Oil-impregnated Paper Samples Research Articles

Evaluating the insulation condition of oil‐impregnated paper bushings based on the time constant database

AbstractAssessing the insulation condition of oil‐impregnated paper (OIP) bushings is necessary to guarantee the safe operation of power transformers. Moisture is the main threat to the insulation of OIP bushings. Therefore, accurately estimating their moisture value is crucial to keep power systems running smoothly. However, it has been suggested that the ageing status of OIP bushings should be considered while assessing the moisture. Dielectric frequency‐domain spectroscopy (FDS) has been extensively employed for assessing insulation condition of OIP insulations due to its benefits. A new approach was proposed to assess the insulation condition of OIP bushings using FDS results. Various OIP samples with various moisture levels and degrees of polymerisation (DP) were selected. The extended Debye model (EDM) parameters related to each sample were calculated using their tanδ − f curve. A time constant database (TCD) was created using the obtained time constants of EDM, which includes the dominant time constant and time constant related to the R0 – C0 branch, and their corresponding insulation condition. Then, a method was introduced based on the created TCD to estimate the insulation condition of OIP bushings. The results showed that the average relative error of moisture and DP estimation is 8.33% and 6.32%, respectively.

Evaluating the insulation condition of oil‐impregnated paper bushings based on the time constant database

AbstractAssessing the insulation condition of oil‐impregnated paper (OIP) bushings is necessary to guarantee the safe operation of power transformers. Moisture is the main threat to the insulation of OIP bushings. Therefore, accurately estimating their moisture value is crucial to keep power systems running smoothly. However, it has been suggested that the ageing status of OIP bushings should be considered while assessing the moisture. Dielectric frequency‐domain spectroscopy (FDS) has been extensively employed for assessing insulation condition of OIP insulations due to its benefits. A new approach was proposed to assess the insulation condition of OIP bushings using FDS results. Various OIP samples with various moisture levels and degrees of polymerisation (DP) were selected. The extended Debye model (EDM) parameters related to each sample were calculated using their tanδ − f curve. A time constant database (TCD) was created using the obtained time constants of EDM, which includes the dominant time constant and time constant related to the R0 – C0 branch, and their corresponding insulation condition. Then, a method was introduced based on the created TCD to estimate the insulation condition of OIP bushings. The results showed that the average relative error of moisture and DP estimation is 8.33% and 6.32%, respectively.

AC-DC breakdown characteristics of oil-paper insulation at low temperature

New energy sources such as wind energy are mainly distributed in low temperature areas such as high altitude and high dimensionality in inland China, which has higher requirements for the low-temperature insulation characteristics of power equipment. This poses higher demands for the low-temperature insulation capabilities of power equipment. However, the existing research on the low temperature breakdown characteristics of oil-impregnated paper is limited. In this study, the authors constructed an AC-DC combined power supply and a specialized low-temperature breakdown testing platform. Various oil-impregnated paper samples with different moisture contents were prepared. The paper investigated the AC-DC breakdown characteristics of these samples under low-temperature conditions, focusing on the effects of temperature, moisture content, and DC component on the breakdown strength. The findings serve as a valuable design reference for the application of oil-impregnated power equipment in low-temperature environments.

Lifetime of Oil-Impregnated Paper under Pulse Stress at Different Frequencies

With the increasing penetration of power electronic interfaces in the power grid, insulation materials will begin to experience stresses at higher frequencies than the conventional 50 Hz AC. This article studies the lifetime curves of oil-impregnated paper (OIP) under pulsed stresses and compares them at 10 kHz and 50 kHz. A pulse modulator is constructed consisting of a rectified DC supply feeding an H-bridge pulse driver connected to a 4:200 pulse transformer. The modulator is used to apply medium voltage pulse waveforms with rise-times of Tr ≈ 1.8 µs across single-layer OIP samples. The results clearly show that an increase in pulse frequency significantly accelerates insulation ageing. However, it is also observed that below a certain threshold of field strength, the slope of the lifeline decreases dramatically thereby indicating decelerated ageing. Possible reasons for this phenomenon are also discussed in this article.

