Oil-paper Insulation System Research Articles

Revealing the Intrinsic Correlation between Cu Scales and Free Radical Chain Reactions in the Regulation of Catalytic Behaviour.

Defining the copper-based catalysts that are responsible for the catalytic behaviour of oil-paper insulation systems and implementing effective regulation are of great significance. Accelerated ageing experiments were conducted to reveal variations in copper scales and deterioration in insulation properties. As ageing progressed, TEM images demonstrated that copper species were adsorbed and aggregated on the fibre surface in the form of nanoparticles (NPs). The scale of NPs exhibited a continuous increase, from 27.06 nm to 94.19 nm. Cu(I) and Cu(II) species were identified as the active sites for inducing intense free radical reactions, which significantly reduced the activation energy, making the insulating oil more susceptible to oxidation. The role of the antioxidant di-tert-butyl-p-cresol (DBPC) in extending the insulation life was regulated by determining the optimal addition time based on variations in the interfacial tension. After the second addition of DBPC, the ageing rates of the dissipation factor, acidity, micro-water and breakdown voltage in the Cu+DBPC group decreased by 28.8%, 43.2%, 52.9% and 46.7%, respectively, compared to the Cu group. This finding not only demonstrates the crucial role of DBPC in preventing the copper-based catalyst-induced oxidation of insulating oil, but also furnishes a vital foundation for enhancing the long-term stability of transformer insulation systems.

The Catalytic Effect of Low Molecular Weight Acids on the Physicochemical and Dielectric Properties of Oil-Paper Insulation Systems.

In most industrialized countries, power transformers built several decades ago are approaching the end of their operational lifespan. The ongoing energy transition, focused on developing 100% renewable energy sources and accelerating global transportation electrification, further exacerbates these assets. Combined with rising electricity demand, there is an increasing risk of critical transformers' degradation acceleration. In this context, understanding the aging mechanisms of the insulation system inside these essential assets, which form the core of every energy network, becomes paramount for today's managers and engineers responsible for their operations. The acids generated through oil oxidation can be classified into two categories: low molecular weight acids (LMAs), which are inherently more hydrophilic and consequently have a greater impact on the degradation rate of solid insulation through hydrolysis, and high molecular weight acids (HMAs), which do not significantly contribute to the degradation of paper insulation. This study specifically addresses the impact of acids generated through oil oxidation-focusing on LMAs. New oil samples were infused with different ratios of LMAs before impregnation. The impregnated paper samples underwent thermal aging at 115 °C. Different physicochemical and dielectric properties were investigated. The investigations revealed that oils blended with formic acid exhibited more adverse effects on the insulation system compared to other LMAs. This information is essential for industry professionals seeking to mitigate the risks associated with transformer degradation and extend the lifespan of these critical assets during the energy transition.

Advancing Thermal Stability in Natural Ester Oil-Paper Insulation Systems via Precision Nanostructuring with Parylene Films: Experimental and Molecular-Level Comprehensive Assessment

The oil-paper insulation system in eco-friendly fire-retardant transformers depends on hydrophilic natural ester insulating oil. Moisture within the system synergistically interacts with aging, worsening oil-paper insulation degradation and hastening overall system aging. Chemical vapor deposition was used to create parylene surface-modified insulating paper as a strategy to inhibit moisture-induced aging in natural ester oil-paper insulation. The effectiveness of the approach was identified by a comprehensive assessment of the physicochemical and electrical properties of the parylene surface-modified insulating paper. The findings from accelerated thermal aging at 130 °C for 90 days on the natural ester oil-paper insulation system reveal the outstanding lipophilic and hydrophobic properties while maintaining electrical characteristics of the parylene surface-modified insulating paper. After 90 days of aging, the parylene surface-modified insulating paper exhibited a 56.76% higher degree of polymerization and a 19.36% significantly lower moisture content than conventional cellulose insulating paper. In the natural ester oil-paper insulation system, the parylene surface-modified insulating paper led to a notable 63% reduction in insulating oil acid value, a 60.50% decrease in dielectric loss, and a substantial 20.35% increase in AC breakdown voltage. Molecular-level investigations revealed the inhibitory mechanism of the parylene film, offering a promising solution to enhance the thermal stability and aging resistance of natural ester oil-paper insulation systems.

