Showing papers on "Pressboard published in 2021"

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

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

Estimation of Moisture Content in Oil-impregnated Pressboard through Analyzing Dielectric Response Current under Switching Impulse

Abstract: This paper proposes an advanced technique for estimating the paper moisture content within oil-impregnated pressboard by analyzing its dielectric response current signature under switching impulse voltage. The present work is oriented towards development of a method which can assess the condition of transformer insulation through measuring the dielectric response current during its switching on or off operation to the grid. In order to investigate the effectiveness of the method, the dielectric response current of a few oil-impregnated pressboards containing different moisture contents, are measured using switching impulse in laboratory. Based on the applied switching impulse and corresponding response current, the branch parameters of the equivalent circuit of insulation are evaluated which have been employed for formulating its transfer function. The zero and pole calculated from the transfer function are found to be sensitive to the paper moisture of insulation. Correlation of transfer function zero and pole with the paper moisture of insulation are derived using the experimental results. These correlations are validated through the experimental investigation on few other test samples.

Precise Measurements of the Temperature Influence on the Complex Permittivity of Power Transformers Moistened Paper-Oil Insulation

Abstract: The reference characteristics of complex permittivity of the transformers insulation solid component were determined for use in the precise diagnostics of the power transformers insulation state. The solid component is a composite of cellulose, insulating oil and water nanoparticles. Measurements were made in the frequency range from 10−4 Hz to 5000 Hz at temperatures from 293.15 to 333.15 K. Uncertainty of temperature measurements was less than ±0.01 K. Pressboard impregnated with insulating oil with a water content of (5.0 ± 0.2) by weight moistened in a manner maximally similar to the moistening process in power transformers was investigated. It was found that there are two stages of changes in permittivity and imaginary permittivity components, occurring for low and high frequency. As the temperature increases, the frequency dependencies of the permittivity and imaginary permittivity component shifts to the higher frequency region. This phenomenon is related to the change of relaxation time with the increase in temperature. The values of relaxation time activation energies of the permittivity ΔWτe′ ≈ (0.827 ± 0.0094) eV and the imaginary permittivity component ΔWτe″ = 0.883 eV were determined. It was found that Cole-Cole charts for the first stage are asymmetric and similar to those described by the Dawidson–Cole relaxation. For stage two, the charts are arc-shaped, corresponding to the Cole-Cole relaxation. It has been established that in the moistened pressboard impregnated with insulating oil, there is an additional polarization mechanism associated with the occurrence of water in the form of nanodrops and the tunneling of electrons between them.

Thermal Aging Effect on Properties of Pure and Doped Nano-TiO 2 Cellulose Pressboard

Abstract: 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.

Study of Oil/Pressboard Creeping Discharges under Divergent AC Voltage—Part 1: Fundamental Phenomena and Influencing Factors

Abstract: This paper presents an experimental study of the fundamental phenomena of progressive creeping discharge as well as the influences of pressboard ageing and voltage amplitude under divergent AC voltages. A ramp-stress test is used to determine the partial discharge inception voltage (PDIV) and the flashover voltage of oil/pressboard interface versus pressboard ageing. Constant-stress tests are carried out to investigate the partial discharge (PD) behaviors, the pressboard surface temperature, and the oil-dissolved gases of progressive creeping discharges. It is found that the pressboard ageing affects only the interface PDIV. The progressive creeping discharges concentrate at the triple-junction and comprises probably oil corona discharges and surface discharges in pressboard dimples. If no pressboard damage is present, the progressive discharges exhibit in general a constant PD pattern but a diminishing intensity, and nevertheless can be sustained for the severely-aged pressboards under strong voltages. The underlying causes are correlated with the competing effects of pressboard ageing on interface moisture and the triple-junction field intensification. Gas analyses indicate the hydrogen is the principal faulty gas and the Duval's Triangle diagnoses the progressive creeping discharges as either low-energy or high-energy discharge fault. No pressboard overheating is observed during the discharges.

