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Showing papers on "Pressboard published in 2020"


Journal ArticleDOI
Jiefeng Liu1, Xianhao Fan1, Yiyi Zhang1, Hanbo Zheng1, Chaohai Zhang1 
TL;DR: In this paper, a fitting fingerprint database (FFD) technique was used for condition prediction of aging and moisture content of cellulose insulation in transformers, which can be used as a powerful tool for predicting the condition of transformers.
Abstract: The results of frequency domain spectroscopy (FDS) will be affected by moisture which often leads to an unreliable condition prediction of oil-immersed cellulose insulation in transformers. In view of this, a solution is proposed in this study by adopting a fitting fingerprint database (FFD) technique, which can be utilized for condition prediction of aging and moisture content. In the current work, the relevant fingerprints that could characterize the insulation aging condition and moisture content are extracted from the FDS curves and the DC conductivity of the transformer oil. The FFD is then constructed by fitting fingerprints. The accuracy of the reported FFD technique is verified from tests on oil-immersed pressboard prepared in the lab and a transformer from the field. In this respect, the present contribution highlights FFD might be used as a powerful tool for condition prediction of oil-immersed cellulose insulation in transformers.

32 citations


Journal ArticleDOI
01 Jun 2020
TL;DR: In this article, the authors compared the AC breakdown properties of insulation paper (pressboard) immersed by the new mixed oil and naphthenic mineral oil and found that the mixed oil is comprehensively superior to the mineral oil due to its larger relative permittivity.
Abstract: Insulation oil is an important dielectric in power devices, and many studies on mixed insulation oil have been conducted in recent years to improve the performance of insulation oils. To replace mineral oil directly, the authors previously developed a novel three-element mixed insulation oil successfully; the main parameters of which satisfy the IEC 60296-2012 standard for mineral oil. In the present study, the AC breakdown properties of insulation paper (pressboard) immersed by the new mixed oil and naphthenic mineral oil were compared. For both insulation oils, the increase in temperature cannot significantly reduce the breakdown strength of oil-immersed insulation paper (pressboard) at a low moisture content, and the increment of moisture content cannot reduce the breakdown voltage at a low temperature (25°C). The breakdown voltage decreases only when the two factors increase simultaneously. For the mixed and mineral oils, the AC breakdown voltage of oil-immersed paper (pressboard) with different thicknesses has significant difference. The mineral oil has a high breakdown voltage for the thin insulation paper, whereas the mixed oil has a high breakdown voltage when the thickness of the insulation paper (pressboard) exceeds 0.2 mm. This phenomenon is mainly caused by the breakdown field strength that varies with the increase of dielectric thickness and the different change trends of paper (pressboard) immersed with the mixed and the mineral oils. Moreover, the stack of thin multilayer insulation paper enables the mixed oil-immersed paper to have a higher breakdown strength than the mineral oil-immersed paper. For the AC breakdown voltage of oil-immersed pressboard with an oil-gap structure, the mixed oil is comprehensively superior to the mineral oil due to its larger relative permittivity.

25 citations


Journal ArticleDOI
TL;DR: In this article, a numerical model of bubble evolution is established by studying the microstructure of pressboard and the physical process of bubble generation, where the porous structure of fibers is equivalent to microtubules, the gases in the bubbles are classified into two categories, and the ideal gas state equation is used to describe the bubble state.
Abstract: In this paper, a numerical model of bubble evolution is established by studying the microstructure of pressboard and the physical process of bubble generation. In the model, the porous structure of fibers is equivalent to microtubules, the gases in the bubbles are classified into two categories, and the ideal gas state equation is used to describe the bubble state. Then the pressure conditions at the bubble boundary and the evaporation rate of water are taken as solution conditions to solve the physical quantities in the state equation. The numerical model provides a new way to calculate the initial temperature of bubble escape (ITBE). And the bubble evolution inside the pressboard is obtained which is hard to be observed in experiment. After that, the influences of moisture content of pressboard and cellulose aging on ITBE in the numerical model are discussed. The model results show that the expansion rate of bubble is not uniform. Bubble growth is slow in the early stage of temperature rise, when the temperature reaches a certain threshold the growth rate of bubble radius increases significantly, which is of great significance to the temperature limit on oil-immersed power transformers.

