Cellulose Insulation In Transformers Research Articles

Prediction of the Degree of Polymerization in Transformer Cellulose Insulation Using the Feedforward Backpropagation Artificial Neural Network

The life expectancy of power transformers is primarily determined by the integrity of the insulating oil and cellulose paper between the conductor turns, phases and phase to earth. During the course of their in-service lifetime, the solid insulating system of windings is contingent on long-standing ageing and decomposition. The decomposition of the cellulose paper insulation is strikingly grievous, as it reduces the tensile strength of the cellulose paper and can trigger premature failure. The latter can trigger premature failure, and to realize at which point during the operational life this may occur is a daunting task. Various methods of estimating the DP have been proposed in the literature; however, these methods yield different results, making it difficult to accurately estimate a reliable DP. In this work, a novel approach based on the Feedforward Backpropagation Artificial Neural Network has been proposed to predict the amount of DP in transformer cellulose insulation. Presently, no ANN model has been proposed to predict the remaining DP using 2FAL concentration. A databank comprising 100 data sets—70 for training and 30 for testing—is used to develop the proposed ANN using 2-furaldehyde (2FAL) as an input and DP as an output. The proposed model yields a correlation coefficient of 0.958 for training, 0.915 for validation, 0.996 for testing and an overall correlation of 0.958 for the model.

Study on Aging Assessment Model of Transformer Cellulose Insulation Paper Based on Methanol in Oil

Methanol is considered to be a new chemical marker to estimate the aging state of transformer cellulose insulation paper. The key to improve the assessment accuracy is to establish the precise relationship between the concentration of methanol in oil and degree of polymerization (DP) of insulation paper. The previous research in this area has shortcomings and needs to be further improved. In this article, first, an aging assessment basic model (temperature 20 °C, moisture content of insulation paper 0.637%) is established based on the theoretical derivation of cellulose degradation kinetics, and is calculated by 130 °C thermal aging experiment. Furthermore, the effects of different temperatures (20 °C–90 °C) and moisture contents of insulation paper (0.6%–3.4%) on methanol in oil are studied by diffusion experiment. Based on the above effects, the temperature and moisture correction factors are established by mathematical reduction. What is more, a modified model is proposed by integrating the basic model and the correction factors. Eventually, the verification experiment is randomly prepared to test the accuracy of modified model and the result shows that the relative error of DP is within 7%. The physical meaning and accuracy of aging assessment are improved by model derivation and modification of influencing factors.

A review on the research progress and future development of nano‐modified cellulose insulation paper

Focussing on the insulating material of power transformer—cellulose insulation paper, this paper first introduces the basic characteristics and engineering application background of insulation paper. Then, the research progress of improving the mechanical properties, thermal stability and electrical properties of insulation paper by nano modification in recent years is compared, and the promotion mechanism is analysed. In addition, in practical engineering application, the oil–paper composite insulation system composed of insulation paper and insulating oil is used for the internal insulation of oil immersed transformer. Therefore, the influence of modified insulation paper on the performance of oil–paper composite insulation system is analysed. Finally, the newly reported new insulation paper is introduced, and the future development trend of modification of traditional insulation paper is analysed.

Assessment of Transformer Cellulose Insulation Life Expectancy Based on Oil Furan Analysis (Case Study: South African Transformers)

Assessment of Transformer Cellulose Insulation Life Expectancy Based on Oil Furan Analysis (Case Study: South African Transformers)

Aging Assessment Model of Transformer Insulation Based on Furfural Indicator under Different Oil/Pressboard Ratios and Oil Change

Oil change, as an effective method, could improve the performance of liquid insulation. However, due to the furfural loss in oil after the oil change, the aging assessment results of cellulose insulation based on the traditional furfural-degree of polymerization (DP) model is not accurate. Besides, the different oil/pressboard mass ratio of the transformer also has a significant impact on the aging assessment results. Current research only focuses on the aging assessment based on furfural analysis under the single oil/paper mass ratio and oil change conditions. In view of this, the aging assessment model for transformer insulation based on furfural indicator under different oil/pressboard ratios and oil change conditions was studied. First, oil/pressboard insulation systems with different oil/pressboard mass ratios were performed for total oil changes. Then, the initial furfural-DP equation and the furfural corrected equation were obtained. Finally, the modified furfural-DP model extended to different oil/pressboard mass ratios under oil change was established. The proposed model is expected to promote the accuracy of the aging assessment of transformer cellulose insulation under different oil/pressboard mass ratios and oil change conditions.

