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Measuring the furan concentration in transformer oil is one of the most common methods of monitoring and evaluating insulation paper and estimating its degree of polymerization (DP). Here, methods for measuring furan concentration, especially ultraviolet-visible (UV-Vis) spectroscopy method, are reviewed on the old transformers oil. The results show that the excessive turbidity of old oils and noise at low wavelengths are the main disadvantages of this method, which reduce the measurements accuracy. The following solutions are proposed to overcome these disadvantages: 1) diluting insulation oil with hexane to reduce absorption intensity and 2) using average absorption at a given wavelength to reduce noise impact. These solutions are applied to the oil samples of ten old distribution transformer. It is observed that the correlation between 2-furaldehyde (2-FAL) concentration and average absorption intensity in 190–600 nm wavelength range is equal to ${R}^{{2}} =0.98$. A mathematical equation for calculating 2-FAL concentration based on average absorption is presented. It has been shown that to reduce the test time, a 190–400 nm range is the optimal measurement wavelength range. The results show that the average difference between the values of DP in the proposed method and the direct measurement method is equal to 77.5. [ABSTRACT FROM AUTHOR]

It is now possible to detect dangerous arcing faults in the paper insulation of transformers and distinguish them from arcing faults in oil, using new sub-zones of Duval Pentagons 1 and 2. [ABSTRACT FROM AUTHOR]

This study conducts thermal aging treatment to study the effect of thermal aging on the PD degradation process of the oil-paper insulation. The partial discharge inception voltages, extinction voltages, carbon mark, and microscopic morphology of samples are analyzed. The results show that thermal aging does not reduce the partial discharge inception voltages of oil-paper but can make the spark produced in the oil-paper more difficult to extinguish. At the initial stage, thermal aging has little effect on the development of partial discharge in normal direction of samples, while in later stages, the effect begins to appear; thermal aging reduces the insulation performance between layers of oil impregnated paper and accelerates the carbon trace growth rate in the horizontal direction. In this model, the first layer is most severely bombarded by electrons, and this layer is most obviously affected by the electric field in the normal direction at the contact point between the electrode and the insulating paper. The lower layer is affected by the horizontal and vertical electric fields; therefore, a large area of fiber looseness occurs on the surface of the oil impregnated paper. The degree of deterioration is proportional to the endurance time and aging time, and the damage degree of oil impregnated paper decreases with the increase of the number of layers. [ABSTRACT FROM AUTHOR]

The corrosive sulfur phenomenon has decades of history and current research is mainly focused on DBDS. However, the transformer oil usually contains multiple sulfides. Up to now, there is still a lack of appropriate quantitative methods to characterize the degree of multiple sulfur corrosion. In this paper, accelerated thermal aging experiments on oilpaper insulation containing dibenzyl disulphide (DBDS), dibenzyl sulfide (DBS) and dodecyl mercaptan (DDM) with different concentrations at 140 °C are described. During the aging process, samples are regularly taken characteristic indexes, such as the color of copper sheet, the degree of polymerization of the insulating paper, the furfural in the oil, the moisture in the oil and paper, and the acid value in the oil are measured. Data are processed by the entropy-weight method and the contribution of each group of sulfides to the deterioration of the insulation system are comprehensively evaluated. Finally, a general formula for the relationship between the aging time and the deterioration degree under different sulfides is obtained, which can quantitatively predict the degree of sulfur corrosion. This study can provide some reference for monitoring the protection of oil-paper insulation and prevent the occurrence of sulfur corrosion failure. [ABSTRACT FROM AUTHOR]

The paper presents a technique for determining the time index of growth of the slope of reverse voltage for double insulated cables, based on the body of the theory of series. It is proved that in the vicinity of the extremum point (maximum) the function of the reverse voltage is approximated by polynomials of the nth power. It is proposed to use second-degree polynomials for practical calculations. The method for calculating relevant indicators is illustrated using real data. Analysis of deviations made it possible to conclude that the calculation method proposed in the paper is far more accurate. In the final part of the study, it was concluded that there is a promising outlook for further development of methodological guidelines for determining complex indices of the remaining life of the cable, including but not limited to the use of various mathematical methods. [ABSTRACT FROM AUTHOR]

Transformer insulation paper is a key indicator for transformer remaining operational life. Paper decomposition is evaluated using the degree of polymerization (DP) which calls for samples of insulation paper from operating transformers. Since collecting such paper samples is extremely difficult, other indicators have been used to indirectly reveal the DP value of insulation paper. This includes dissolved gases in transformer oil such as carbon oxides and hydrocarbon gases, furan compounds, methanol, ethanol, and moisture along with some oil characteristics such as interfacial tension. However, for the same oil sample, these individual parameters lead to different DP values. This is attributed to the lack of accuracy of the established mathematical and artificial intelligence models correlating DP with each of the above mentioned individual parameters. This paper presents a self-learning method to estimate the DP value of transformer insulation paper based on multiple transformer oil aging parameters. The proposed method comprises data processing, fuzzy c-means and linear regression. Results reveal that estimating the DP value based on multiple aging parameters is more accurate than estimating it using one single parameter as per the current practice. The proposed method not only helps to understand the correlation between multiple oil aging parameters and the DP value of paper insulation, but also promotes the establishment of more accurate life assessment models for power transformers based on these oil aging parameters.

