation is proposed based on a microstrip ring resonator (MRR) designed with a fundamental frequency of 1.16 GHz. A calculation model for the dielectric response analysis using MRR is established to characterize dielectric properties of the material layered on the substrate. ANSYS HFSS electromagnetic simulation software was used to study the impact of dielectric material thickness and the substrate on the MRR test results. Experimental measurement demonstrated that the maximum deviation in resonance frequency was merely 0.542 %. The dielectric properties of uniformly and non-uniformly damp insulating pressboard were analyzed across the frequency band from 1 to 6 GHz. The MRR test results were compared with those of the standard resonant cavity method and frequency domain dielectric spectroscopy (FDS) test. Using the regression algorithm and curve analysis, moisture assessment models for both uniformly and non-uniformly damp oil-paper insulation were established. The experimental results show that the MRR test can clearly distinguish between the pressboard samples with varying moisture levels under uniform conditions, with a maximum moisture evaluation error of 8.19 %. The results demonstrate that the MRR test method, requiring only 56 s, can quickly and effectively evaluate the moisture content of insulating pressboard.

Migration and Bridging Characteristics of Cellulose Impurities in Oil-Paper Insulation

Migration and Bridging Characteristics of Cellulose Impurities in Oil-Paper Insulation

Thermal aging performance of soybean insulating oil mixed with glyceryl triacetate

A method of delaying the aging of natural ester oil-paper insulation systems by adding glyceryl triacetate is proposed in this work. A study was conducted on the thermal aging performance of soybean insulating oil mixed with glyceryl triacetate at a volume fraction of 7.5%. Through a 42-day accelerated thermal aging experiment, it was found that the addition of glyceryl triacetate effectively delayed the increase in kinematic viscosity of the insulating oil and reduced the generation of aging products such as moisture and acid. However, the polar compounds in glyceryl triacetate reduced the breakdown voltage of the mixed oil and made the dielectric loss factor higher than that of pure oil. In addition, tests were conducted on the insulating paper immersed in two types of oil, and it was found that the addition of glyceryl triacetate helps to distribute mixed oil molecules on the surface of insulating paper, which effectively helped absorb moisture from the paper, weakening the hydrolysis reaction of cellulose and delaying the aging of insulating paper. In addition, the insulating paper immersed in the mixed oil maintains good power frequency breakdown voltage and dielectric properties throughout the thermal aging experiment.

AC-DC breakdown characteristics of oil-paper insulation at low temperature

New energy sources such as wind energy are mainly distributed in low temperature areas such as high altitude and high dimensionality in inland China, which has higher requirements for the low-temperature insulation characteristics of power equipment. This poses higher demands for the low-temperature insulation capabilities of power equipment. However, the existing research on the low temperature breakdown characteristics of oil-impregnated paper is limited. In this study, the authors constructed an AC-DC combined power supply and a specialized low-temperature breakdown testing platform. Various oil-impregnated paper samples with different moisture contents were prepared. The paper investigated the AC-DC breakdown characteristics of these samples under low-temperature conditions, focusing on the effects of temperature, moisture content, and DC component on the breakdown strength. The findings serve as a valuable design reference for the application of oil-impregnated power equipment in low-temperature environments.

Surface enhanced substrate simulation optimized coupled PLS model for quantitative detection of acetone in transformer oil

AbstractAcetone is a reliable index to evaluate the aging state of oil paper insulation. It is of great significance to study a fast, highly sensitive and accurate method of detecting acetone in oil to ensure the safe and stable operation of oil-immersed transformers. In this paper, a method based on surface enhanced Raman spectroscopy for the detection of acetone in oil is investigated. Based on the mechanism of surface-enhanced Raman spectroscopy (SERS), a simulation model of surface-enhanced substrate was constructed using COMSOL software, and it was determined that the enhancement effect was better when silver nanowires/zinc oxide nanorods (Ag/ZnO) composite structure was selected. Ag/ZnO substrates were prepared by solvothermal method and experimentally tuned and characterized to obtain SERS substrates with high enhancement properties. The substrate coupled partial least square (PLS) model was used to establish a method for the detection of acetone in insulating oils, and its limit of quantification for acetone was 0.003 mg/g, which meets the requirements for engineering testing.