Assessment of Power Transformer Paper Ageing Using Wavelet Texture Analysis of Microscopy Images

This paper presents a wavelet texture analysis of microscopy images acquired from artificially-aged samples of power transformer winding insulation paper. Oil-impregnated Kraft paper samples are prepared for experimental tests where they are thermally stressed in an oven at temperatures above what would normally occur in operation. This accelerated ageing test arrangement is used to produce a set of paper samples with varying levels of insulation deterioration. From the paper samples, microscopy images of the paper surface are obtained using a standard optical microscope. These microscopic images are analyzed using a texture analysis method that utilizes a two-dimensional wavelet transform to extract detailed information from the wavelet decomposition coefficient matrices. These features are analyzed to assess how the texture of paper changes in response to thermal deterioration and ageing. The results suggest that the texture features are correlated to the commonly-used degree of polymerization test. Additionally, statistical classification is performed on the wavelet texture features using both supervised and unsupervised machine learning methods. The results demonstrate that differentiation between oil-impregnated paper samples with different levels of thermal degradation using wavelet texture analysis is effective.

Modeling oil-paper insulation frequency domain spectroscopy based on its microscopic dielectric processes

The present study sets out to devise a universal function model to explain the characteristic curve obtained in the frequency domain spectroscopy (FDS) test on oil-paper insulation, based on its microscopic conduction and relaxation processes, and thus to enhance the accuracy and applicability of the test. First, from the analysis of the relationship between the real and imaginary parts of the dielectric's complex permittivity, it is demonstrated that a relaxation peak co-exists with the conduction process in the low-frequency band of an FDS curve obtained for oil-impregnated paper sample. Second, values for polarization barrier heights, essential to the determination of the microscopic polarization mechanisms, are presented as results of FDS and thermally stimulated depolarization current experiments carried out on oil-impregnated paper samples. The polarization peaks obtained in the imaginary permittivity frequency spectrum are determined as, respectively, space charge polarization and interface polarization. Finally, a function model in good agreement with experiment data is proposed, which quantitatively describes the FDS curve in oil-impregnated sample, including two relaxation processes and one conduction process.

Influence of thermal aging on the breakdown characteristics of transformer oil impregnated paper

For a long time, thermal effect, as a major factor for insulation aging, is a great challenge to the safe operation of power transformers. During the decades-long operation, cellulose fibers contained in the oil-paper insulation, break and deform, accompanied by the generation of new chemical products and variation of mechanical/electrical properties. Therefore, in this work, the influence of thermal aging on the electrical performance of the transformer oil paper insulation was investigated. 50% breakdown voltage and average AC breakdown voltage of oil impregnated paper were experimentally determined, and the water content measurement, surface morphology observation and frequency domain spectroscopy measurement were carried out on the oil impregnated paper samples with different aging degrees. The test results indicate that the electrical performances of aged oil impregnated paper improved during the aging process, which could be attributed to the forming of micro globules caused by the effect of thermal aging.

Classification of degradation in oil-impregnated cellulose insulation using texture analysis of optical microscopy images

Oil-impregnated Kraft paper samples normally used for power transformer electrical insulation have been artificially aged thermally to produce a sample set with varying levels of insulation deterioration. The samples were aged in an oven at a temperature above the normal operating temperature of a power transformer.Digital images obtained from optical microscopy measurements on the paper samples have been analyzed using a texture analysis method which converts each image into a spatial gray level dependence matrix (SGLDM). The SGLDM contains information about the statistical variation of pixels gray-level intensities in an image and thereby information about the sample texture. Mathematical operators applied to the SGLDM have been used to extract 22 different statistical texture features for each sample image.Optical microscopy images of the thermally aged samples show that thermal deterioration of the insulation paper produces changes in morphology and physical structure. These changes are detectable by the statistical texture features extracted from the SGLDM texture analysis. Statistical classification is performed on the feature set to demonstrate that differentiation between oil-impregnated paper samples with different levels of thermal degradation is reliable with low error rates. Therefore, development of a practical method to assess condition of oil-impregnated paper insulation using optical microscopy and texture analysis is promising.

Study of the Impact of Initial Moisture Content in Oil Impregnated Insulation Paper on Thermal Aging Rate of Condenser Bushing