The reliability of power transformers operation largely depends on the insulation system condition, especially on the condition of paper insulation. Determining the condition of paper insulation of transformers in use cannot be made by direct measurements. The most common way of assessing the paper insulation condition is by analyzing the content of 2-furfural dissolved in the insulating oil and by applying correlations between the degree of paper polymerization and the 2-furfural content in the insulating oil. The concentration of 2-furfural in the oil depends on the moisture content of the insulation system and the oxygen and acid content in the oil. In order to gain a better insight into the cumulative content of 2-furfural produced by paper degradation, excluding changes in the 2-furfural content caused by migration between paper and oil at different operating temperatures, dependence of the degree of polymerization of kraft paper to 2-FAL content in the oil, corrected to a reference temperature of 20°C, for a moisture content in cellulose of 1-1.5% and 3-4% is presented in this paper. This diagram has been verified on 4 transformers according to available data from the database of the Electrical Engineering Institute Nikola Tesla. In the future, this diagram should be completed for moisture content of 2.5% in cellulose insulation.

The paper presents the results of research and analysis of literature data on the paper insulation's thermal aging. An exponential decrease in the degree of polymerization of paper insulation from the aging time is established, at the starting time of transformers’ operation the aging rate of paper insulation is maximal and after that its slowing down. It was found out that the degree of polymerization at the starting time of the transformer's operation can be in the range of 900 units, this is due to its drying during the manufacturing process.

In this paper, the PD properties of an ester oil/PB and a mineral oil/PB system undergoing a moisture transient are investigated using a needle-plane electrode under natural cooling. The temperature, moisture concentration and water activity of the oil are continuously monitored. Moreover, an improved moisture diffusion model is developed by using COMSOL Multiphysics to verify the experimental results. The measurement results show that there is a sharp reduction of partial discharge inception voltage (PDIV) or a sudden increase of PD intensity in the process of natural cooling for both ester oil and mineral oil system. However, the PDs extinguish gradually after the occurrence of the minimum of PDIV. For the cooling mode selected in this study, the minimum of PDIV for the ester oil system appears in the third hour after cooling begins, while it is about in the second hour for the mineral oil system. In addition, the simulation results are consistent with the experimental data. [ABSTRACT FROM AUTHOR]

This article deals with the application of synthetic ester for drying cellulose insulation of distribution transformers. The most important advantages and disadvantages of the commonly used methods of drying transformer insulation on site are discussed. The emphasis is placed on the analysis and research of the most important factors affecting the drying efficiency of cellulose insulation using a synthetic ester, such as: insulation temperature, water content in ester, weight ratio of cellulose materials to ester, thickness of pressboard, and water content in solid insulation. The choice of values range of the individual factors which are used during tests on drying efficiency in laboratory conditions are justified. It is found on the basis of the obtained test results that to ensure the satisfactory drying effect an average insulation temperature should be at least of 70°C and the water content in the ester should not exceed 150 ppm. The results give the opportunity to estimate the drying efficiency depending on the conditions of the process. Moreover, the results are used to develop the procedure of drying of distribution transformers by means of synthetic ester. [ABSTRACT FROM AUTHOR]

High-voltage DC (HVDC) cables show more efficiency than high-voltage AC cables over the long distance power transmission and have attracted significant attention recently due to rapid developments in off-shore wind farm. Polypropylene laminated paper (PPLP) insulation based HVDC cables have provided a reliable operation for many years and are the highest voltage level in operation at the moment. In this study, the authors intend to understand the electrical performance of PPLP insulation system by examining space charge in detail, especially at the interfaces. It has been found that the interface between polypropylene and Kraft paper acts as trap for charge carriers, resulting in an optimal electric field distribution in PPLP insulation system. The optimal electric field distribution enhances overall electrical performance of PPLP insulation, including higher dc breakdown strength and lower electrical conductivity.

