Search

The oil-paper insulation system is widely used for the insulation purposes of power transformers as a result of its ability to withstand high thermal and electrical stresses. The presence of a gas cavity between impregnated papers layer is a serious problem. When localized breakdown takes place within this cavity, partial discharge (PD) occurs and ages the rest of the insulation system. This aging phenomenon declines the expected life time of the insulation system and subsequently the life time of the power transformers. Dissolved gas analysis is a recommended technique that is used in order to assess the state of the insulation system inside the transformers. In this paper, PD is recorded in the case of gas cavity presence between the layers of the impregnated papers under the effect of AC and DC voltage stress. This record is extended from the instance of PD inception until the breakdown of the whole insulation system. Furthermore, the concentration of the gases that dissolve in the insulating oil due to the aging of the impregnated paper is measured eight times during the record of the PD. Finally, the degradation of the insulating papers is studied optically through the use of a microscope at different stages of the aging test. Depending on PD measurements, dissolved gas analysis (DGA) results, and the optical study of the paper aging, the evolution of the aging of oil-paper insulation system as a result of internal PD is declared when AC and DC voltage stresses are applied. It is obtained that the aging evolutionary process totally differs with the alteration of the applied voltage.

Sandwich-structured nanocomposites have the potential to improve the mechanical and electrical properties of materials simultaneously. Nanofibrillated cellulose (NFC) modified paper sheet (denominated as N) and softwood paper sheet (denominated as P) are used to prepare insulating paper. Different kinds of insulating paper with the structures of PPP, NNN, NPN and PNP are prepared. Tensile strength, DC conductivity, dielectric response, space charge and breakdown strength are measured. The activation energies of prepared materials are calculated through DC conductivity at 30, 50, 70, and 90 °C. The mechanism for sandwich structure regulating the mechanical and breakdown properties of insulating paper is discussed. Oil impregnated N/P/N paper has a remarkable improvement on tensile strength, conductivity, and DC breakdown strength, which provides a promising scheme to obtain the insulating paper with outstanding tensile and breakdown strength. [ABSTRACT FROM AUTHOR]

In this research, nano-TiO 2 of various diameters is added to cellulose and pressboard is made to enhance its anti-thermal aging characteristics. After thermally aging of the pressboard samples, partial discharge inception voltage (PDIV), AC breakdown strength and relative permittivity are measured. The results show that the dielectric properties of the pressboard made with 10 nm TiO 2 has the highest AC breakdown due to closer match of the relative permittivity. The mechanical properties are higher than the unfilled cellulose samples. The change observed of the surface microstructure of the samples show that aging resistance of the filled samples are better than the unfilled samples. [ABSTRACT FROM AUTHOR]

Due to the long-term effect of the electric field and the heating from the conductor heating, many bubbles will precipitate on the surface of the oil-paper inside the bushing. When the bubbles move to the strong electric field, the partial discharge is caused, which seriously threatens the safe operation of the transformer hushing. Therefore, the conditions for bubble separation were analyzed based on the physical analysis of the liquid. The test platform for bubble observation inside the bushing oil-paper insulation was established, and lest samples with different water content were tested subsequently. The rapid heating method was employed to study the bubble generation over the oil-paper surface, and the variation of initial temperature of bubbles was obtained. The bubble initiation temperature between 0.5 °C/min and 5.5 °C/min was explored through experiments. The results show that the bubble initiation temperature differed by a maxi-mum of 30 °C at different temperature rise rated. When the rising rate is greater than 3 °C/min, the bubble initial temperature begin to decrease significantly; while when the temperature rising rale is less than 3 °C/min, the bubble generation initial temperature is more dispersive hut the result is normally distributed; When the moisture content increases, the initial temperature of bubble generation decreases significantly. When the moisture content increases from 2. 7% to 6. 8% , the initial temperature of the bubble drops by 44 °C. The research results have engineering application value for analyzing whether there are bubbles inside the bushing during operation, evaluating the internal insulation status of the transformer bushing, and ensuring the safe and stable operation of the power transformer. [ABSTRACT FROM AUTHOR]

The article presents issues related to the aging behavior of oil-paper insulations in transformers using different oil- and ester-based insulating fluids. Despite numerous conducted studies on the subject of oil-paper aging, the use of new insulating fluids is creating open questions. In addition, new liquids such as synthetic and natural esters, as well as oil of the newest generation, are being used. Furthermore, there is still little research on the formation of aging markers with this form of the dielectric. For this reason, in this contribution, oil-paper insulations with mineral oil-based insulating fluids, natural and synthetic esters, as well as oil from natural gas, are aged thermally accelerated at 130 °C over a duration of 15 weeks, by considering two cases of free-breathing and hermetically sealed transformers. Therefore, various aging markers are investigated to allow a condition assessment. The results show that differences exist between the fluids and design of the transformer, as in the aging rate of the paper and the formation of aging markers in the insulating liquid such as acids. These findings can be used to improve asset management strategies by a more precise determination of the aging state depending on the transformer type as well as the type of insulating fluid.