Modified furfural-DP equation with different oil-paper-pressboard mass ratios under oil replacement condition

The furfural analysis is an effective method to evaluate the aging state of transformer cellulose insulation. However, the existing research on the furfural analysis mainly focuses on ‘oil-paper’ system, without considering the oil-paper-pressboard system. In fact, the mass of insulating pressboard occupies a large proportion in the transformer, and there are obvious differences in the mass ratio of oil-paper-pressboard among transformers with different voltage levels, which will affect the effectiveness of furfural analysis. In addition, oil replacement is an oil treatment method to improve the insulation performance of oil, which will lead to the loss of furfural content in oil. In view of this, a series of experiments and studies were carried out to obtain the modified equation between furfural and degree of polymerization (DP) of paper/pressboard insulation with different oil-paper-pressboard mass ratios under oil replacement condition. Firstly, accelerated thermal aging experiment of insulation samples with various oil-paper-pressboard mass ratios was conducted in the lab, and the effect of oil-paper-pressboard mass ratio on furfural was analyzed. Then, the oil replacement experiments were performed, and the oil replacement compensation factor was proposed. Finally, a modified furfural-DP equation was established. This paper attempts to report a method for analyzing the effect of oil-paper-pressboard mass ratio and oil replacement on furfural-DP equation. These findings are expected to improve the application of furfural analysis in field transformers.

Assessment of Concentration of Mineral Oil in Synthetic Ester Based on the Density of the Mixture and the Capacitance of the Capacitor Immersed in It

This research was carried out during the realization of a project with the aim of developing a method of drying cellulose insulation in power transformers by using synthetic ester. Unfortunately, during the drying process, the ester is systematically contaminated with mineral oil, which reduces its water absorption. Information on the oil concentration in the mixture is needed in two cases: when deciding how to treat the mixture, and during the treatment. The article presents two methods: (1) based on the measurement of the mixture density, and (2) based on the measurement of a capacitor immersed in the mixture. The most important scientific achievement of the work is the proof, by way of experiment, of the existence of a relationship between the concentration of mineral oil and (1) the density of the mixture, and (2) the capacity of the capacitor immersed in it. These relations are presented in the form of equations for which the error calculus showed that the uncertainty of measurement for both methods did not exceed 3 p. %. Due to the similar measurement error of both methods, the authors recommend the capacitance method as easier to use, especially online.

Aging state assessment of transformer cellulosic paper insulation using multivariate chemical indicators

Compared with a single chemical indicator, the use of multivariate chemical indicators in aging status assessment of transformer cellulose paper insulation can improve the reliability of the evaluation. In this study, a non-destructive method based on multivariate chemical indicators was proposed to evaluate the remaining life of cellulosic paper insulation. The selected chemical indicators closely related to the aging of the insulating paper were analyzed. Additionally, by applying the subjective and objective weighting method, the evaluation information of each indicator was fully extracted to obtain reasonable comprehensive evaluation weights. Besides, the comprehensive evaluation index of multivariate chemical indicators was obtained by using the technique for order preference by similarity to ideal solution method, which is consistent with the deterioration law of paper insulation. Then, the correlation lifespan model between the comprehensive evaluation index of multivariate chemical indicators and the degree of polymerization of insulating paper under different aging stages was established. The effectiveness of the proposed model was verified by experiments. This promising model can increase the accuracy of the condition assessment for the transformer cellulosic paper insulation.