Calculation of Charge Transport Phenomena and Mobility Analysis in the Insulating Oil Using First Principle and Marcus Theory.

Oil-paper insulation is widely used as a reliable composite insulation system in power transformers. The dielectric property of oil insulation plays an important role in the reliable operation of power equipment. To recognize the charge transfer process in composite insulation, the mobility of the charge in aged insulating oil is studied. However, few studies have been conducted on the microscopic mechanism of charge transport phenomena at the molecular level. In this research, we have studied the molecular electronic structure and the distribution of holes and electrons in the insulating oil by first-principles calculation. By combining with Marcus theory, the corresponding electron coupling energy, reorganization energy, and free energy are obtained. The corresponding charge hopping model is chosen by the parameter relation, and the hopping rate is calculated. At last, the mobility of holes and electrons in insulating oil within the insulation is simulated by the Monte Carlo method. Other possible charge migration methods are also studied and discussed for the comparison. It is observed that the transfer integral of electrons is 2 orders of magnitude larger than that of holes, which is mostly due to the localization of lowest unoccupied molecular orbitals (LUMO). The hole and charge transfers accord with Marcus hopping, the adiabatic charge transfer model, and the charge hopping rate is obtained. The actual free energy action under an external electric field is obtained by calculating polarizability and permittivity. Monte Carlo simulation is used to obtain the charge transfer image and mobility under an actual electric field. Possible types of traps and mobility of ions and clusters in the insulating oil are also studied.

Evaluating the Effects of Temperature on Moisture Dynamics and Relaxation Mechanism in Transformer Oil-Paper Insulation

Evaluating the Effects of Temperature on Moisture Dynamics and Relaxation Mechanism in Transformer Oil-Paper Insulation

Model moisture transport in oil‐paper insulation of transformer: Theory and experiment

AbstractThe insulation performance of oil paper insulation is significantly affected by moisture such that monitoring moisture content is important. However, it is difficult to obtain the moisture content accurately due to its dynamic change under multi physical fields. Studying the moisture transportation in oil paper insulation under multi physical fields becomes a vital method in solving the problem. A multi physical model describing moisture migration was proposed, which considered the effects of temperature on moisture in different existing states in oil‐immersed paper (OIP). The temperature distribution formed a vapour pressure gradient to drive moisture migration and affected migration speed. Then, experiments and simulations of moisture migration were performed, which showed that the experiments were in good agreement with simulation. The results revealed that the temperature gradient caused uneven moisture distribution and the increased temperature reduced OIP moisture content. The established model could fully characterise moisture migration under temperature gradient, which provided a theoretical reference for predicting the risk of partial dampness and evaluating insulation performance.

Synthetic Ester-Based Oils and Their Application in Power Industry

Suitable (biodegradable) substitution of classic mineral petroleum-based oils used in power transformers is studied. Paper presents test results of six paper-oil insulation systems varying by different type of oil. Three common transformer oils and three new synthetic oils (never used in power transformers before) were selected for the purpose of experiment. The oils have been exposed to accelerated thermal aging (at the temperature of 90 °C) and their properties were measured in time intervals of 0, 50, 125, 225, 500, 1000 and 2000 hours.

Instruments for On-line Monitoring of Transformers

The main objective of the paper is to describe the experience with newly developed monitoring system for transformers with oil-paper insulation system. From the view of operation reliability, it is necessary for operators to provide a monitoring of transformer’s failure free operation. The application of monitoring system enables to warn timely of the rising failure and also decrease of degradation of single subsystems of machine. When failure is detected, it is important to support this detection by other measurements that would lead to exact determination of the failure. SFRA analysis (Sweep Frequency Response Analysis) is such additional method in our monitoring.