Characteristics and Kinetics of Cellulose Degradation in Oil-Impregnated Pressboard under Simultaneous Thermal-Electrical-Mechanical Stress Accelerated Aging

Abstract: This study aims to investigate the characteristics and kinetics of cellulose degradation in oil-impregnated pressboard (OIP) under simultaneous thermal-electrical-mechanical stress accelerated aging. Thus, the multi-stress aging experimental setup is designed and established. OIP multi-stress aging experiments are performed at 130 °C, a DC electric voltage of +6 kV, vibration amplitude of ${10}{-}{50}\ \mu\mathrm{m}$ and vibration frequency of 100–300 Hz. The aging characteristics of pressboard and oil are then measured, including the degree of polymerization and aging by-products (i.e., furfural, moisture, acidity and dissolved gas). Investigations on aging characteristics reveal that thermal stress is still the dominant factor of cellulose degradation, while electrical and mechanical stresses serve as aging acceleration factors. The simultaneous thermal-electrical-mechanical aging of OIP is 10-24% faster than the single thermal aging. Moreover, the kinetics of cellulose degradation is also proposed to predict the bond scission fraction of cellulose with a goodness-of-fitting over 0.97. Finally, the correlations between the parameters in the kinetics of cellulose degradation and mechanical stress reveal that the increase of vibration amplitude generates more weak and amorphous links available for degradation, while the increase of vibration frequency accelerates the reaction rate.

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

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

Partial Discharge Detection Using Distributed Acoustic Sensing at the Oil-Pressboard Interface

Abstract: This paper investigates novel, initial experimentation in detecting and analysing Partial Discharge at the Oil-Pressboard interface using a continuous fibre-optic-based Distributed Acoustic Sensing (DAS) system. Discharge was successfully detected at a minimum of 223 pC despite the sample rate of DAS being lower than the spectra of acoustic emission. DAS presents multiple advantages over conventional Partial Discharge techniques including inherent localisation, immunity to electrical and magnetic noise, as well as much greater detection distances.

Partial Discharge Simulation of Air Gap Defects in Oil-Paper Insulation Paperboard of Converter Transformer under Different Ratios of AC–DC Combined Voltage

Abstract: A converter transformer is important primary equipment in a DC transmission project. The voltage on the valve side winding is complex when the equipment is running, including DC, AC, and AC–DC combined voltage. The insulation structure of the valve side winding of a converter transformer is an oil-paper insulation structure, which may have a variety of defects in the manufacturing stage and daily use, resulting in partial discharge. Therefore, it is the key to studying the partial discharge characteristics and mechanism of oil-paper insulation under AC–DC combined voltage. In this paper, we build a two-dimensional air gap model of oil-paper-insulated pressboard considering the actual particles and actual reaction based on the fluid model. The characteristics and evolution mechanism of partial discharge (PD) in pressboard under different AC/DC combined voltages are studied by numerical simulation. The results show that when the DC component increases, the polarity effect of partial discharge is more obvious, while the potential and discharge intensity in the air gap decrease. Further analysis revealed that the DC component in the combined voltage accumulated a large number of surface charges on the surface of the air gap, and the space charge distribution was more uniform and dispersed, which generated an electric field with opposite polarity to the DC component in the air gap and, then, inhibited the development of local discharge in the paperboard. The results of the simulation are consistent with the previous experimental phenomena, and the mechanism analysis of the simulation results also verifies the previous analysis on the mechanism of experimental phenomena. This will lay a theoretical foundation for the further study of partial discharge phenomenon of oil-paper insulation structures in practical operation in the future.

Effect of Partial Oil Change on Furfural Partitioning in Oil-Paper-Pressboard Insulation System

Abstract: The total/partial oil change treatment can affect the furfural partitioning in oil-paper-pressboard insulation, resulting in poor accuracy of furfural analysis. Moreover, due to the high cost of total oil change, partial oil change is more practical during transformer operation. This paper investigates the furfural partitioning in the oil-paper-pressboard insulation under partial oil change conditions. First, the initial furfural partitioning equation with non-oil change is proposed based on the obtained furfural content in oil, paper, and pressboard. Then, the modified furfural partitioning model under partial oil change condition is established by introducing a correction factor. Moreover, a preliminary correction scheme for the furfural partitioning model considering the influence of moisture and acid is provided. Finally, furfural content in oil-Kraft paper and oil-thermally upgraded Kraft (TUK) paper insulation are discussed, and results indicate that the proposed modified furfural partitioning model is not suitable for the oil-TUK paper insulation. This work can provide theoretical support for furfural analysis of oil-immersed transformers.