This paper studied the impact of moisture on the correlated characteristics of the condenser bushings oil-paper insulation system. The oil-impregnated paper samples underwent accelerated thermal aging at 130 °C after preparation at different initial moisture contents (1%, 3%, 5% and 7%). All the samples were extracted periodically for the measurement of the moisture content, the degree of polymerization (DP) and frequency domain dielectric spectroscopy (FDS). Next, the measurement results of samples were compared to the related research results of transformer oil-paper insulation, offering a theoretical basis of the parameter analysis. The obtained results show that the moisture fluctuation amplitude can reflect the different initial moisture contents of insulating paper and the mass ratio of oil and paper has little impact on the moisture content fluctuation pattern in oil-paper but has a great impact on moisture fluctuation amplitude; reduction of DP presents an accelerating trend with the increase of initial moisture content, and the aging rate of test samples is higher under low moisture content but lower under high moisture content compared to the insulation paper in transformers. Two obvious “deceleration zones” appeared in the dielectric spectrum with the decrease of frequency, and not only does the integral value of dielectric dissipation factor (tan δ) reflect the aging degree, but it reflects the moisture content in solid insulation. These types of research in this paper can be applied to evaluate the condition of humidified insulation and the aging state of solid insulation for condenser bushings.

Effect of space charge on the accumulative characteristics of oil paper insulation under repeated lightning impulses

Repeated impulses have long been considered to affect the insulation properties of electrical equipment. This interesting but rarely studied phenomenon may pose great risks to the reliable operation of power equipment. Therefore, in this work, we focus on the accumulative effect of repeated lightning impulses on the oil-impregnated paper (OIP) used in power transformers. Specifically, we study the effect of space charge on the accumulative characteristics of OIP. A repetitive impulse accumulating test platform was set up, and the tests were conducted to obtain the accumulative characteristics of OIP under repeated lightning impulses. The accumulative characteristics of OIP with various interval time were experimentally identified to study the influence of interval time on the accumulative effect. Then, space charge in OIP samples were investigated using the pulsed electro-acoustic technique. Charge behavior in OIP was analyzed under different conditions, i.e., changing the applied times, amplitude and interval time of the applied lightning impulses. Charge injection and transport occurred during lightning impulse accumulation. The increased application of lightning impulses, high applied voltage, and short interval time contributed to the accumulation and transport of space charges. At last, the possible mechanisms of the accumulative effect are discussed in this article based on the experimental results.

Analysis of thermally aged insulation paper in a natural ester-based dielectric fluid

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.

Study on the accumulative effect of repeated lightning impulses on insulation characteristics of transformer oil impregnated paper

Oil-paper insulation, as a major insulation mode used in power transformers, is continuously and unavoidably subjected to repeated lightning impulses, which may bring potential risk to the insulation system. Therefore, in this work, accumulative effect of repeated lightning impulses and its damage mechanism in oil-impregnated paper (OIP) are investigated. Firstly a repetitive impulse accumulating test platform was set up, and tests were performed to confirm the existence of accumulative effect of repeated lightning impulses. Then atomic force microscope observation and frequency domain spectroscopy were carried out on OIP samples with different times of lightning impulses, based on which the influence of lightning impulses accumulative effect on surface morphology and dielectric properties of OIP was investigated. The results indicate that repeated lightning impulses produce accumulative damage to the OIP and hence lead to the morphology change and generation of translucent gelatinous substance on the sample surface. Both the surface roughness and dielectric parameters increase with the accumulating of repeated lightning impulses, which can be attributed to the effects of polar products, fiber deformation and the translucent gelatinous substance together. At last, the possible mechanism of the accumulative effect was discussed and relevant tests were carried out to verify the speculation.

Effects of paper-aged state on space charge characteristics in oil-impregnated paper insulation

Oil-paper compound insulation has been widely used in power transformers for quite a long time because of its good performance. The paper insulation degrades under the combined stresses of thermal, electrical and chemical stresses during routine operations, resulting in the reduction of its degree of polymerization (DP) and hence its mechanical and electrical strength. Space charges in a dielectric material have a close relation to its electrical performance. Therefore, in this work, the effect of paper-aged states (with different DP values) on the space charge behavior in oil-impregnated paper insulation is investigated, and the pulsed electro-acoustic (PEA) method is used for the space charge distribution measurement. In order to achieve this purpose, new oil-impregnated paper samples are thermally-accelerated aged for different time interval to obtain different DP values, and then the residual oil in each sample is removed, and the Kraft paper samples are dried in a vacuum dry oven. After that, all paper samples are impregnated with new oil, and PEA measurements are performed at room temperature. The experimental results show that, when the external dc voltage is applied on the oil-paper sample, homo-charge injection occurs for all samples. The samples with different aging states exhibit quite different space charge characteristics for both voltage-on and voltage-off conditions. When the aged states are worse, for voltage-on condition, more charges are induced at cathode and anode, as well as more space charges are accumulated in the bulk. For voltage-off condition, the space charge decay rate shows no obvious change for different aged samples.