An investigation of the Behaviour of Copper Electronic State Evolution on the Failure of Oil-paper Insulation Systems

Abstract Copper, as one of the most hazardous metals to transformer insulation systems, has attracted a great deal of interest from scholars at home and abroad. Unfortunately, the specific type of copper that causes catalytic aging of oil-paper insulation is still controversial. Therefore, in this paper, the effect of the electronic state evolution of copper in different oxidation states on the aging performance of the insulation systems was investigated by characterization techniques and performance evaluation. Cu2O has a stronger catalytic oxidizing effect on insulation system than CuO. This study elucidates the electronic state evolution of different oxidation states of copper and its influence on the insulation performance, which lays a good foundation for the advancement of transformer fault warning technology.

Research on infrared spectroscopy detection of furfural content in transformer oil based on acetonitrile extraction

Abstract The stable operation of power transformers depends on the oil-paper insulation system, in which the aging degree of insulating paper is the key to evaluate the remaining life of the transformer. Currently, methanol extraction is widely used to detect the furfural content in oil to evaluate the aging state of insulating paper, but it ignores the influence of methanol produced during the operation of the transformer on the extraction results. To address this issue, this paper proposes a new method that can replace methanol extraction for detecting furfural in oil through simulation and experiment. Firstly, through simulation and experiment, it is proved that the strong vibration absorption peaks at 1677 cm-1 for furfural carbonyl and 2240 cm-1 for acetonitrile cyano can be used to establish a new method for extracting furfural content in oil using acetonitrile. A quantitative model between the concentration x of furfural in the furfural-acetonitrile mixed solution and the area y of the infrared absorption peak at 1677 cm-1 is established, with a goodness of fit of 0.9974. Secondly, a comparison between direct detection and acetonitrile extraction methods is conducted. The results show that direct detection is simple to operate, but the minimum detection concentration is 40mg/L, which is difficult to meet practical requirements. Acetonitrile extraction can reduce the minimum detection concentration to 0.1mg/L. At the same time, the extraction conditions are analyzed to determine the extraction ratio of 30, extraction times of 5, and extraction rate of 60%. Finally, the proposed detection method is applied to thermal aging tests on insulating paper of different types. The experimental results show that the proposed method has good repeatability and improves the detection resolution of furfural content. This paper combines infrared spectroscopy with acetonitrile extraction technology to open up a more efficient and practical new method for detecting furfural content in transformer oil.

The diffusion behavior of H3O+ in insulation systems composed of paper cellulose and modified natural ester insulating oil

The oil-paper insulation system will be affected by the synergistic effects of moisture and acids during long-term stable operation of the transformer. The moisture in the oil can be combined with the H+ionized from the acid molecules in the form of H3O+ by coordination, thus changing the proportion of acid distribution in the insulating oil and paper cellulose. Natural ester insulating oil, as a new development trend to replace mineral oil, can be significantly improved by adding nanoparticles to them. The diffusion behavior of H3O+ in paper cellulose and nano-SiO2-modified soybean oil-based natural ester insulating oil insulating system has been investigated using molecular dynamics in this article. The results indicate that the incorporation with nanoparticles is effective in mitigating the impact of temperature on the diffusion behavior of H3O+ in oil-paper insulation. Compared with the unmodified model, the nano-modified model results in a significant reduction in the concentration distribution of H3O+ in paper cellulose. The hydroxyl and ether bonds in paper cellulose will interact with H3O+ to generate hydrogen bonds and destroy its original hydrogen bonding network, while nanoparticles can inhibit diffusion by adsorbing H3O+ through hydroxyl oxygen atoms.