18 citations


Journal ArticleDOI
22 Sep 2020
TL;DR: In this paper, a measuring stand designed and built for testing direct and alternating current properties of power transformers basic insulation component is presented, which is characterized by high accuracy of temperature stabilization and maintenance during several hours of measurements.
Abstract: 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.

15 citations


Journal ArticleDOI
TL;DR: In this paper, the authors investigated the dissolved gases evolution in transformer oil under combined DC/AC electrical-thermal stress and proposed a carrier recombination coefficient to explain the oil DGA results under various aging stresses.
Abstract: This paper investigates the dissolved gases evolution in transformer oil under combined DC/AC electrical-thermal stress. Dissolved gases detected in transformer aged insulation oil reveal that oil under AC electric field combined with thermal stress can produce more dissolved gases than oil under DC electric field with thermal stress but less than the gases produced in oil under distorted AC or combined AC/DC voltage stress. This is attributed to the divergent migration properties of the charged components under different types of electric fields. To further understand this behavior, carrier recombination coefficient is proposed to explain the oil DGA results under various aging stresses. Results show that diagnostic parameters such as breakdown voltage, oil interfacial tension, and moisture content in pressboard should be used along with DGA results in order to accurately diagnose the insulation condition within converter transformers that impose a combined AC/DC voltage stress on the insulation system.

15 citations


Journal ArticleDOI
TL;DR: In this article, a novel effort of suspension of nanorods (NRs) into mineral oil has been carried out and the results have exhibited improved insulation characteristics of transformer oil.
Abstract: The mineral oil (MO) in conjunction with paper is the main insulation components in oil-immersed transformers; their insulation properties play a significant role in the safe and stable operation of power transformers. To strengthen the insulation level of ultra-high voltage transformer and to reduce its size and weight, it is imminent to enhance the insulating performance of transformer oil and oil-impregnated cellulose. Recently, a unique novel effort of suspension of nanorods (NRs) into MO has been carried out and the results have exhibited improved insulation characteristics of transformer oil. The Al2O3 NRs with favorable features were prepared in our laboratory to develop transformer oil/paper insulation system with better insulation performance. The transformer oil-based nanofluids (NFs) were prepared with nanorod shape, oleic acid surface modification and 0.8 g/L concentration of NRs. The impregnated pressboards were prepared by impregnating them into dried oil and NF under vacuum beneath 1 kPa at 80 °C for almost 48 h to obtain the oil-impregnated pressboard (OIP) and nanofluid-impregnated pressboard (NIP), respectively. The interface at solid/liquid is considered weak link and is the main reason of oil-filled transformer collapse. The effect of Al2O3 NRs on creeping discharge and flashover traits of oil/pressboard (OP) interface under AC and impulse voltages were studied. Partial discharge and creeping flashover test of OIPs and NIPs were conducted. TSDC and PEA tests were applied to examine the space charge properties of OP interface before and after suspension of Al2O3 NRs.

14 citations


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

12 citations


Journal ArticleDOI
TL;DR: In this article, the aging of cellulose pressboard was evaluated using frequency domain spectroscopy, a promising non-destructive method for power equipment diagnosis. But, the application of a pressboard aging assessment method was not discussed.
Abstract: 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.

10 citations


Journal ArticleDOI
TL;DR: In this paper, the flashover strength of nanofluid impregnated pressboard was tested for different weight fractions of nanoparticles and was compared to that of the base mineral oil.
Abstract: One of the weakest paths for electrical discharge in power transformers is oil/pressboard interface. So, in this study it is aimed to enhance the flashover resistance of such interface through using the concept of nanofluids. Nanofluid impregnated pressboard were prepared using two different types of nanoparticles. These types are Alumina (Al2O3) and Titania (TiO2) which have different values of zeta (ζ) potential that had been experimentally measured. Flashover tests are carried out at pressboard interface with nanofluids and mineral oil. Three conditions of oil flow are considered, static and turbulent dynamic flow with two different circulating velocities. Flashover strength of nanofluid impregnated pressboard was tested for different weight fractions of nanoparticles and was compared to that of the base mineral oil. In each test, the value and polarity of applied AC voltage at the instant of flashover was recorded. Weibull distribution was used for analyzing flashover strength data. The polarity at which flashover occurred was positive in all tests. Physical mechanisms have been discussed considering electron trapping and double layer model of nanoparticles.