Behavior of positive streamers in ester liquids and mineral oil in a non-uniform field with and without insulating pressboard barrier

Abstract: This article presents the results of the studies on positive streamer behavior in non-uniform field the insulating system, in which the free liquid gap is divided on parts by pressboard barrier. The streamer characteristics were assessed mainly on the basis of registration of light emitted during streamer development and additionally on the basis of propagation velocity evaluated. Three liquids were tested during the studies namely mineral oil, synthetic ester and natural ester. A non-uniform electric field was created by point-to-sphere electrode system, a standard practice in laboratory, although very much unlikely taken place in real transformers. Two liquid gaps 20 mm and 40 mm were considered. The studies included LIBV determination of free liquid gaps, acceleration voltage determination in each case and streamer behavior assessment when 4 mm in thick pressboard barrier was placed 10 mm below HV tip. The results showed the differences in LIBV of the liquids considered in favor of natural ester for 20 mm gap and in favor of mineral oil for 40 mm gap. However when comparing acceleration voltage much better properties indicated mineral oil tested, what was expected based on literature data. When introducing the pressboard barrier the streamers significantly changed their characteristics in relation to the system without barrier. In most cases, at the values of the voltage at which the streamers developed as fast, they have been slowed down. Additionally, the differences in breakdown voltage when barrier introducing were significantly reduced. This fact is a benefit in assessment of ester liquids at lightning impulse stresses and in consideration for their use in power transformers.

Influence of Replacing Oil with Different Natural Esters on the Thermal Ageing Behavior of Mineral Oil-Paper Insulation

Abstract: Natural ester has high flash point, ignition point and good electrical strength, which could improve the overload capacity of transformer and delay the ageing of insulation pressboard. In this paper, the accelerated thermal aging test of oil-paper samples which composed of 25# mineral oil, FR3 oil, PFAE oil, cellulose insulation pressboard and thermally upgraded paper (TUP) are conducted at 120°C. In the middle ageing period, aged mineral oil in 6/33 samples were replaced with new PFAE and FR3, while aged PFAE in 3/33 samples were replaced with FR3, and then the accelerated thermal ageing test continued. The research on changing laws of DP of thermally upgraded paper (TUP) impregnated in PFAE oil, FR3 oil and mineral oil has been performed. The measurement of the insulation paper breakdown direct current (DC) and alternating current (AC) breakdown voltage is carried out. Comparison on kinematic viscosity and AC breakdown voltage of insulating oils are performed. Result shows that FR3 would increase over 1 time the lifespan of TUP in mineral oil, while PFAE shows a similar performance to mineral oil and could not obviously decrease the ageing rate of insulation paper. The kinematic viscosity of insulating oils did not change much after oil replacement. The AC breakdown voltage of natural esters are higher than that of mineral oil and could keep good insulation properties. This research would be helpful for the status evaluation of natural ester replacement transformer in the future.

Characteristics of Creepage Discharge in Oil/Pressboard Composite Insulation

Abstract: This paper deals with partial discharge characteristics in oil/pressboard composite insulation system, which is used in oil-immersed power transformer. It is found that the discharge propagates in the same route on the pressboard surface as in the previous discharge every time under repeated impulse voltage application. We also revealed that the white trace remained along the path of discharge and affected the subsequent discharge progress because of being a weak point part on insulation. This study presents an investigation of the effect of the white trace on partial discharge inception voltage (PDIV) of the composite insulation system with high-speed optical observation and electrical measurements for the discharge propagation. The discharge propagation mechanism is also discussed with comparing the discharge progress characteristics before and after the white trace formation.

A New Approach to Estimate Activation Energy of Oil-impregnated Pressboard Stressed under Switching Impulse at Different Temperatures

Abstract: This paper presents an advanced technique for calculating activation energy of oil-impregnated pressboard through dielectric response current measurement under switching impulse voltage at different temperatures. The proposed technique analyzes the measured impulse voltage and current signals to formulate its equivalent circuit at different temperatures. The equivalent circuit parameters are utilized to determine the shifting of ac conductivity curve along frequency axis with variations in temperatures. Based on the shifted ac conductivity curves at different temperatures, the corresponding activation energy of the insulation is estimated. The effectiveness of the proposed technique is investigated through a few laboratory prepared test samples.