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.

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.

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.

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.

Insulation aging condition assessment of transformer in the visual domain based on SE-CNN

The aging of transformer oil–paper insulation (OPI) contributes significantly to transformer failure and reduced service life. Therefore, reliable and precise evaluation of the aging status of transformer insulation condition is crucial in averting transformer failure. This article proposes a novel deep learning-based aging condition assessment technique of transformers by combining FDS (Frequency domain spectroscopy) and squeeze-and-excitation-enabled convolutional neural network (SE-CNN). An intrinsic product-based transformation using a Gramian angular field is employed to dimensionally expand the 1-D FDS signals acquired from OPI samples into 2D images. Each feature extraction channel in the SE-CNN-based aging diagnosis framework is assigned a specific weight, enabling the model to concentrate on the most relevant features while discarding the rest. Furthermore, the effect of moisture variation on the model is also considered under three different conditions to ensure the consequence of compensation for the effect of moisture on FDS. A comparison with traditional machine learning models has also been made to ensure the viability of the proposed methodology. Additionally, the model is interpreted by creating explanation weight plots of the feature coefficients using a local interpretable model-agnostic explanations (LIME) method in Python. The findings demonstrate that the novel aging diagnosis framework visually assesses the transformer’s OPI system with the highest accuracy of 99.12% in the test set. Furthermore, it facilitates the automatic extraction of deep aging features, thus providing better generalization and convergence capability.

Centrifugal Extraction-Assisted Fiber-Enhanced Raman Spectroscopy for Online Detection of Trace Furfural in Oil Power Equipment.

Oil-paper insulated equipment is integral in power conversion and supports low-loss electricity transport. As a characteristic byproduct of the oil-paper insulation system, the realization of efficient detection of furfural in oil is crucial to the safe operation of the power grid. We proposed a novel approach using dual-enhanced Raman spectroscopy for sensing trace liquid components. This method employs a centrifugal extractor to separate and enrich the targeted components, achieving selective enhancement. The optimal phase ratio was determined to be 30:1. A liquid-core fiber was used to optimize the laser transmission efficiency and Raman signal collection efficiency, resulting in a nonselective signal enhancement of 44.86. It also investigated the impact of intermolecular interactions on the shift of Raman spectra, identifying the reasons for the differences in Raman signals between pure furfural, furfural in oil, and furfural in water. A batch of samples with furfural dissolved in insulation oil was measured using this system and achieved a limit of detection of 0.091 mg/L. The stability of the dual-enhanced Raman platform was experimentally verified with a spectral intensity fluctuation of 0.68%. This method is fast, stable, adaptable, and suitable for the detection of a wide range of liquid ingredients.

Effect of amino-modified SiO2 nanoparticles on micro-water diffusion behaviour in soybean oil-based natural ester oil-paper insulation system

Nanoparticles can significantly improve the insulation property of oil-paper insulation systems. In this paper, amino-modified SiO2 nanoparticles were used as nano-additives to establish a soybean oil-cellulose paper oil-paper insulation model, and the effect of amino-modified SiO2 nanoparticles on micro-water diffusion in oil-paper insulation systems was systematically studied. Simulation experiments show that amino-modified SiO2 nanoparticles can effectively delay the diffusion of water molecules in oil-paper insulation. As the temperature rises, compared with unmodified SiO2 nanoparticles, the diffusion motion of water molecules in amino-modified SiO2 nanoparticles oil–water model is slower. When the temperature is 383 K, the water molecule diffusion coefficient of the unmodified model is 1.22 times that of the modified model, and amino-modified SiO2 nanoparticles can form special hydrogen bonds with water molecules, preventing water molecules from further diffusing into cellulose paper, breaking the cellulose chain leads to a decrease in mechanical property, and further generating small molecules such as water and acid, which accelerate the aging of oil-paper insulation systems. Indirectly improve the electrical performance and aging performance of oil-paper insulation.