High entrance capacitances of ultrahigh-voltage (UHV) transformers make it difficult to generate impulses with standard front time during the lightning impulse tests. In this paper, we discuss the feasibility of examining the longitudinal insulation design of a transformer via impulses with a front time longer than the standard value. We consider the following features: transient voltage distributions and insulation characteristics of longitudinal insulation. The transient voltages of two UHV windings are measured with a capacitive voltage sensor under impulses with different front times. The results demonstrate that the front time of the applied impulse has little effect on the gradient voltage distributions and the oscillatory component of the transient voltage, but it determines the front time of transient voltages. Therefore, the relationship between breakdown characteristics of the turn-to-turn insulation and the front time is studied. Experimental results revealed that whatever the insulation thickness, an increase in the front time of the applied impulse decreases the breakdown voltage, but this variation is within acceptable tolerance when the front time changes from 1.2 μs to 6 μs. Thus, as long as the front time is restricted within 6 μs, we can ensure the validity of lightning impulse tests on the examination of longitudinal insulation designs. [ABSTRACT FROM AUTHOR]

The breakdown and streamer behavior in double-layer oil-paper insulation under impulses of varying front time were investigated in this study to determine the prebreakdown process in oil-paper insulation. The relationships between front times and breakdown characteristics were investigated and impulse-polarity-dependent variations were observed. The complete impulse breakdown process and precise role of front time were assessed as per streamer initiation and streamer propagation based on emitted light signals and shadow images. The re-initiation of streamers in non-first oil gaps was determined to be the key factor in breakdown as-dominated by applied voltage and space charge accumulation. The initiation and propagation times of streamers were observed to find that the streamers tend to propagate at phases with higher voltage amplitude as front time increases, which facilitates streamer re-initiation and thus decreases the breakdown voltage. Equivalent applied voltages during streamer propagation were further calculated to quantitatively investigate the effect of front time on streamer propagation. [ABSTRACT FROM AUTHOR]

The paper presents a technique for determining the time index of growth of the slope of reverse voltage for double insulated cables, based on the body of the theory of series. It is proved that in the vicinity of the extremum point (maximum) the function of the reverse voltage is approximated by polynomials of the nth power. It is proposed to use second-degree polynomials for practical calculations. The method for calculating relevant indicators is illustrated using real data. Analysis of deviations made it possible to conclude that the calculation method proposed in the paper is far more accurate. In the final part of the study, it was concluded that there is a promising outlook for further development of methodological guidelines for determining complex indices of the remaining life of the cable, including but not limited to the use of various mathematical methods. [ABSTRACT FROM AUTHOR]

In this paper, seven groups of sheet insulation for use in liquid nitrogen immersed power transformers have been tested under three different standard electrodes from ASTM D149-97a. The seven groups consist of three different types of paper/film, nonadhesive Kapton 100 HN (Kapton), THERNOMID 14.20 paper (NMN), and Nomex 410 paper (Nomex). The porous paper, Nomex 410, is found to have ac dielectric strength of 62–68 kV/mm, which is likely to be independent from its thickness. The nonporous film, Kapton, shows the dielectric strength of 288 kV/mm, whereas NMN paper has a dielectric strength of 74–78 kV/mm, larger than Nomex with similar thickness. Liquid nitrogen gaps near the triple junction of electrodes as well as liquid nitrogen pockets between the fibers are the main sources of partial discharge (PD) for NMN and Nomex. For Kapton, PDs are more likely to occur in the cavities filled with liquid nitrogen, either on the surface of the electrode or around the impurities on the surface of the film. Electrodes with a small stressed volume are more appropriate for dielectric tests of thin paper/film insulation, such as Kapton, due to the lower influence of cavities on the electrode surface. In contrast, electrodes with a large stressed volume and uniform electric field distribution are suitable for the papers such as Nomex and NMN. [ABSTRACT FROM AUTHOR]

The paper presents the results of investigations on the thermal properties of cellulose and aramid paper impregnated with various liquid insulations. The liquid insulations investigated were mineral oil, synthetic ester, and natural ester. The heat transfer coefficient of the liquid was calculated on the basis of measurements of the thermal expansion, specific heat, density, thermal conductivity and kinematic viscosity of the liquid insulations. Temperatures in the paper and liquid insulations were calculated. The results allow to select the best paper and liquid insulations which provide the most effective cooling in transformers. [ABSTRACT FROM AUTHOR]

