Pressboard Samples Research Articles

A Particularly Dangerous Case of the Bubble Effect in Transformers That Appeared in a Large Mass of Pressboard Heated by Mineral Oil

The topic concerns the so-far-unknown mechanism of the bubble effect (b.e.) in a large mass of moist cellulose heated with mineral oil. The well-known b.e. occurs in the Hot Spot area, i.e., in the place where the hot metal of the windings is in contact with the insulation paper. The authors first showed that cyclic heating of a windings model causes the drying of both the insulation paper and pressboard, but the paper dries faster. For this reason, the bubble effect inception temperature can be lower in the pressboard than in the paper. Next, the authors showed that the bubble effect in the pressboard is very intense and causes a sudden and very large increase in pressure in the tank. Moreover, if the tank seal is suddenly damaged because of this, the number and volume of bubbles will increase dramatically. Next, the influence of the mass of cellulose to the mass of oil ratio on the pressure increase dynamics was tested. This experiment showed that the greater the mass of cellulose to the mass of oil, the greater the increase in pressure in the test chamber. The authors also determined that the characteristics of the bubble effect initiation temperature in the pressboard samples, depending on their moisture content, ranged from 2.0 to 4.8%. The experiment showed that the b.e. in the pressboard proceeds in the same way as in paper insulation. The research results showed that, in addition to the well-known b.e. in the winding paper in the Hot Spot area, the b.e. can occur in a large mass of pressboard cellulose, which can be much more dangerous for the transformer.

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

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

Exploring the electrical properties of pressboard and kraft paper insulation in power transformers: A dielectric spectroscopy and principal component analysis approach

<p>We explored the aging effects on insulating materials used in power transformers through dielectric spectroscopy. Frequency domain spectroscopy (FDS) was conducted for both aged and new Kraft paper and pressboard samples to determine their dielectric properties across a temperature range of 40–100 ℃ for pressboard and 40–70 ℃ for Kraft paper. Principal component analysis (PCA) was employed to classify the data, identify key factors contributing to variance, and elucidate the relationships between different parameters. The analysis revealed positive correlations between conductivity and frequency, as well as between permittivity and tan delta, with distinct differences in behavior observed between aged and new samples at lower frequencies. Additionally, the impact of temperature was evident, with increased temperature leading to an upward shift in dielectric behavior. At lower temperatures, aging had a reduced effect on the materials' properties. These findings provide significant insights into the dielectric behavior and aging mechanisms of insulation materials in power transformers.</p>

Study on Micro Topography of Thermal Aging of Insulation Pressboard Based on Image Processing

The insulation pressboard is very important in the insulation of oil-immersed transformers, which determines the useful life of the transformer. In order to analyze the change in the micro topography of the pressboard under the condition of thermal aging in detail, the insulation pressboard samples of different thermal aging periods were made. Image enhancement, denoising, segmentation, optimization, edge detection, expansion, and corrosion were used to process SEM images of the pressboard after thermal aging, so as to extract the fiber width, hole size, and surface roughness. By comparing the relationship between the micro topography and the degree of polymerization of the insulation pressboard in different thermal aging periods, it can be concluded that when the fiber width was 91% of the unaged pressboard fiber width, and the corresponding roughness was 0.162, the insulation life was only approximately half of its original value, and when the fiber width was 79% of that of the unaged pressboard, the corresponding roughness was 0.225, and the insulation life was nearing the end of its life. This shows that the image processing method used in this paper can observe the micro topography changes during thermal aging and can estimate the thermal aging degree of the insulation pressboard.

ELECTRICAL CONDUCTIVITY OF PRESSBOARD AND THE INFLUENCE OF MOISTURE CONTENT

The electrical conductivity σ is an important parameter for material condition evaluation at AC applications and is responsible for electrical field distribution in DC equipment. With a focus on HVDC equipment design, the influence of moisture content in oil-impregnated pressboard is determined in this preliminary investigation. The electrical conductivity of pressboard samples, which have been wetted artificially in the laboratory, is investigated within this work. Moisture contents between <0,3% and 5,5% could be achieved artificially through increasing pressboard moisture content levels in a climate chamber. The electrical conductivity was determined by voltage-current measurements at 20°C in the style of IEC 60093 with measurement times up to and longer than 24 hours. For these investigations, the pressboard samples with a thickness of 1 mm have been placed in an (mineral) oil-filled test vessel and stressed by a DC field with E = 3 kV/mm. It could be demonstrated that the moisture content of pressboard has a strong influence onto the electrical conductivity: An increase of electrical conductivity by a factor of around 10 for each percentage point of moisture increase up to moisture levels of around 3,5% was observed. At higher moisture contents (>5%), other mechanisms seem to govern the electrical current and the conductivity respectively, which is also discussed within the work.