Experimental Validation of a Method of Drying Cellulose Insulation in Distribution Transformers Using Circulating Synthetic Ester

The article concerns a method of drying cellulose insulation of distribution transformers with the use of synthetic ester. This method uses the high solubility of water in the ester compared to other dielectric liquids. The method is based on the striving of the oil-paper insulation system for a state of moisture equilibrium. Water migrates from the cellulose insulation of the high moisture content to the synthetic ester which is subjected to continuous drying. The research was carried out on a complex laboratory model reflecting the transformer insulation system. In this model, the drying of the ester was carried out using a molecular sieve with an appropriately selected adsorber weight. To check the drying efficiency of the cellulose insulation, the water content in the pressboard strips taken from the model, both before and after the drying process, was determined. The water content was measured using the Karl Fischer titration method. The research showed the high efficiency of the proposed method of drying. With the ester moisture in the range of 110-130 ppm and the insulation system temperature of about 70 °C, the loss of water in the samples dried for 7 days was over one percentage point. The obtained test results constitute the basis for the validation of the method on distribution transformers.

Investigation on Formation Mechanisms of Methanol During Cellulose Insulation Aging Based on Molecular Dynamics Simulation

In recent years, methanol has been proposed as a chemical indicator to assess the aging condition of cellulose insulation in oil-immersed power transformers. However, the formation mechanisms of methanol during cellulose degradation are not clear enough. In this paper, such formation mechanisms were studied using molecular dynamics simulation. Three main formation pathways of methanol were found and discussed. Analysis of the Mayer bonds of cellulose revealed that each atom of cellulose contributes differently to methanol formation. The methylol groups on the 5-carbon atom of the 1-pyran ring and 4-pyran ring of cellulose are more readily detached to form methanol molecules. In addition, changes in the amount of methanol molecule with simulation time were investigated. The results indicated that the separate existence of initial moisture and oxygen has no significant effect on the formation of methanol, while the co-existence of initial moisture and initial oxygen catalyzes the conversion of cellulose degradation products to methanol. The findings reported in this paper can provide a valuable theoretical basis for further studies on methanol as an indicator to evaluate the residual life of cellulose insulation.

Power transformer cellulosic insulation destruction assessment using a calculated index composed of CO, CO2, 2-Furfural, and Acetylene

Power transformer insulation is degraded and decomposed over time. Due to the importance of cellulose, indicators such as 2-Furfural, CO, CO2, and CO2/CO ratio are used to investigate cellulose aging status because these are cellulose degradation products. However, various studies have shown that using each of these indicators alone may lead to incorrect cellulose degradation diagnosis. Therefore, a new calculated index called CFA index (an abbreviation of Carbon, Furfural, and Acetylene) that considers the effects of four indicators (CO, CO2, 2-furfural, acetylene) together will be presented in this paper. The CFA index has been obtained from the analysis of 216 power transformer measurement data using the correlation coefficients calculation. The reason for considering acetylene is the importance of its production and the high correlation coefficient between acetylene and transformer age. In this method, if the transformer has the CFA index between −2 and −5.3, then it can be ensured that the transformer does not suffer from severe cellulose aging or decomposition. The normal range for the CFA index is obtained from investigating data from many power transformers, all with CO2/CO ratio between 3 and 10. Finally, to verify the accuracy of the CFA index and its ability to detect cellulosic insulation degradation and the inefficiency of other methods in some cases, dissolved gas analysis (DGA) test results of 306 power transformers will be investigated, and results will be fully presented.

Prediction of Moisture and Aging Conditions of Oil-Immersed Cellulose Insulation Based on Fingerprints Database of Dielectric Modulus.

Frequency-domain spectroscopy (FDS) is demonstrated to be affected by electrode polarization and conductance behavior in the low-frequency ranges, which causes the unreliable prediction results of transformer cellulose insulation. In order to solve this issue, a fingerprint database based on the dielectric modulus is reported to predict the degree of polymerization (DP) and moisture content of cellulose insulation. In the current work, the relevant fingerprints that characterize the insulation conditions are obtained by studying the dielectric modulus curves of cellulose insulation with various insulation conditions, as well as the DC conductivity of transformer oil. Then, the dielectric modulus fingerprint database is established in the lab, and the accuracy of the reported fingerprint database is later verified. As a potential tool, the dielectric modulus fingerprint database is tested by several samples, and the results demonstrate that the accuracy of this method is more than 80%. In that respect, an interesting discovery of this paper is that the dielectric modulus fingerprint database may be a helpful tool for conditions prediction of the transformer cellulose insulation system.