This paper investigates thermal overloading, voltage dips and insulation failure across a distribution transformer (DT), under residential and battery electric vehicle (BEV) loadings. The objective of this paper is to discuss the charging impact of BEVs on voltage across consumer-service points, as well as across the life of paper insulation under varying ambient temperatures (during winter and summer), with and without a centralized battery energy storage system (BESS). This study contributes in two ways. The first part of this study deals with coordinated and uncoordinated BEV charging scenarios. The second part of this study deals with maximum utilization of a test DT rated under dynamic thermal rating (DRoDT). The DRoDT integration with BESS is carried out to flatten the load spikes, to obtain maximum DT utilization, to achieve active power and voltage supports in addition to an enhanced DT lifespan. The obtained results indicate that, when test DT operates under the proposed hybrid technique (combining both dynamic transformer ratings and a centralized BESS), it attains maximum utilization, lower hot-spot temperature, enhanced lifespan, less degraded paper insulation and an improved voltage across each consumer service point. The proposed technique is furthermore found effective in maintaining the loading across the distribution transformer within the nominal limits. However, under excess loading during peak hours, the proposed technique provides relief to the DT to a certain extent. To achieve an optimal DT operation and an enhanced BESS lifespan, the BESS is operated under nominal charging and discharging cyclic limits. Under the proposed DRoDT integration with BESS, DT attains 25.9% more life when loaded with coordinated BEV charging, in comparison to no BESS integration under the same loading scenario. The worst loading due to uncoordinated BEV charging also brings 51% increase in DT life when loaded under the proposed technique.

Frequency domain spectroscopy is a non-invasive, offline test performed to estimate moisture content, conductivity and aging status of the composite oil-paper insulation of a transformer. In conventional frequency domain spectroscopy test, sinusoidal excitation voltage is applied on the insulation under test for two cycles starting from 0.1 mHz to 1 kHz. One important aspect of conventional dielectric response measurement is the choice of excitation voltage. The insulation in real life operation is often stressed by voltage waveforms, which are non-stationary in nature. Therefore, modification of excitation voltage waveform is required for reliable assessment of insulation condition. Another important factor is the measurement time. Conventional frequency sweep from 0.1 mHz to 1 kHz, makes frequency domain spectroscopy measurement inadvertently lengthy, hence development of new measuring technique is necessary to reduce measurement duration for diagnosis of insulation condition. Considering these two facts, this contribution reports the feasibility of conducting dielectric response measurement in frequency domain using chirp excitation voltage waveforms. Instead of applying sinusoidal excitation at discrete measurement frequencies, chirp signals (band limited from 0.1 mHz to 1 Hz) were used to conduct frequency domain spectroscopy measurement on three laboratory prepared test samples and on two transformers. The dielectric dissipation factors evaluated using chirp waveforms showed a very good agreement when compared to that obtained by using sinusoidal excitation. Besides, investigations also revealed that the proposed method offered least amount of measurement time in comparison with conventional measurement as well as with some existing state of the art methods. [ABSTRACT FROM AUTHOR]

Seed-based oil has been identified as a viable alternative insulating fluid and a number of vegetable oils has been studied for that purpose. Among the seed-based oils studied is Jatropha oil which was reported to have suitable breakdown strength for high voltage application. However, most of the reports did not study the ageing characteristics of the oil and its compatibility with cellulose insulation. In this present study, the properties and ageing behaviour of a freshly prepared Jatropha oil and its compatibility with cellulose insulation paper were evaluated. The oil was purified through modified Dijkstra and Opstal purification method. The thermo-physical and dielectric studies of the oil was performed and thermal ageing of Kraft paper in the prepared ester fluid was performed. Thermally accelerated ageing was performed with an open beaker containing ageing catalysts, thermally upgraded insulation paper, and the fluid at 140°C for 120, 240, 360 and 480 h. Jatropha oil is a high temperature fluid with high specific heat capacity of 2.59 kJ kg-1 K and low dielectric loss of 0.009. The oil became polymerised with ageing in open cup. This may be due to thermo-oxidative degradation of the oil that progressed with ageing time. This is an indication that the oil is not suitable for use in free breathing power equipment. [ABSTRACT FROM AUTHOR]

Thermal and electrical degradation properties in low-viscosity (20 mm2/s, (20 cSt) at 25 °C) silicone liquid and insulation paper were investigated to improve reliability and to enable preventive maintenance technology in transformers. Heat-accelerated degradation in samples of 20 cSt silicone liquid and insulation paper in sealed glass tubes with copper, silicon steel, and pressboard was examined to replicate the environment of the transformer tank. After the examinations, the tensile strength of the paper was measured, and the quantity of decomposition compounds was also measured. It was concluded that diagnosing faults by analyzing decomposition compounds with silicone liquid should be possible in the same way that it is with conventional mineral oil. [ABSTRACT FROM AUTHOR]