Analysis of the Impregnation Process of Cellulose Materials by Using Ester-Based Liquids and Mineral Oil

Nowadays, new alternative of dielectric liquids with biodegradable characteristics has been considered to replace conventional mineral oil in both distribution and power transformer applications. However, there are some manufacturing processes that need to be modified to incorporate these ecological fluids, mainly in power transformers. Ester liquids, either natural or synthetic, show higher viscosity than mineral oil; therefore, a significant impact on the impregnation process of insulation materials is expected. In this work, the impregnation process of cellulose insulation is investigated by using three different liquids: natural ester (NE), synthetic ester (SE), and mineral oil. Laboratory tests were carried out using insulation blocks and pressboard samples to evaluate the influence of each fluid in the cellulose material. It is shown that the impregnation process with ester liquids is longer than mineral oil, at room temperature; however, it could be reduced by increasing the temperature of the ester liquid.

Compatibility Study of Silicone Rubber and Mineral Oil

In this study, three types of silicone rubbers, namely, insulative silicone rubber, conductive silicone rubber and silicone rubber with conductive as well as insulative layers are investigated for their compatibility with mineral oil. Mineral oil with different silicone rubber samples is thermally aged at 130 °C for 360 h, 720 h and 1080 h and at 23 °C, 98 °C and 130 °C for 360 h. At the end of each ageing interval, mineral oil and oil-impregnated silicone rubbers are investigated for their dielectric properties. Aged mineral oil samples are investigated for their moisture content, breakdown voltage, colour number, dissolved gases and total acid number, whereas solid insulation samples are investigated for their moisture content. Additionally, pressboard samples in mineral oil and mineral oil without any solid insulation materials are also aged under the same conditions and are investigated for their dielectric properties. From the obtained results, it can be assessed that the presence of carbon particles in conductive silicone rubber negatively impacts the dielectric properties of mineral oil. Among the investigated silicone rubbers, the insulative silicone rubber exhibits good compatibility with mineral oil and a strong potential for being used in mineral oil.

Thermal Aging Effect on Properties of Pure and Doped Nano-TiO2 Cellulose Pressboard

In this research, nano-TiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> 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 <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> 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.

Measurement Stand, Method and Results of Composite Electrotechnical Pressboard-Mineral Oil Electrical Measurements

The paper presents a measuring stand designed and built for testing direct and alternating current properties of power transformers basic insulation component i.e. electrotechnical pressboard impregnated with transformer oil. Measurements of direct and alternating current parameters are performed using the frequency domain spectroscopy and polarization depolarization current methods.The measuring station includes a specially developed climatic chamber which is characterized by high accuracy of temperature stabilization and maintenance during several dozen hours of measurements. The uncertainty of temperature maintaining during several dozen hours of measurements does not exceed ± 0.01 °C. The computer software developed to control the station allows for remote measurements, changes in supply voltage and temperature settings and acquisition of the obtained results. A new type of measuring capacitor was developed and manufactured, the structure of which significantly reduces the chance of samples contamination during measurements. By increasing the accuracy of temperature stabilization during measurements, the resolution of measurement temperatures was increased, at which it is possible to perform measurements with the frequency domain spectroscopy and polarization depolarization current methods. This allowed to reduce the step of measurement temperature change and thus to increase the accuracy of determining the activation energy of the measured parameters.The article also contains basic information on the analysis of the direct and alternating current electrical parameters of the composite electrotechnical pressboard-mineral oil-water nanoparticles. The results of several direct and alternating current parameters measurements of a transformer oil impregnated pressboard sample with a moisture content of (5.2 ± 0.1) % by weight obtained by the use of a measuring stand are presented as examples.