Laboratory Model Studies on the Drying Efficiency of Transformer Cellulose Insulation Using Synthetic Ester

This paper presents the results of laboratory tests of cellulose insulation drying with the use of synthetic ester. The effectiveness of the drying process was investigated depending on the initial moisture of cellulose samples (2%, 3%, and 4%), ester temperature (55, 70, and 85 °C), initial moisture of the ester (70, 140, and 220 ppm), drying time (48, 96, and 168 h), and the weight ratio of cellulosic materials to ester (0.067 and 0.033). A large influence of temperature and time of drying on the efficiency of the drying process was found. This is important information due to the application of the results in the transformers drying procedure. The heating and drying ester unit should provide the highest possible temperature. For the assumed experiment conditions the initial moisture of the ester had little effect on the drying efficiency. An ester with a moisture content below 140 ppm can still be considered as meeting the requirements for drying cellulose with significant moisture. The weight ratio of cellulose products to ester has no major effect on drying efficiency during cellulose drying by circulating dry ester.

Failure mechanism of transformer oil-immersed cellulosic insulation induced by sulfur corrosion

The life of transformer oil-paper insulation is determined by the insulation property of transformer cellulosic insulation. Existing studies have shown that sulfur corrosion poses a serious threat to the properties of transformer oil-immersed cellulosic insulation due to the reaction of corrosive sulfur dissolved in oil with copper winding. This paper investigated the effect of sulfur corrosion on the characteristics of transformer cellulosic insulation, cellulosic insulation winding samples with different degrees of sulfur corrosion were obtained through an accelerated thermal aging experiment. Then, short-/long-term withstand voltage tests for the cellulosic insulation winding were conducted. Experimental results showed that the contamination of sulfur corrosion had a minimal effect on the power–frequency breakdown voltage of the cellulosic insulation winding but considerably decreased its long-term electrical life. Combined with the analysis of the electrical/physical and chemical parameters of oil-immersed cellulosic insulation, the failure mechanism of transformer oil-immersed cellulosic insulation induced by sulfur corrosion was synthetically discussed and analyzed.

Aging evaluation and moisture prediction of oil-immersed cellulose insulation in field transformer using frequency domain spectroscopy and aging kinetics model

This work aims to propose an improved model for aging evaluation and moisture prediction of transformer cellulose insulation, which is based on the combination of the advantages of both frequency domain spectroscopy (FDS) and cellulose aging kinetics model. In the present work, the FDS was firstly employed to achieve the prediction of the inside moisture according to its moisture-sensitive property. Then, the predicted moisture is applied to construct the improved aging kinetics model, so that the aging condition of the transformer cellulose insulation can be predicted as well. The presented lab tests, as well as the field tests, have demonstrated the accuracy and feasibility of the proposed model. In that respect, the present contributions attempt to present a novel idea for the aging evaluation and moisture prediction for transformer cellulose insulation.

Condition prediction for oil-immersed cellulose insulation in field transformer using fitting fingerprint database

The results of frequency domain spectroscopy (FDS) will be affected by moisture which often leads to an unreliable condition prediction of oil-immersed cellulose insulation in transformers. In view of this, a solution is proposed in this study by adopting a fitting fingerprint database (FFD) technique, which can be utilized for condition prediction of aging and moisture content. In the current work, the relevant fingerprints that could characterize the insulation aging condition and moisture content are extracted from the FDS curves and the DC conductivity of the transformer oil. The FFD is then constructed by fitting fingerprints. The accuracy of the reported FFD technique is verified from tests on oil-immersed pressboard prepared in the lab and a transformer from the field. In this respect, the present contribution highlights FFD might be used as a powerful tool for condition prediction of oil-immersed cellulose insulation in transformers.