The main insulation system in high-voltage direct current (HVDC) transformer consists of oil-paper insulation. The formation of space charge in insulation paper is crucial for the dielectric strength. Unfortunately, space charge behavior changes because of the corrosive sulfur substance in oil. This paper presents the space charge behavior in insulation paper induced by copper sulfide generated by corrosive sulfur in insulation oil. Thermal aging tests of paper-wrapped copper strip called the pigtail model were conducted at 130 °C in laboratory. Scanning electron microscopy (SEM) was used to observe the surface of copper and paper. Pulse electroacoustic (PEA) and thermally stimulated current (TSC) methods were used to obtain the space charge behavior in paper. Results showed that both maximum and total amount of space charge increased for the insulation paper contaminated by semi-conductor chemical substance copper sulfide. The space charge decay rate of contaminated paper was significantly enhanced after the polarization voltage was removed. The TSC results revealed that copper sulfide increased the trap density and lowered the shallow trap energy levels. These results contributed to charge transportation by de-trapping and trapping processes. This improved charge transportation could be the main reason for the decreased breakdown voltage of paper insulation material.

The oil-paper insulation in a transformer gradually ages due to the combined effects of thermal and electrical stress that occur during its operation. In this paper, for investigating the changes that occur within the oil-paper during the aging process with different aging stresses, partial discharge (PD) characteristics and trap parameters are measured. An internal discharge cavity model is designed for the electrical stress, and the thermal stress is generated by a baking oven simultaneously. The experiments are designed such that the dominant factor on the aging process is either thermal, electrical, or a unity of both stress factors. The phase resolved partial discharge (PRPD) spectrograms are recorded at different aging time, and the trap parameters are extracted by thermally stimulated depolarization current (TSDC) method. Additionally, the surface morphologies of these samples are observed by using a scanning electron microscope (SEM). The results show that the trap energy presents a decreasing tendency when the electrical stress plays the dominant role, and the discharge energy becomes larger over the aging time until pre-breakdown. When the thermal stress dominates the aging process, the trap energy shows an increasing tendency over the aging time, and there exists a process of ‘material conditioning’ that occurs at the initial stage of the aging process. The change of trap parameters in aged oil-paper will further affect PD characteristics. Correlation analysis of PD and trap characteristics can promote more accurate condition estimation of aged oil-paper. [ABSTRACT FROM AUTHOR]

Electric power equipment has complex construction. Therefore, it is very important to have enough information about the state of equipment. High voltage transformers play a very important role in the electric power system. One of the most important parts of electric power equipment is the insulation system. Insulation system must be in a good condition for reliable and safe operation of electrical devices. Insulation system of electrical equipment is exposed to various factors which could have negative influence on its condition. Oil impregnated insulation paper is one of the oldest insulation systems used in electrical power equipment. Mineral oils have been used for decades as transformer fluids because of their excellent dielectric properties and availability. However, performance of mineral oil starts to be limited due to environmental consideration. The aim of this paper is to simulate a real insulation system of transformer and to show the influence of accelerated thermal ageing on the insulation system. Properties such as relative permittivity, dissipation factor and the breakdown voltage will be described and analysed. [ABSTRACT FROM AUTHOR]

Insulation system is one of the most important part of electrical devices. This insulation system is usually composed by solid part and liquid part. Oil impregnated insulation paper is used in transformers for many years. There are a lot of factors which have significant influence on the insulation system during the life time of electrical devices. Temperature is one of the factors which has strong influence on condition of oil paper insulation. This paper is focused on measuring of electrophysical properties of oil paper as relative permittivity, AC breakdown voltage and polarization index of insulation before and after thermal ageing. Vegetable oil paper insulation and mineral oil paper insulation were investigated and analyzed. [ABSTRACT FROM AUTHOR]

Paper materials are used as the insulation of power transformers. Over time these materials slowly degrade, until they reach a point when they no longer function effectively as transformer insulation, and the transformer has then reached its end of life. The aging rate of paper is affected by temperature, water, oxygen and acids. Investigations have been performed previously on Kraft and Kraft thermally upgraded types of paper. However, a different type of paper used in transformer insulation, Grade 3 presspaper (which contains cotton), has not been extensively tested and compared to Kraft paper. In these experiments we studied the aging rate of Grade 3 presspaper and compared it to our previous studies of Kraft paper. Traditionally, the aging rate of paper has been studied in sealed vessels. The problem with this approach is that the chemical environment within the vessel will change during aging, and so the aging rate will be affected. In our experiment setup we controlled the water and oxygen content to more accurately determine the aging rate. Similarly to Kraft paper, the aging rate of Grade 3 presspaper with the same water content increased with oxygen content in the oil. Life curves were developed based on the water content of the paper and the oxygen content of the oil. [ABSTRACT FROM AUTHOR]