Analysis of deterioration characteristics in oil-immersed insulation pressboard with different durations of aging based on an image-processing method

Insulation pressboard samples were obtained by thermal aging (according to Montsinger’s formula, at 130 °C, the pressboard is heated for 0 to 32 days) and discharge experiments. SEM images of samples were analysed. Image segmentation was applied to calculate the fibre width, cross-sectional porosity, and carbon-trace area. Inter-layer fibre models were established to observe fibre morphology using 3-D reconstruction. The initial discharge voltage decreased with age, and the discharge amounts increased. After 16 days of aging, the fibre width had decreased to between 68.1% and 81.8% of unaged pressboard. As the aging increased, cellulose hydrogen bonds were broken, which affected the expansion of interlayer pores, increasing the porosity of the pressboard. After 32 days of aging, the porosity increased to 2.38 times that of a new pressboard. In addition, the longer the aging, the larger the area of carbon marks caused by the discharge breakdown. With the aggravation of thermal aging, the insulating property of pressboard decreased due to the decrease of fibre width and increase of porosity that further accelerated the damage to the fibre structure. It was concluded that the fibre width and porosity could be used as criteria to judge the degradation of pressboard.

Quantitative aging assessment method for cellulose pressboard based on the interpretation of the dielectric response mechanism

Cellulose pressboard is used as a key element in various power equipment insulation structures and determines the operational safety of electric equipment and power systems. This paper aims to increase the application of a pressboard aging assessment method based on frequency domain spectroscopy, a promising nondestructive method for power equipment diagnosis, through in-depth theoretical analysis and accurate data fitting. To achieve this purpose, first, based on classic theoretical models in the field of dielectric physics, a mathematical deduction is proposed to separate each dielectric process in the overlapping frequency band. Second, a novel analysis approach that considers both the pressboard dielectric response and the thermally stimulated current is proposed and demonstrated to be effective in determining the physical meaning of pressboard microscopic dielectric processes. Third, a function model based on a specific microscopic mechanism is established to fit the frequency domain spectra of pressboard samples with different aging degrees. Based on this approach, the variations in charge carrier motion characteristics in the frequency domain with increasing pressboard aging degree are clarified. Moreover, the function model provides characteristic parameters that are clearly related to the degree of polymerization of the material, which represents a quantitative method for evaluating the aging degree of cellulose pressboard by indirectly obtaining its frequency response curve.

Research on the electrical aging characteristics of oil-impregnated pressboard under partial discharges

The insulating performance of oil-impregnated pressboard decreases due to partial discharges (PDs). Although methods of pressboard life assessment and its dependency on PD have been extensively investigated, little is known about the microscopic process and the physical mechanism of PD-induced pressboard failure. This study investigated the electrical aging characteristics of pressboard from the perspective of physico-chemical analysis. Pressboard samples were treated with the chosen numbers of PDs to induce electrical aging and the chemical composition of the PD-treated pressboard was analyzed. The results revealed the generation of carbonyl groups and the consumption of hydroxyl groups on the surface of the pressboard after PD treatment. These groups were generated by the cracking of the C2-C3 and C1-O bonds in the pyran ring, and the oxidation of hydroxyl group in cellulose molecules, respectively. Subsequently, the effect of PDs on the surface topography of the pressboard was examined. The results showed that the surface roughness of the pressboard changed with a U-shaped trend under the influence of PDs, first decreasing and then increasing immediately after. In addition, pits were found on the surface of the pressboard. Breakdown of the pressboard happened when a micro pit grew to a critical depth.

Power frequency breakdown characteristics of new 3-element mixed insulation oil and oil-impregnated pressboard

Mixed insulation oil has attracted widespread attention in recent years due to its excellent performance. In our previous works, we proposed a novel 3-element mixed insulation oil with superior physical and electrical properties. To promote the application of the mixed insulation oil to transformers, we studied the AC breakdown characteristics of the 3-element mixed insulation oil and the mixed insulation oil-impregnated pressboards and then performed a comparison to Karamay 25# mineral oil. Oil or oil-impregnated pressboard samples with different moisture contents were obtained by moisture absorption or by controlling the drying time. The breakdown tests were carried out at different temperatures in an oil bath. The results showed that the AC breakdown characteristics of the 3-element mixed insulation oil are better than those of mineral oil and that the mixed insulation oil can maintain breakdown strength over a range of moisture contents. The breakdown mechanism of insulation oil is discussed according to the experiment results. Moreover, the breakdown characteristics of the pressboard impregnated with the 3-element mixed insulation oil are also better than those of the pressboard impregnated with mineral oil. These advantages of mixed oil promote application of the 3-element mixed insulation oil to power transformers.