10 citations


Journal ArticleDOI
TL;DR: In this article, the authors investigated the electrical aging characteristics of oil-impregnated pressboard from the perspective of physico-chemical analysis and showed that the surface roughness of the pressboard changed with a U-shaped trend under the influence of partial discharges.
Abstract: 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, "micro pits" were found on the surface of the pressboard. Breakdown of the pressboard happened when a micro pit grew to a critical depth.

9 citations


Journal ArticleDOI
26 Jun 2020
TL;DR: In this article, the authors present a test stand for measuring the penetration of transformer oil through electrotechnical pressboard, which consists of a pipe, to the lower end of which is glued a pressboard plate.
Abstract: The paper presents a new test stand for ivestigating the rate of penetration of transformer oil through electrotechnical pressboard. The stand consists of a pipe, to the lower end of which is glued a pressboard plate. The pipe is filled with insulating oil. A mirror is placed under the plate, which directs its image to the lens of the camera, which takes a series of photographs at a given time interval. After being saturated with the insulating oil, the pressboard changes colour from light to dark yellow. The absorbing time is defined as the time in which a dark yellow spot appears on the lower light surface of the pressboard after the pipe is filled with oil. A new way of determining capillary diameters has been developed when the number of capillaries is unknown and the volume of liquid flowing through them is not measurable. The distribution of the times of penetration of transformer oil through 2 mm thick electrotechnical pressboard was determined, the values of which range from about 220 min to about 550 min. It was found that the radii of capillaries through which the insulating oil penetrates are within the range from about 45 nm to about 70 nm. Due to the structure of the pressboard, which consists of cellulose fibres, arranged more or less tightly, there are capillaries in the structure of the board, each of which has sections of varying lengths of radii. This means that short sections of a single capillary can have radii both smaller than 45 nm and larger than 70 nm. The developed stand and the new analysis method can be used for testing various porous materials for penetration by various liquids.

Journal ArticleDOI
TL;DR: In this article, the surface discharge inception voltage (SDIV) with thermally aged EOIP material is higher under negative DC voltage compared with positive DC and AC voltage, and a minimal reduction in voltage was observed under harmonic AC voltages with different total harmonic distortion (THD).
Abstract: This paper reports the key findings from an experimental study carried out on thermally aged natural ester fluid impregnated pressboard (EOIP) material. The results show that the surface discharge inception voltage (SDIV) with thermally aged EOIP material is higher under negative DC voltage compared with positive DC and AC voltage. The SDIV increases with AC voltage frequency and a minimal reduction in voltage was observed under harmonic AC voltages with different total harmonic distortion (THD). The rate of change in voltage due to ripple content in DC voltage affects the SDIV of EOIP material, initiating discharges at lower voltages. The UHF signals radiated during the surface discharge process have a bandwidth in the range 0.9 to 2 GHz. Surface potential variation studies indicated the formation of high trap sites and trap density due to thermal aging. A reduction in the oxidation onset temperature and the activation energy of thermal decomposition were observed with thermally aged EOIP material. A pyrolysis study showed high composition of levoglucosan and acidic residue with the thermally aged specimen, which reduces its degree of polymerization. Impedance spectroscopic analysis indicates a non-Debye type of relaxation with thermally aged EOIP material. The mobility and charge transfer characteristics were also inferred from the modulus spectroscopy which showed less change for thermally aged pressboard material.