In this study, a new multi‐criterion based fuzzy logic model has been proposed to determine the overall health index of transformers. The method relies on the concentrations of individual dissolved gasses, significant diagnostic test results of transformer oil and paper insulation. Real field data of 200 working transformers of a state owned power utility have been tested to validate the accuracy and reliability of the proposed fuzzy logic condition assessment model of transformers. Results obtained from the present proposed model have been compared with the previously proposed condition monitoring models. Comparison of the results shows that the output of present proposed model is more reliable and accurate. Integration of multi‐criterion analysis in the fuzzy logic model has overcome the shortcomings of the previous fuzzy models requiring higher number of inputs and large set of rules. The proposed fuzzy model is flexible, accurate and easy to implement for determining the overall health index of the working transformers. It shall prove to be very useful to the utility managers and power utilities. [ABSTRACT FROM AUTHOR]

The paper presents the results of studies of the generation of methanol in the accelerated aging of paper insulation in transformer oil under the influence of an elevated temperature. All investigated materials (cable paper, transformer oil) are of Russian origin. A description is given for the used models of paper insulation samples, specially designed test cells and a circulating bath. The results of a comparison of studies conducted by authors and other researchers from countries are analyzed. [ABSTRACT FROM AUTHOR]

Careful monitoring of high voltage equipments and diagnosing the critical conditions before they lead to a disaster are prerequisites of condition-based maintenance. Moisture content in a transformer is regarded as one of the major factors in diagnosing its conditions. It causes many problems for a power transformer including electrical breakdown between either its windings or one winding with neutral, increase in the amount of partial discharge and sundry minor problems. Since paper insulation of a power transformer carries large portion of water content, determining moisture content in this part of the transformer is essential. However, the problem is that the direct measurement of moisture in paper is impossible. Therefore, various methods have been proposed to measure the moisture content in a transformer but each one has its limitations. In this study, an approach is introduced to measure water content in a transformer by analysing the moisture dynamics in oil, tracking its variations and analysis of parameters such as temperature, without necessity of disconnecting the transformer from the power grid.

The article concerns the drying of power transformer cellulose insulation by means of synthetic ester. The introduction describes the negative influence of cellulose insulation moisture on transformer operation reliability. It presents the operation limitations resulting from high water content in cellulose insulation. The article presents currently applied methods of drying of cellulose insulation and a new proposed method which makes possible effective and safe water reduction in the transformer. This method consists in replacing mineral oil by synthetic ester during the drying time of cellulose. This electro-insulating liquid is being more widely applied as liquid insulation in transformers mainly due to its fire safety properties and biodegradability. Synthetic ester is characterized by high water solubility, which was used by the author of this article to dry cellulose insulation. The article presents the research results which confirmed the possibility of using synthetic ester for effective drying of cellulose insulation. The author compared the effectiveness of cellulose insulation drying by means of synthetic ester and mineral oil. [ABSTRACT FROM AUTHOR]

Bubbling phenomenon usually arises from overheated cellulose insulation in oil, which is known for deleterious effects on the insulation strength of oil-immersed transformers. In this paper, the experiment was conducted on a self-heating conductor wrapped with insulation paper. The inception temperature of bubbling phenomenon negatively correlates to the rising rate of temperature and moisture content of paper, which is in accordance with the results of previous research. Besides, bubbles formed under different conditions could be different in morphology and locations. Thereafter, a method for temperature rise was proposed, based on which the models were heated for component research. It is found that the component changes in oil and blanket gas are significant in a short time interval under bubbling conditions. Comparison trials were then conducted on naked conductors and paper-wrapped conductors. Based on the theory of mass transport, it is revealed that the water vapor and carbon oxides contribute a major part in bubble composition, and their proportions vary with the bubbling temperature. This study concludes that bubbling phenomenon at temperature below 120 ?C can be recognized as the vaporization of moisture in paper. Moreover, bubbles formed at temperatures around 160 ?C can be attributed to the combined effect of water vaporization and gases from cellulose degradation. [ABSTRACT FROM AUTHOR]

This article presents an algorithm for modelling an Adaptive Neuro Fuzzy Inference System (ANFIS) for power transformer paper conditions in order to estimate the transformer's expected life. The dielectric characteristics, dissolved gasses, and furfural of 108 running transformers were collected, which were divided into 76 training datasets and another 32 testing datasets. The degree of polymerization (DP) of the transformer paper was predicted using the ANFIS model based on using the dielectric characteristics and dissolved gases as input. These inputs were analyzed, and the best combination was selected, whereas CO + CO2, acidity, interfacial tension, and color were correlated with the paper's deterioration condition and were chosen as the input variables. The best combination of input variables and membership function was selected to build the optimal ANFIS model, which was then compared and evaluated. The proposed ANFIS model has 89.07% training accuracy and 85.75% testing accuracy and was applied to a transformer paper insulation assessment and an estimation of the expected life of four Indonesian transformers for which furfural data is unavailable. This proposed algorithm can be used as a furfural alternative for the general assessment of transformer paper conditions and the estimation of expected life and provides a helpful assistance for experts in transformer condition assessment. [ABSTRACT FROM AUTHOR]