Comparison of creeping discharges propagating over pressboard immersed in olive oil, mineral oil and other natural and synthetic ester liquids under DC voltage

This paper presents an experimental study on the characteristics of creeping discharges propagating over pressboard immersed in olive oil, rapeseed oil, tetra-ester, methyl oleate, and mineral oil, under both negative and positive DC voltages. The investigated characteristics are mainly the shape of discharges and their stopping lengths. Two experimental test cells are used: in the first one, the pressboard is inserted between pointe and plane electrodes so that the tip is perpendicular and at the center of the pressboard sample; and in the second one, the pressboard is placed between a point and bar electrodes in a way that allow a propagation in one direction (tangentially to pressboard). It is observed that the creeping discharges under DC voltage are not radial. Their shapes are different from those observed in previous work under AC and lightning impulse voltages. On the other hand, for given voltage and pressboard thickness, the stopping length L <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">f</sub> is shorter when the point is negative than when it is positive indicating that the flashover voltage will be greater with a negative point This also evidences a difference in the processes involved in each polarity especially the space charge resulting of the injected charges and the charge accumulated at the interface (including the charges resulting of double layer) as well as the evolution of the electric field in the vicinity of the point electrode and along the pressboard surface. For a given pressboard thickness, L <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">f</sub> increases with the voltage; it decreases when the thickness of pressboard is reduced.

Effect of the oil‐paper insulation aging on partial discharge characteristics in a hemispherical surface model

To investigate the effect of aging on surface discharge of the pressboard on the shielding electrode at the end of bushing, a thermal aging preparation methods of different aging degrees of pressboard samples, surface defect model, and the associated hemisphere are designed and the step-up method is used for subsequent experimental work. Finally, the partial discharges (PDs) under different degrees of aging are compared and combined with Fourier Transform Infrared spectra and electric field simulation. Results show that the pre-discharge occurs mainly in the oil gap between the hemispherical electrode edge and the pressboard. The aging of the pressboard exerts no obvious effect on the discharge characteristics in the initial stage of PD; with the development of the discharge, the development of aging of pressboard leads to a higher electric field intensity acting over a greater area, the more intense the discharge, the faster the relative discharge increases and the larger its amplitude. This discharge will cause decomposition of the oil in the pores of the aging pressboard thus generating gas bubbles which will move as discharge develops. When the bubbles reach the surface of the pressboard, the local electric field intensity will increase and the discharge will become more intense.

Effect of aging on material properties and partial discharge characteristics of insulating pressboard

Pressboard samples with different aging degrees were prepared to study the material and discharge characteristics of pressboards with a shielding electrode. The step-up method was used for the subsequent experiment. The partial discharges of the pressboards with different aging degrees were compared and combined using Fourier transform infrared spectra, as well as electric field simulations and analysis. The microstructure of the aged pressboards was observed with scanning electron microscopy. The results showed that pre-discharge occurred mainly in the oil gap between the hemispherical electrode edge and pressboard. Aging the pressboard had no obvious effect on the discharge characteristics at the initial partial discharge stage. As the partial discharge developed, aging the pressboard resulted in a higher electric field intensity acting over a greater area, which led to a more intense discharge. Microscopic observation showed that aging and discharge destroyed the fibre, reduced the fibre width, and caused holes and fractures, which promoted further development of the discharge. The spectroscopic analysis showed that aging destroyed O-H functional groups and reduced the intermolecular forces and mechanical properties of the pressboard.

Thermal aging of cellulosic pressboard material and its surface discharge and chemical characterization

Cellulosic pressboard is a key insulation material in oil filled transformers. Surface discharge or partial discharge degrades the insulation material and reduces the life time of the transformer. This study is aimed at understanding the effect of thermal aging of pressboard on the variation of its surface discharge and chemical characteristics. Thermal degradation of pressboard was carried out at 180 °C for 500 h in presence of ester oil under simulated aging conditions. The variation of salient electrical characteristics like surface discharge inception voltage (SDIV), charge measurement and optical emission spectra of the discharges were assessed at different aging periods. A significant reduction in SDIV was observed due to thermal aging. The charge measurement indicated that total charge deposition on pressboard increased with aging duration. Optical emission spectroscopy yielded valuable information regarding the elements evolved during discharge activity. Chemical characterization of the virgin and aged pressboard samples was carried out using analytical pyrolysis coupled with gas chromatograph-mass spectrometric technique (Py-GC/MS). The production of anhydrosugars, furan derivatives and oxygenates from the pressboard increased with aging time, while the esters in the oil were degraded to carboxylic acids with aging. Owing to the weakening of the hydrogen bonding network in oil impregnated pressboard with thermal ageing, high yield of end-chain depropagation products like anhydrosugars was observed during pyrolysis. The weak sites in cellulosic pressboard are expected to act as charge deposition sites.