AC Breakdown Characteristics of Different Moisture Content Pressboards Impregnated with Natural Ester

Abstract: -Biodegradable materials are necessary for saving the environment. Natural ester is alternative liquid insulation that can be biodegraded and used in a transformer. Insulation system in a transformer, liquid insulation cooperates with solid insulation such as cellulose. Pressboard is the main kind of cellulose used in the transformers. Normally, the characteristics of solid insulation can be investigated and analyzed by diverse methods. This paper represents the study of AC breakdown voltage of non-impregnation, 8-hours, and 16-hours impregnated pressboard with natural ester with different moisture content. To vary the moisture content of the pressboards, they were heated at 100 degrees Celsius for 12 hours and 24 -hours in a vacuum oven. After that, the Coulometric Karl Fischer method was used to determine the moisture content. Before impregnation, the natural ester was heated at 100 degrees Celsius for 12-hours in another vacuum oven. To investigate AC breakdown voltage, electrode configuration and test circuit were set-ups according to IEC 60243-1. From the test result, higher’ moisture content caused clearly lower AC breakdown voltage of the pressboards.

Influence of Water and Powder Contaminants on Dielectric Strength of Transformer Oil

Abstract: Transformer failure statistics reveal that most of the transformer failures are due to insulation failures, where a considerable fraction of that percentage is caused by oil contamination. In this paper, the influence of water and powder contaminants on the electrical performance of transformer oil is investigated. The powder contaminant used is the fibrous dust obtained from the pressboard insulation of high voltage transformers. In a transformer, pressboard powder can easily contaminate the oil during the operation due to electrical and thermal stresses. Levels of powder contaminants selected for the research are 20mg, 40mg, and 80mg and added water levels were 1 ppm, 2 ppm, 3 ppm, and 4 ppm for an initial water level of 26ppm. Effect of contamination on AC breakdown voltage and visual streamer inception voltage of transformer oil was measured, and the results were statistically analyzed. This consists of Pearson correlation analysis, one-way multivariate analysis of variance and one-way analysis of variance. The correlation between the measured parameters of powder and water contamination levels was calculated with the Pearson correlation coefficient. The effect size which is a parameter that indicates how much the variance of the response (dependent) variable is described by the predictor(independent) variable was calculated using one-way multivariate analysis of variance and one-way analysis of variance methods. The results obtained from one-way multivariate analysis of variance showed an 86.1 % effect size for the case of powder contamination. For the same case, with univariate one-way analysis of variance results, 88.8% and 96.2% effect sizes were calculated for breakdown voltage strength and visual streamer inception voltage respectively. For the water contamination case, one-way analysis of variance statistical method was utilized, and the effect size was calculated as 99.34% for the response variable (average of breakdown voltage and visual streamer inception voltage). Results of both cases made it evident that there is a significant effect of contamination on the dielectric strength of transformer oil. With a higher number of experimental results, the accuracy of the statistical analysis could be improved and with an increased number of response variables the analysis can be extended.

Investigation on Dynamic Diffusion Behavior of Furfural in Oil-Pressboard Insulation under Partial Oil Replacement Condition

Abstract: Furfural, as a typical chemical indicator, is used to evaluate the aging state of the cellulose insulation in the transformer. However, partial oil replacement will lead to the loss of furfural in oil and disturb the equilibrium distribution of furfural between oil and cellulose insulation, which could affect the evaluation results of the aging state of the cellulose insulation. Unfortunately, the influence of partial oil replacement has not been focused on. Given this issue, this work studies the influence of partial oil replacement on dynamic diffusion behavior of furfural in oil-pressboard insulation. First, the dynamic diffusion behavior of furfural between insulating pressboard and oil after partial oil replacement is discussed, and the theoretical diffusion model of furfural is established. Second, based on the mass gain curve of furfural in oil, the diffusion coefficients under different oil replacement ratios are obtained. Finally, a formula for calculating the diffusion equilibrium time is proposed. This paper attempts to report a method for analyzing the impact of the oil replacement ratio on the dynamic diffusion of furfural. These findings are expected to improve the accuracy of the aging assessment results by using furfural analysis.