Generation of hydrogen from oils with the addition of passivators such as 1, 2, 3-benzotriazole (BTA) and Irgamet® 39 for suppressing copper sulfide deposition was investigated by heating tests. Large amounts of hydrogen were generated from the oils with the addition of Irgamet® 39 compared to the oils with the addition of BTA. BTA is suitable in terms of less generation of hydrogen. For Irgamet® 39 added oils, the amounts of hydrogen increase with an increase in oxygen concentration. Irgamet® 39 was consumed by thermal decomposition and oxidation. Consumption by oxidation of Irgamet® 39 is considered to be affected to the hydrogen generation. Amounts of hydrogen increase with increase in heating temperature for a certain period of time. After conducting heating test, Irgamet® 39 was remained in oil. Total amount of hydrogen, which is defined by sum of the amount of hydrogen generated by consumed Irgamet® 39 and the amount of hydrogen generated by residual Irgamet® 39, are considered to be independent of heating temperature. Effects of additives such as dibenzyl disulfide, which is one of the prime compounds that cause the copper sulfide deposition, and 2, 6 ditertiary-butyl paracresol as antioxidant on hydrogen generation were significant. Consequently, in case of adding Irgamet® 39 for the purpose of suppressing copper sulfide deposition, installation of a conservator system and rubber bag which reduces the dissolution of oxygen into the oil is effective to minimize the amount of hydrogen. [ABSTRACT FROM PUBLISHER]

To improve the thermal stability of the cellulose insulation paper used in power transformers, Al2O3 nanoparticles were used to modify traditional cellulose insulation paper. Molecular simulations and experimental methods were used to analyze the enhancement and the microscopic mechanism of Al2O3 nanoparticles in the thermal aging process of insulation paper. Molecular simulations were performed for an unmodified and a Al2O3 nanoparticle-modified cellulose model to explore the changes in mechanical properties, cohesive energy densities and the glass transition temperatures of the two models in a temperature range of 70?150 ?C. The results of these simulations showed that the elastic modulus of the cellulose model modified with Al2O3 nanoparticles was larger than that of the unmodified model. Further, the addition of Al2O3 nanoparticles increased the glass transition temperature, signifying that the number of hydrogen bond connections was increased and the thermal stability of the modified model was improved. In addition, physical samples of modified insulation papers mixed with Al2O3 nanoparticles were made, and accelerated thermal aging experiments on these samples showed that the decrease in the degree of polymerization for the modified insulation paper was slower than that of the unmodified insulation paper. Further, breakdown tests suggested that the addition of Al2O3 nanoparticles has a certain positive effect on the AC breakdown voltage. Both the simulated and experimental results indicate that the addition of Al2O3 nanoparticles improves the thermal aging performance of cellulose insulation paper. [ABSTRACT FROM PUBLISHER]

Dissolved gas analysis (DGA) remains one of the best non-intrusive methods of assessing transformer health and internal faults. However, few approaches identify consistently the incipient faults initiated by low energy degradation processes. The Low Energy Degradation Triangle (LEDT) method formulates three low energy dissolved gases hydrogen (H2), methane (CH4) and carbon monoxide (CO) into a triangular representation to identify early state changes from normal to unhealthy. The approach has been applied to records of DGA from over 200 generator step-up transformers, to large transmission transformers and industrial units. A degradation parameter derived from the LEDT gave early prediction of the failure of more than 90% of transformers that failed or were removed from service. The approach appears to have benefits for transformer health assessment. It also illustrates an association in some transformers between degradation and geomagnetic disturbances. Additionally, the LEDT indicates there is little benefit from oil treatment after low energy degradation has commenced. [ABSTRACT FROM AUTHOR]

The power transformer is one of the most expensive pieces of equipment in a high-voltage AC power system. An adequate monitoring system for power transformers can help in reducing the failure rate and thereby enhance system reliability and economic efficiency. Long-term and accelerated aging is not possible in real transformers, so prorated models of real transformers are preferred. This paper presents a study aimed at the aging of different cellulose papers and pressboards impregnated with transformer oil subjected to accelerated thermal aging at an oil temperature of 120 ° C and winding temperature of 150 ° C for time intervals extending from 0 to 500 h. The study manifests a close relationship between the degree of polymerization and the tensile strength of paper and pressboard that could be used in predicting the condition of solid insulation at various elevated temperatures. The graphical representation of various solid insulation properties and their correlation analysis has also been carried out. [ABSTRACT FROM AUTHOR]