Aging assesment of transformer pressboard insulation by micro-strip ring resonator at GHz frequencies

Condition assessment of insulation is widely used to identify the state of aging of power transformers. Traditionally, condition assessment is done using dielectric response measurements conducted at low frequencies. However, due to long testing time required for these methods, attention has now been paid towards measurements at microwave (above 1 GHz) frequencies. In this paper, we establish a correlation between dielectric properties and the state of aging of transformer pressboard samples. This is useful as it paves way to develop techniques to assess the state of aging by measuring dielectric properties at high frequencies. Dielectric measurements are made by using a microstrip ring resonator. The state of aging is determined by measuring the tensile index. Tests were carried out on dry and wet oil-impregnated pressboard samples thermally aged at laboratory conditions for durations up to 45 days. The results were compared with field aged samples collected from a 33 kV sealed type distribution transformer of 18 years in service. It was found that the permittivity values had a good correlation with respective tensile index values whereas loss tangent values showed limited correlation for both dry and wet samples. The results of field aged samples matched well with the established correlation curves. It can be concluded that the permittivity of pressboard samples estimated from the ring resonator can be effectively used to evaluate the state of aging of transformer insulation.

Degradation characteristics of oil-immersed pressboard samples induced by partial discharges under DC voltage

To investigate partial discharge-induced degradation characteristics of oil-immersed pressboard under DC voltage, an experimental platform with needle-plane oil-pressboard insulation model is established. The surface condition and DC breakdown voltage of pressboard samples after PD pretreatment are observed and measured. In addition, a fitting formula to evaluate the degradation extent of pressboard samples is proposed. The results show that under constant PD pretreatments, surface roughness of pressboard samples decreases first and increases afterwards. The changing rate is positively correlated with PD intensity. During PD pretreatment, a “whitish-colored circular region” appears at samples' surface and grows with PD intensity. For PD pretreatments of 500 pC and 1000 pC, DC breakdown voltage of pressboard samples remains unchanged. The breakdown spot has no clear carbonization nor creepage trace. For PD pretreatments of 2000 pC and 3000 pC, DC breakdown voltage decreases. The breakdown spot is obviously carbonized and creepage paths emerge around spots. Further analysis indicates that degradation of oil-immersed pressboard samples due to DC PDs is the result of motion activation process induced by electrical and thermal effects under DC electric field.

INVESTIGATION ON THE DEGRADATION BEHAVIOR OF CREEPAGE DISCHARGE ON PRESSBOARD IMMERSED IN PALM FATTY ACID ESTER (PFAE) OIL

Creepage discharge at the oil-pressboard interface is known as the development of a conducting path which is characterized by white and carbonized marks. This phenomenon tends to cause damage on the cellulose pressboard insulation which is subsequently promote to the catastrophic failure of a transformer. One of major defects that may cause the creepage discharge to occur along the pressboard surface is an excessive moisture content in pressboard insulation. Previously, there is an extensive research concerning the effect of moisture content in pressboard on the degradation of creepage discharge at mineral oil-pressboard interface. However, none has been found on the study of the effect of moisture content in pressboard on creepage discharge using ester oil. Therefore, this paper attempts to present the investigation on the degradation behavior of creepage discharge at Palm Fatty Acid Ester (PFAE) oil-pressboard interface by using a needle-bar electrode configuration under a constant AC voltage. Dry and wet pressboard samples (moisture content of 3 %) are used in this experiment to differentiate the degradation behavior. The development of creepage discharge is analyzed by correlating the visual records of creepage discharge and phase-resolved partial discharge (PRPD) pattern. An unexpected result has been found that the number of partial discharges (PDs) for wet pressboard is lower than the dry one as the formation of white mark with bush-like pattern is observed. Eventually, as the white mark is seen about halfway of the gap distance, the PDs number for wet pressboard sample significantly dropped, i.e. by 99 % compared to the dry board which is only 38 %. Variations in the PRPD data is due to constructive and destructive superpositions of electric field. The results suggest that the destructive effect is higher on the wet pressboard compared to the dry one.