Moisture Adsorption Mechanism of Molecular Sieves in Oil–Paper Insulation: Simulation on Static Adsorption Characteristics

Oil-paper insulation structures are widely utilized inside power transformers. Moisture speeds up the deterioration of the paper insulation and significantly degrades the performance of the transformers’ oil-paper insulation. The molecular sieve is a widely used material for moisture removal from gas and liquid, but its mechanism in moisture removal in transformer insulation is still not clear, especially from a microscopic perspective. In this paper, the density functional theory and molecular dynamics simulation were adopted to calculate the forces of water molecules in the oil-paper insulation system. Also, the models of different A-type molecular sieves were constructed, and the steady-state adsorption characteristics of water molecules in the sieves were obtained by the Monte Carlo method. The results show that the method of calculating the adsorption energy to characterize the adsorption capacity of moisture by different objects is reasonable. The basic reason why molecular sieves can effectively adsorb moisture in the oil-paper structure is that the adsorption energy between the water molecule and the molecular sieve structure is higher than that between the water molecule and other molecules in the oil-paper insulation. Compared with 3A molecular sieves, 4A molecular sieves have a larger free volume, more adsorption sites, and higher adsorption energy, which indicates that 4A molecular sieves have better moisture adsorption characteristics. This research gives a microcosmic explanation of the moisture adsorption characteristics of the oil-paper insulation structure and the moisture adsorption mechanism of molecular sieve, which may provide a theoretical basis for the application and selection of molecular sieves in the online moisture removal of oil-paper insulation structure.

Natural Ester Replacement Effect on Thermal Degradation of Cellulose Insulation From Macroscopic Behavior to Atomic-Scale Mechanism

The accelerated thermal ageing test was conducted with samples composed of mineral oil (MO), FR3 natural ester, kraft insulation pressboard (KIP) and thermally upgraded paper (TUP) at 120 °C. In the middle ageing period, aged MO was replaced with new FR3. Molecular dynamics was used to simulate the mechanism of delaying cellulose aging by replacing natural esters. The experiment results show that, compared to the controls, KIP and TUP impregnated in FR3 had higher degrees of polymerization (DP) values and lower ageing rates. After the FR3 replacement, the relative chain scission rate of KIP and TUP was significantly slowed. FR3 has better aging retarding effect on TUP than KIP. Atomic scale mechanism analysis shows that FR3 natural ester could inhibit the diffusion of water and acid in the oil-paper insulation system before and after oil change, which has a significant ability to delay the acid hydrolysis of cellulose. These is the main reason why KIP and TUP exhibited a higher DP in natural ester. This paper provides theoretical support for transformer replacement of natural ester.

Bubble Formation and Motion Characteristics in the Transformer Oil-Paper Insulation System Under the Extremely Inhomogeneous Electric Field

Bubble Formation and Motion Characteristics in the Transformer Oil-Paper Insulation System Under the Extremely Inhomogeneous Electric Field

A Comparison Between Bubble Formation on Kraft and Thermally Upgraded Papers in Synthetic Ester

Bubble formation endangers the integrity of the oil-paper insulation system of power transformers. In this research, a test setup is created to detect bubbling inception temperature (BIT) for Kraft and thermally upgraded papers (TUPs). The paper samples are dried, prepared at six different moisture levels, and immersed in synthetic ester for the experiments. The paper strips are wound around a cartridge heater, and a controller unit raises its temperature. Results of the experiments revealed that BITs for Kraft paper are comparable to those reported in the literature. Usage of synthetic ester is shown to cause insignificant changes in the BITs. The causes of discrepancies among the BITs obtained via different test setups are discussed in detail, and an equation is presented to estimate BIT in Kraft paper based on four experimental data sets. The tested TUP is shown to have significantly lower BITs than Kraft paper, which is in contrast to the previously reported data. Scrutinizing TUP at temperatures beyond BITs revealed that the behavior of bubble formation differs from that in the Kraft paper. In the test setup with Kraft paper, bubbles are formed progressively at temperatures higher than BIT. In contrast, occasional sporadic bubbles emerge from TUP beyond the BIT until a stream of bubbles, similar to the case of Kraft paper, is observed at several degrees higher than BIT in the TUP.