Journal ArticleDOI
TL;DR: In this paper, a method to monitor the growth process of electrical trees in pressboards based on solution of the inverse problem for the electrostatic field is described. But the method is limited to the case of tree branches.
Abstract: There is still a lack of effective means of observation for electrical trees in opaque materials, which hampers experimental and theoretical research on the growth of such trees in the oil-impregnated pressboards of power transformers. This presents a great obstacle to the accurate diagnosis and prediction of partial discharge (PD) defects. This paper describes a method to monitor the growth process of electrical trees in pressboards based on solution of the inverse problem for the electrostatic field. The three-dimensional structure of the carbonized branches is reconstructed from the positions of PDs. Each PD is located by the charge remaining in the material. This charge is calculated from the electric field intensity or potential at some points in the field using the mirror image method and solution of the inverse problem for the electrostatic field. The electric field intensity is measured using an array of metal strips near the zero-potential border while PDs are occurring. High-voltage PD experiments are performed on oil-impregnated pressboard strips to verify the feasibility of the method. From the results of these experiments, the accuracy of PD site location is found to be as good as 0.35 mm in the χ direction (the horizontal coordinate), which is sufficient to describe the general structure of electrical trees in oil-impregnated pressboard. The movement of the PD positions indicates the growth process of tree branches. The calculated positions and lengths of the branches are consistent with observations of carbonized tracks after the pressboard has been torn open. The results obtained here will be of benefit to establishing a real-time monitoring method for three-dimensional reconstruction of electrical trees to reveal their internal growth mechanism in opaque materials such as oil-impregnated pressboards.

Journal ArticleDOI
TL;DR: In this article, the effect of casein and polyvinyl-based adhesives used in winding cylinders was investigated through standard PD measurements, and the results indicated that pressboards bonded with casein-based adhesive exhibit PD values 20-25 pC lower than those bonded with polyvinel-based AA at identical applied voltages (7.5-25 kV or 3-10 kV/mm).
Abstract: In this study, 238 power transformers (PTs) with operating voltages between 150­500 kV and capacities in the range of 60–167 MVA are tested for partial discharge (PD). Thirty-eight of the PTs exceeded the pre-determined value of 70 pC at 1.5 maximum voltage of the PT. Failure analysis of the failed 38 PTs showed that 21% of the PD sources originate from the winding cylinder, which is formed by bonding the edges of a pressboard with an adhesive. Therefore, in this study, the effect of casein- and polyvinyl-based adhesives used in winding cylinders was investigated through standard PD measurements. The results indicate that pressboards bonded with casein-based adhesives exhibit PD values 20–25 pC lower than those bonded with polyvinyl-based adhesives (50–400 pC) at identical applied voltages (7.5–25 kV or 3–10 kV/mm). The excellent performance of casein-based adhesives is mainly attributed to its porous structure, which accelerates the impregnation of oil into the adhesive and pressboard, as revealed by scanning electron microscopy observations. This practical study will be beneficial in the process of designing and manufacturing PTs and highlight materials for producing PTs with low PD values.

Journal ArticleDOI
Zhou Peng1, Guangning Wu1, Bo Gao1, Yan Yang1, Guangcai Hu1, Liu Cheng1 
TL;DR: A needle-plate model was developed to explore different creepage discharge paths in oil-pressboard insulation under AC-DC combined voltage and observed that development of white marks corresponded to developmental process of discharge parameters, suggesting that creepage discharges can be diagnosed based on discharge parameters.
Abstract: The main dielectric medium in converter transformers is the oil-pressboard. The valve windings of converter transformers are subjected to AC-DC combined voltage. Creepage discharge is a common defect of oil-pressboard insulation. The degree of the oil-immersed pressboard (OIP) damage is directly affected by different creepage discharge paths. In this study, a needle-plate model was developed to explore different creepage discharge paths in oil-pressboard insulation under AC-DC combined voltage. A high-speed camera was used to record the developmental process of white marks on the OIP. We explored reasons behind the different creepage discharge paths under different voltage types. Results revealed that the creepage discharge path and the degree of OIP damage were influenced by the DC component. Although the damaged OIP in converter transformers may cause catastrophic flashover inside the transformers, it is difficult to replace it. This implies that transformer insulation designers should pay attention to the influence of the DC component on OIP damage. In this study, we used pulse current method to detect discharge patterns. Discharge parameters were then extracted from discharge patterns. We observed that developmental process of white marks corresponded to developmental process of discharge parameters. This suggests that creepage discharges can be diagnosed based on discharge parameters. Based on these conclusions, important information was provided for the on-line monitoring design of transformer insulation status.