Life Expectancy Estimation of Thermally Aged Cu Contaminant-Diffused Oil Impregnated Pressboard

Abstract: Life expectancy of a thermally aged oil impregnated pressboard (OIP) in the presence of corrosive dibenzyl disulphide (DBDS) is studied in this work using a fast estimation method based on level of diffusion of Cu contaminant. Data from laser induced breakdown spectroscopy (LIBS) is used as the degradation parameter. Thermogravimetric analysis (TGA) is used for rapid evaluation of life expectancy at various temperatures. Aged OIP material is tested for its surface flashover breakdown strength and its thermal stability is studied by TGA and differential scanning calorimetry. Thermal activation energy is estimated using Flynn-Wall-Ozawa method and is agreement with isothermally calculated values. Aging law is derived and found to be nearly first order based on which temperature dependent life expectancy law is formulated. LIBS data of OIP collected from a failed transformer is considered to be end-of-life condition. Estimated life expectancy values using proposed fast method are compared with traditional long-aging evaluation and are in good agreement. Increase in corrosive DBDS concentration and change in aging medium using proposed method and are found to have a significant impact on life expectancy of the OIP insulation.

Application and Comparative Analysis of Fuzzy Inference System for Transformer Fault Diagnosis with Dissolved Gases in Oil

Abstract: Oils are used for cooling and insulation in transformers. The fault currents occurred on transformers can damage windings and insulation. As a result of this incoming fault current, gas emission occurs in materials such as oil and pressboard. The type of these gases released gives information about the fault of the transformer. In this study, it has been studied on the determination of the fault by analyzing the gases released as a result of the failure in 63 transformers using FIS. As a result of the analysis, the results obtained with FIS were compared with analyzed results with IEC 60599 and fault reports from the corporation, which these values obtained. When 63.5% true results were obtained compared to IEC 60599, 97.6% correct results were obtained compared to fault reports of the corporation because 22 of 63 transformer gas values were not actually suitable for applying the IEC Ratio method.

Understanding the Flow Electrification of Synthetic Ester fluid Adopting Spinning Disc Method

Abstract: Accelerated thermal aging of synthetic ester fluid is carried at 120°C and their subsequent variation in charging tendency and dielectric properties with oil and pressboard were studied. Spinning disc system simulates the electrification phenomenon in laboratory conditions. The Flow electrification process is simulated with three configurations as, virgin oil with pressboard, aged oil with pressboard and aged pressboard with the fresh oil. The generation of current magnitude increases with the rotational speed of disc and with temperature. In addition, drastic reduction is observed with aged specimen. Surface potential measurement in pressboard is performed with +/-DC voltage profiles. Higher accumulation of charge and relatively lower charge decay performance is exhibited by aged specimens. In addition, Dielectric relaxation spectroscopy (DRS) studies exhibits the dielectric characterization of oil, the aged specimen is found to have a higher loss factor. Dielectric parameters are responsible for the electrification at the interface of the medium.

Experiment and Simulation of Pressboard Barrier Effect on the Streamer Propagation in Natural Ester During Positive Pulsed Voltage

Abstract: In this study, the effect of the pressboard barrier (PB) on streamer pattern in natural ester under the non-uniform electric field is investigated by experiment and simulation. The propagation patterns and characteristics of the streamer in the oil with/without a pressboard barrier are obtained by the shadowgraph technique. The mechanism of the pressboard barrier effect on the streamer pattern and propagation is revealed using a multi-streamer branch simulation method. Results show that the positional relationship between the needle electrode and the pressboard barrier significantly affects the streamer propagation and branching. The barrier effect of the pressboard, the distribution of space electric field affected by the pressboard barrier, and the image force of charge on the pressboard surface are all critical factors for affecting the propagation and branching of the streamer. The greatest threat to the pressboard failure is the z-axis streamer or those branches close to the z-axis that is severely inhibited by other streamer branches. It raises the risk of insulation failure caused by the breakdown of the insulating pressboard.