State-of-the-art review on the performance of cellulosic dielectric materials in power transformers: Mechanical response and ageing

Power transformers are essential elements in electrical systems and the end of their useful life is frequently conditioned by the state of the cellulosic insulation materials inside them. These materials are subjected to high temperatures and mechanical stresses, primarily generated by short-circuits or inrush currents. Moreover, both the cellulosic materials and the dielectric oil in which they are impregnated are affected by chemical reactions. In order to achieve a better understanding of ageing kinetics and improve the management and planning in electrical systems from a technical and economical point of view, it is important to better understand the behaviour of cellulosic insulation. The purpose of this review is to analyse the current state-of-the-art with reference to the modelling of the mechanical behaviour of paper materials used as insulation in power transformers, as this topic has been previously analysed in many different publications but only fragmentarily. A suitable model for transformer cellulosic insulation, in combination with engineering measurements, could help to optimise the design of the insulation and manufacturing processes, as well as to accurately predict the end-of-life of power transformers.

Improved method for aging assessment of winding hot-spot insulation of transformer based on the 2-FAL concentration in oil

2-furfuraldehyde (2-FAL) is widely accepted as a chemical marker to indicate the degradation of cellulose insulation in oil-impregnated transformers. However, the application of 2-FAL in the aging diagnosis of transformers in operation is ineffective. The excessive dispersion of 2-FAL concentration measured in transformers is caused by many influence factors. In addition, the 2-FAL concentration in oil cannot indicate the aging of the most serious point given the temperature distribution along the winding height, that is, the insulation at the hot spot of a winding. To optimize the assessment, an improved method for the hot-spot insulation assessment based on the 2-FAL concentration in oil is proposed in this work. The 2-FAL generation in cellulose chain scission is firstly analyzed theoretically, and a new linear relationship between (1/DPt-1/DP0) and 2-FAL total generation is obtained. Moreover, the 2-FAL partitioning between an insulation paper and mineral oil influenced by temperature, moisture, and aging status in total is considered. Second, by considering the linear relationship and the 2-FAL partitioning, an equation between the insulation at hot-spot temperature and the 2-FAL concentration in oil is established for the thermal aging under unequal temperature distribution condition. Finally, to verify the proposed method, a testing platform is introduced to conduct the thermal aging experiment with temperature gradients. The proposed method demonstrates better assessment results than the conventional thermal aging test at constant temperature.

Aging condition assessment of transformer oil-immersed cellulosic insulation based upon the average activation energy method

It is widely accepted that the service life of transformer oil–paper insulation is determined by the aging condition of transformer cellulosic insulation. Therefore, it is rather meaningful to accurately and reliably evaluate the aging condition of transformer cellulose insulation. Existing studies have shown that activation energy can eliminate the temperature effect on the frequency dielectric spectroscopy, however, the traditional activation energy might not be suitable for assessing the aging condition of transformer cellulosic insulation. The aim of the work is to assess the aging condition of cellulosic insulation by using average activation energy technique. The experiment process is firstly discussed. Then, the second-order dynamic equation formed by Arrhenius equation and Ekenstam equation is strictly investigated to accurately calculate the degree of polymerization value of transformer cellulosic insulation. The experimental results preliminarily show that the average activation energy can effectively assess aging condition of transformer cellulosic insulation.

Study on Quantitative Correlations between the Ageing Condition of Transformer Cellulose Insulation and the Large Time Constant Obtained from the Extended Debye Model

Polarization-depolarization current (PDC) measurements are now being used as a diagnosis tool to predict the ageing condition of transformer oil-paper insulation. Unfortunately, it is somewhat difficult to obtain the ageing condition of transformer cellulose insulation using the PDC technique due to the variation in transformer insulation geometry. In this literature, to quantify the ageing condition of transformer cellulose insulation using the PDC technique, we firstly designed a series of experiments under controlled laboratory conditions, and then obtained the branch parameters of an extended Debye model using the technique of curve fitting the PDC data. Finally, the ageing effect and water effect on the parameters of large time constant branches were systematically investigated. In the present paper, it is observed that there is a good exponential correlation between large time constants and degree of polymerization (DP). Therefore, the authors believe that the large time constants may be regard as a sensitive ageing indicator and the nice correlations might be utilized for the quantitative assessment of ageing condition in transformer cellulose insulation in the future due to the geometry independence of large time constants. In addition, it is found that the water in cellulose pressboards has a predominant effect on large time constants.