Creeping flashover test methods, characteristics, and mechanisms of oil-paper insulation structures of power transformers

Power transformers are critical equipment responsible for the transmission and transformation of electrical energy, and they are an important component of power system. Under the combined effects of electric field, thermal field, and mechanical stress, insulation degradation is prone to occur in oil-paper insulation structures. Among them, the “liquid-solid” interface formed by oil and oil-immersed pressboard is the weak point in transformer insulation, and extensive operational experience has shown that creeping flashover of insulating pressboard is a common fault in power transformers. Creeping flashover of insulating pressboard can rapidly destroy the insulation structure, leading to irreversible damage to the transformer and posing a serious risk of transformer accidents. In recent years, many scholars have conducted in-depth research on the characteristics and mechanisms of creeping flashover in oil-paper insulation structures using various experimental methods. They have obtained significant findings that have important theoretical and practical implications for optimizing the structure of power transformers and improving their reliability. This paper first introduces the characteristics of the electromagnetic environment inside the power transformer and the common forms of creeping flashover accidents in engineering. Then, classified according to the uneven degree of electric field and creepage distance of test electrodes used in relevant research, and the test methods and flashover characteristics of small size (<5 cm) and large size insulating pressboard under extremely uneven and slightly uneven electric fields are introduced respectively. Secondly, the existing breakdown model and mechanism theory of oil-paper insulation structure creeping flashover are summarized. Finally, the limitations of the existing creeping flashover test method of insulating pressboard are analysed, and the key issues to be solved are condensed. This review aims to provide readers with a comprehensive overview of the test methods, characteristics, and mechanisms of insulating pressboard creeping flashover.

Breakdown Characteristics and Damage Mechanism of Typical Defects in Oil-paper Insulation under HEMP

Breakdown Characteristics and Damage Mechanism of Typical Defects in Oil-paper Insulation under HEMP

Bubble Migration and Accumulation in Oil-paper Insulation: Modeling of Coalescence-induced Size Evolution

Bubble Migration and Accumulation in Oil-paper Insulation: Modeling of Coalescence-induced Size Evolution

Aging Characterization of Oil-paper Insulation Based on Fluorescence Characteristics of Suspended Fibers in Oil

Aging Characterization of Oil-paper Insulation Based on Fluorescence Characteristics of Suspended Fibers in Oil

A New Nondestructive Aging Characterization Method for Oil-paper Insulation Based on the Transmission Characteristics of Complementary Split Ring Resonator

A New Nondestructive Aging Characterization Method for Oil-paper Insulation Based on the Transmission Characteristics of Complementary Split Ring Resonator

Analysis of the results of preventive testing of bushings using elements of the theory of statistical hypothesis testing

The article presents the results of a study of the dependence of insulation indicators on the duration of operation for serviceable and faulty high-voltage oil-filled bushings, both of hermetic and non-hermetic construction. The results of periodic tests of oil-filled bushings, with a voltage of 110 kV of hermetic design with paper-oil insulation of the capacitor type, as well as non-hermetic construction, were used as initial data. Three statistical criteria were used to form statistically homogeneous data sets: the Wilcoxon rank test, the Z-test, and the Fisher-Snedecor test. The values of the sample mean, sample variance, as well as coefficients of asymmetry and excess of insulation indicators of high-voltage bushings were obtained for the array of initial data and arrays obtained during statistical processing. The values of the sample averages for the arrays obtained as a result of the statistical processing procedure differ significantly. There is a significant decrease in the sample variance values for tgd1 compared to the original arrays. To check the homogeneity of the data in the obtained arrays, the one-factor variance analysis model is used. The practical use of this algorithm made it possible to significantly reduce the heterogeneity of the results of periodic tests of the state of insulation of high-voltage bushings. The analysis of the composition of the arrays with homogeneous values of the indicators made it possible to establish that the values of the indicators of bushings from different regions of Ukraine, of different types and made with different numbers of factory drawings, got into the homogeneous data arrays. It is noted that for high-voltage bushings that do not have insulation defects, the values of the mathematical expectation of the same indicator for different arrays of preventive tests of capacitor-type insulation have a shift relative to each other. From this it follows that the load factor and the operating time of the transformer significantly affect the determination of the maximum allowable value of the insulation index of the high-voltage bushing.