Using Clustering Methods for the Identification of Acoustic Emission Signals Generated by the Selected Form of Partial Discharge in Oil-Paper Insulation

The article presents the results concerning the use of clustering methods to identify signals of acoustic emission (AE) generated by partial discharge (PD) in oil-paper insulation. The conducted testing featured qualitative analysis of the following clustering methods: single linkage, complete linkage, average linkage, centroid linkage and Ward linkage. The purpose of the analysis was to search the tested series of AE signal measurements, deriving from three various PD forms, for elements of grouping (clusters), which are most similar to one another and maximally different than in other groups in terms of a specific feature or adopted criteria. Then, the conducted clustering was used as a basis for attempting to assess the effectiveness of identification of particular PD forms that modelled exemplary defects of the power transformer’s oil-paper insulation system. The relevant analyses and simulations were conducted using the Matlab estimation environment and the clustering procedures available in it. The conducted tests featured analyses of the results of the series of measurements of acoustic emissions generated by the basic PD forms, which were obtained in laboratory conditions using spark gap systems that modelled the defects of the power transformer’s oil-paper insulation.

Study on the Partial Surface Discharge Process of Oil-Paper Insulated Transformer Bushing with Defective Condenser Layer

Oil-impregnated paper condenser transformer bushings are an important part of transformer equipment, and partial discharge (PD) occurred when defects exist on the condenser aluminum foil layers. Firstly, to study the PD process of the oil-paper insulated capacitance graded bushing with the defect of broken aluminum foil, a defective oil-paper bushing discharge sample is constructed to study the PD parameters and capacitance, and to discharge carbonization traces at different voltage levels. Then, in order to verify the process of condenser aluminum foil layer discharge and the space charge variation in the oil-paper insulation system of a sample model, the surface flashovers of a needle-plane discharge model based on the bipolar charge transport model and the hydrodynamic model was built. The simulation, by Transport of Diluted Species physics of COMSOL Multiphysics software, points out the discharge process of aluminum foil electrode caused by space charge action and electric field distortion under an electric field at different voltages. The results of simulation and sample bushing experiments showed that the PD process of the defective condenser foil layer is mainly divided into three stages: tip corona discharge, streamer in oil, and surface flashovers. The voltage amplitude is larger the more electrical branches are discharged and the shorter the discharge time is. The findings of the article have important implications for the discharge of the foil layer inside the oil-paper bushing.

4-ATP-modified CNTs@NiO–Fe2O3–Ag SERS filter membrane for rapid in-situ detection of furfural in mineral oil

Evaluating the transformer aging state and detecting multi-aging characteristics in transformer oil with high sensitivity and fast speed has become a key challenge. This study introduces a P–N heterojunction (CNTs@NiO-α-Fe2O3) fabricated through electroless nickel plating and a one-step hydrothermal method. Additionally, silver nanoparticles (AgNPs) with adjustable particle sizes are grown on the surface using a chemical reduction method. To obtain high sensitivity and rapid SERS signal, CNTs@NiO-α-Fe2O3–Ag gel is adsorbed on a disposable needle filter (220 nm) surface, and 4-aminothiophene (4-ATP) is grafted onto the surface of SERS substrate. The minimum detection limit was 0.025 mg/L (EF = 5.22 × 104), and the response time of SERS best signal could be shortened to 3 min. Density functional theory (DFT) calculations reveal that by constructing a P–N heterostructure of NiO–Fe2O3 and assessing the adsorption energies of furfural, acetone, and methanol on the surface of the P–N heterojunction. This SERS strategy has a huge application prospect in the aging diagnosis of oil-paper insulation systems in a transformer.