Abstract: Space charge is a key insulation problem of HVDC equipment. In this paper, the influence of voltage polarity and electrode material were studied by the pulse electroacoustic (PEA) method. It had been found that the type of space charge accumulating at the interface was the same with the voltage polarity. With different electrode materials, the space charge injection was consequently different, but not evident. The interface made the drift velocity slower in two-layer samples. It suggested that the interface had a significant impact on space charge distribution and the contact state was complex, so a modified Maxwell–Wagner model was proposed. [Copyright &y& Elsevier]

A substantial quantity of testing has been performed on the degradation of paper insulation over the decades. The aim being to better educate the electrical industry on how to best operate expensive assets, such as transformers. The longevity of paper insulation is frequently tested using accelerated ageing experiments, where the effects of temperature and the chemical environment on paper lifespan can be studied. Such research has resulted in paper life expectancy curves being published by the IEEE and the IEC. The investigations tend to use sealed vessels. However, the disadvantage of using this method is that the water content of the paper changes during the ageing process which then changes the ageing rate. In these ageing experiments the water and oxygen content was controlled using a special test rig to compare the ageing rate to previous work and to determine the ageing effect of paper by combining temperature, water content of paper and oxygen content of the oil. We found that the rate of paper ageing can be more accurately determined by controlling the water and oxygen during the experiment which then produced noticeable changes in predicting life expectancy. [ABSTRACT FROM PUBLISHER]

Oil-paper as a reliable insulation system is widely used in power transformers and cables. The dielectric properties of oil-paper insulation play an important role in the reliable operation of power equipment. However, the formation and dynamics of space charge can affect the performance of insulation material. In this paper, space charge in oil-paper insulation system has been investigated using the pulsed electroacoustic (PEA) technique. A series of measurements were carried out when the insulation system was subjected to different applied voltages at different temperatures. Charge behavior in the insulation system has been analyzed and the influence of temperature on charge dynamics was discussed. The test results show that homocharge injection takes place under all the test conditions, the applied dc voltage mainly effect the amount of space charge, while the temperature has greater influence on the distribution and mobility of space charge inside oil-paper samples. [ABSTRACT FROM AUTHOR]

A thermal aging experiment was conducted for oil-paper insulation to evaluate the insulation's response under thermal stress. Oil-papers at different aging stages were used to create oil-paper bound gas cavity specimens, which were used to collect data on degrees of polymerization (DP) and partial discharge (PD). Evaluation of oil-paper aging was based on statistical operators of PD and factor analysis was used to extract the principle parameters of PD. Three types of fuzzy clustering approaches were used to classify PD of aged oil-paper: the fuzzy c-means, the kernel fuzzy c-means, and the possibilistic fuzzy c-means. The clustering results showed that the possibilistic fuzzy c-means clustering was capable of classifying PD of oil-paper bound gas cavity specimens. The factor analysis method was also verified to be helpful in fuzzy clustering of PD data samples by reducing number of parameters. [ABSTRACT FROM AUTHOR]

The article presents issues related to the aging behavior of oil-paper insulations in transformers using different oil- and ester-based insulating fluids. Despite numerous conducted studies on the subject of oil-paper aging, the use of new insulating fluids is creating open questions. In addition, new liquids such as synthetic and natural esters, as well as oil of the newest generation, are being used. Furthermore, there is still little research on the formation of aging markers with this form of the dielectric. For this reason, in this contribution, oil-paper insulations with mineral oil-based insulating fluids, natural and synthetic esters, as well as oil from natural gas, are aged thermally accelerated at 130 °C over a duration of 15 weeks, by considering two cases of free-breathing and hermetically sealed transformers. Therefore, various aging markers are investigated to allow a condition assessment. The results show that differences exist between the fluids and design of the transformer, as in the aging rate of the paper and the formation of aging markers in the insulating liquid such as acids. These findings can be used to improve asset management strategies by a more precise determination of the aging state depending on the transformer type as well as the type of insulating fluid. [ABSTRACT FROM AUTHOR]

With the development of new insulation materials, vegetable oil?the best substitute for mineral oil?has gradually been widely used in the liquid insulation of transformers. To investigate the influence of vegetable oil on the thermal aging rate of oil paper, transformer paper impregnated with mineral oil and vegetable oil underwent thermally accelerated aging at three different temperatures. Degree Polymerization (DP) of paper was measured to indicate the aging degree of transformer paper. The aging rate of paper in mineral oil and vegetable oil was compared quantitatively, and results showed that vegetable oil retarded paper?s degradation rate and extended its useful lifetime. The reasons contributing to such phenomenon were analyzed using X-ray Photoelectron Spectroscopy (XPS) and molecular simulation software. Transformer paper in vegetable oil had larger activation energy. Due to the larger interaction force between water and natural ester molecules, water molecule was easily bonded with natural ester, weakening the hydrolysis process of cellulose. Cellulose was chemically modified by natural ester during thermal aging process, and the reactive ?OH (hydroxyl) groups on the cellulose became esterified with fatty acid esters. The water molecule was firmly bounded to the ester groups on glucose produced by esterification. The long-chain fatty acid esterified to cellulose was parallel with cellulose chains and acted as a ?water barrier? to further weaken the hydrolysis process. [ABSTRACT FROM AUTHOR]