In order to explore a non-destructive method for measuring the polymerization degree (DP) of paper insulation in transformer, a new method that based on the optical properties of free fiber particles in transformer oil was studied. The chromatic dispersion images of fibers with different aging degree were obtained by polarizing microscope, and the eigenvalues (r, b, and Mahalanobis distance) of the images were extracted by the RGB (red, blue, and green) tricolor analysis method. Then, the correlation between the three eigenvalues and DP of paper insulation were simulated respectively. The results showed that the color of images changed from blue-purple to orange-yellow gradually with the increase of aging degree. For the three eigenvalues, the relationship between Mahalanobis distance and DP had the best goodness of fit (R2 = 0.98), higher than that of r (0.94) and b (0.94). The mean square error of the relationship between Mahalanobis distance and DP (52.17) was also significantly lower than that of r and b (97.58, 98.05). Therefore, the DP of unknown paper insulation could be calculated by the simulated relationship of Mahalanobis distance and DP.

The article presents a new method for determining moisture of solid materials using near-infrared spectroscopy. In this method, the absorbance of a near-infrared wave passing through methanol is measured; water is extracted beforehand from the material in methanol. The mass of water extracted from the sample is determined on the basis of dependence of absorbance on the water concentration in methanol. The determined water mass to dry mass ratio of the sample allows to calculate the percentage moisture of the investigated material. This new method of water content measurement was prepared in order to determine moisture in fibrous electro-insulating materials that are applied in, e.g. power transformers. The investigations described here allow to determine the time necessary to extract water from cellulose and aramid materials. The author also compared the research results of water content obtained by means of this new method with results obtained by using the normalized Karl Fischer titration method and the weight method. It was proven, based on this comparison, that the new method can be used to measure water content in fibrous materials that are either impregnated or unimpregnated with mineral oil. According to the author, the method of measuring water content presented here can be widely applied in different branches of the industry, such as in food processing and pharmaceutical manufacture. [ABSTRACT FROM PUBLISHER]

The article concerns the presence of water in insulating systems consisting of cellulose materials impregnated with different electro-insulting liquids. This is a particularly important issue regarding power transformers with their strategic importance for an electrical power system. Water is a serious threat and can be the reason of a dangerous breakdown. Thus there is a need to have insulating system diagnostics in terms of water content. The article discusses the issue of water solubility in different electro-insulating liquids. The author determined the water saturation limits for these liquids. Moreover, the author determined water sorption isotherms for cellulose materials of different aging degree. On the basis of the obtained results, equilibrium curves were constructed for some selected kinds of insulation. The research results presented here allow to improve the reliability of determining moisture in cellulose insulation of various polymerization degree on the basis of water content measurement in an electro-insulating liquid. [ABSTRACT FROM PUBLISHER]

The water concentration in transformer paper insulation must be monitored because, if it becomes too high, the paper insulation will age rapidly [1]-[4] and the transformer may fail on overload [5]-[9]. A failure can occur on overload if the temperature of the paper rises sufficiently to cause evolution of water bubbles. If these bubbles cause dielectric failure of the oil gap, the transformer may fail catastrophically. Paper insulation becomes brittle as it ages, and eventually becomes too brittle to act as an effective insulator. Oxygen increases the aging rate because the acids created by chemical reactions occurring in the oil, paper, and oxygen react further with water [10]-[12]. A study of the thermal aging of Kraft paper found that wet oxygenated paper degrades 40 times faster than the same paper heated in a dry oxygen-free environment [4]. The paper within a transformer is likely to degrade fastest at the top of the winding, where it is hottest [13]. Since the water concentration in the paper significantly influences its aging, it is important that the water concentration be measured accurately at the top of the winding. [ABSTRACT FROM PUBLISHER]

The adoption of prognostics for critical assets has the potential to advance asset management in the power industry significantly. Whereas diagnostic techniques can identify the presence of incipient faults, prognostics aims to predict the future state of a given asset [1], [2]. Prognostics can therefore be used to estimate the remaining useful life (RUL) of the asset, and help plan maintenance while minimizing the risk of failure in service. Prognostics requires a good model of the process of deterioration, from inception through to failure [1]?[3]. Deterioration may be due to aging, as in the case of paper insulation, or it may be due to a fault. Regardless of the cause of deterioration, prognostics is useful only if the deterioration is slow enough that maintenance (repair or replacement) can be scheduled during the predicted RUL. Thus, prognostics is not superior to diagnostics if the deterioration is so rapid that failure cannot be prevented. [ABSTRACT FROM PUBLISHER]

One of the most detrimental effects associated with transformer insulating materials (paper and oil) and copper windings is the corrosive action of certain sulfur components found in transformer oil. Consequently, transformer oil is typically blended with both antioxidant and anti-corrosive agents in an attempt to prolong ageing effects and hinder copper sulfide deposition respectively. This study focusses on the effect antioxidants have on corrosive sulfur deposition on copper metal surfaces. To this end transformer oil was blended with a natural extract from turmeric powder and tested for its ability to act as an antioxidant and to retard copper sulfide deposition. The efficacy of this blended oil was compared with that of oil blended with either of two commonly used antioxidants, namely, 2,6-di-tert-butyl- 4-methylphenol or 2,6-di-tert-butylphenol. Tests for corrosive sulfur showed that for the turmerone oil blend there was a substantial delay in the onset of copper sulfide deposition on the surface of copper metal. Open circuit potential studies showed that the turmerone fraction that was blended into the transformer oil was adsorbed onto the copper surface. This unique property was not observed for the other two antioxidants in this study and can account for the retardation of copper sulfide deposition. In addition, the turmerone-virgin oil blend did not alter the thermal stability of the Kraft insulation paper in direct contact with the oil. The performance of the turmerone oil blend as a dual antioxidant and retardant of copper sulfide deposition renders it eminently suitable for extending the lifetime of insulation in fluid-filled transformers. [ABSTRACT FROM AUTHOR]

Power transformer is one of the most expensive equipments in a high voltage AC power system. Adequate monitoring system for power transformers can help in reducing the failure rate and thereby enhancing system reliability and economic efficiency. The failure of power transformer has serious consequences on system reliability. Long term and comprehensive testing cannot be done in working transformer, so prorated model of transformer is preferred. In the proposed work, a new dual temperature test cell is developed based on IEC 62332 and IEEE C57.100. The cell provides for a current carrying conductor wrapped with insulating paper and further insulation is provided with high-density pressboard as per arrangements in power transformer. In this paper a new mathematical model is developed using activation energy and preexponential constant calculated from experimental results at different temperatures. The expected life based on degree of polymerization (DP) value, shows nonlinear aging of insulation based on investigation carried out on prorated model. [ABSTRACT FROM AUTHOR]

The paper presents new diagnostic method applied to assess aging degree for paper-oil insulation used in professional electrical power device, primarily including medium and high power transformers. The described method is based on modified diagnostics using the recovery voltage effect occurring in dielectrics (RVM ? Recovery Voltage Methods). Modification of the RVM method applied so far, which is now used only to assess moistening degree for paper-oil insulation in power transformers, consists in a new parametric analysis of recovery voltage time characteristics. Simulation and experimental tests carried out in laboratory conditions for adequately prepared insulation samples have proven that the time-constant of recovery voltage decay (?R_MIN) is the most susceptible parameter as regards quantitative change in the function of aging of the examined insulation. This conclusion is also confirmed by statistical studies, in which a multi-parameter variance analysis method has been applied. An original equivalent circuit diagram of the insulation has been used to simulate aging degree of paper-oil insulation samples in numerical tests. The diagram is based on the Cole-Cole type dielectric response model. [ABSTRACT FROM AUTHOR]

The compatibility of a newly developed ester fluid (PKOAE) with cellulose insulation paper is evaluated by performing thermal ageing of Kraft paper in the new ester fluid. Thermally accelerated ageing was performed in sealed mild steel pressure vessel containing ageing catalysts, thermally upgraded insulation paper, and dielectric fluid (mineral oil and palm kernel oil alkyl ester) at 150°C for 28, 56, and 84 days. Under the same illumination, the color level of ester aged paper samples decreased by 33% after 84 days of ageing as compared with the color level of aged mineral oil impregnated paper samples which decreased by 73% within the same time. Whilst the samples of PKOAE impregnated paper exhibited approximately 5% reduction in tensile strength after 28 days of ageing, the kraft paper samples closer to copper strips were found to have a significant faster degradation rate. The tensile strength of paper samples closer to copper plates dropped to about 35% of the tensile strength of the unaged sample after ageing for 28 days, and the paper layers farther away from the copper plates decreased by about 10%. The mechanisms that could be responsible for the slower ageing rate of kraft paper in PKOAE fluid are discussed. The characteristic breakdown strength of PKOAE impregnated paper is on the average, 42% better than mineral oil impregnated paper. The high breakdown strength of PKOAE fluid may be responsible for the higher breakdown field of ester-paper system since the applied electrical stress is shared by the oil-paper combination. [ABSTRACT FROM AUTHOR]