Journal ArticleDOI
TL;DR: In this article, the influence of clamping pressure relaxation and pressboard aging on windings' overall modal characteristics is studied, and the local vibration characteristics of conductors under hoop tension and axial bending are investigated.
Abstract: This study investigates the overall and local vibration characteristics of disk-type windings in power transformers. The influence of clamping pressure relaxation and pressboard aging on windings' overall modal characteristics is studied. Individual conductors in a winding can vibrate independently and have the local vibration from that of overall windings. The local vibration characteristics of conductors under hoop tension, axial bending and the loss of clamping pressure are investigated. It is found that the clamping pressure relaxation and pressboard aging lead to the decrease of natural frequencies but the faulty conductors result in the increasing number of natural frequencies.

Journal ArticleDOI
TL;DR: Electrical insulation materials are highly exposed to electrical network-based electric and magnetic fields in power systems and a transformer failure which is related to pressboard insulation may lead to total breakdown and hence system malfunction in a total manner.
Abstract: Electrical insulation materials are highly exposed to electrical network-based electric and magnetic fields in power systems. In electrical fields, electrical insulation materials are prone to breakdown and cause failure. A transformer failure which is related to pressboard insulation may lead to total breakdown and hence system malfunction in a total manner. In this study, a test setup is used to conduct discharge tests for pressboards in different thicknesses where main interest is originated magnetic fields on the pressboards. These tests are fulfilled with spherical and rod electrodes in transformer oil where magnetic field sensors are used to acquire discharge-based magnetic field data. By investigating high-voltage stresses with different levels, possible breakdown voltage of a pressboard is predicted and statistically analyzed. In addition to magnetic field measurements, discharge current measurements are taken; however, contrary to conventional studies, this study assesses magnetic field data which are dependent on the thickness of pressboard insulation. For different voltage levels (13 kV and 22 kV for different stress levels), magnetic field measurements and current waveforms are obtained by using magnetic field sensors and high-speed oscilloscope. Magnetic field time series signals are subjected to wavelet analysis, and wavelet coefficients are obtained. Rather than utilizing time series current signals or time series magnetic field signals, wavelet coefficients of magnetic field signals are taken into consideration as a novel approach. These coefficients are processed by multifractal analysis, and finally, the integrity of the pressboard is determined as in proper mode (no failure) or in breakdown mode.

Proceedings ArticleDOI
25 Oct 2020
TL;DR: In this article, the authors presented AC and lightning impulse (positive and negative impulse) breakdown voltage characteristics of natural ester impregnated pressboards with different impregnation period.
Abstract: The paper represents AC and lightning impulse (positive and negative impulse) breakdown voltage characteristics of natural ester impregnated pressboards with different impregnation period. The pressboard specimen was prepared as a rectangular shape (the length and the width are 120 mm as well) and thickness dimension of the pressboard in the experiment was approximately 1.6 mm. The pressboards were dried up in the vacuum oven with temperature at 80°C for 12 hours under 200 mbar pressure. In addition, these pressboards were divided into 3-group according to the impregnation periods. The 1st group, the pressboards were impregnated with natural ester at 60°C for 8 hours under 200 mbar pressure. The 2nd and 3rd group, the pressboards were impregnated at 60°C for 16 and 24 hours, respectively, under 200 mbar pressure-controlled conditions. The plane-plane electrodes inserted with the specified number of the pressboard specimens were prepared. These electrodes were installed in the test cell filled with the natural ester. A 24 kV AC Voltage was applied to the test cell for 3 months. At the end of the first month, half of the pressboard samples was token from the test cells. Then, the AC breakdown voltage and impulse breakdown voltage characteristics of the impregnated pressboard were investigated in accordance with IEC 60243-1 and IEC 60243-3, respectively. At the end of the third month, another half of the pressboard samples was token from the test cells to perform AC and lightning impulse breakdown voltage test also. From the test results, all pressboards which are impregnated for a long period provided higher AC breakdown voltage than the pressboard which are impregnated for a short period. In the other hand, the impulse breakdown voltage values of each condition are not very different. Obviously, the period for impregnation significantly affects the AC breakdown characteristic of the natural ester impregnated pressboard. On the otherwise, it is not affecting the impulse breakdown characteristic of the natural ester impregnated pressboard.

Journal ArticleDOI
TL;DR: In this paper, the influence of nano modification on dielectric characteristics of modified pressboards were investigated, and their partial discharge characteristics in pin-plane electrode system were also studied, and the inhibition mechanism to partial discharge of nano-modification was analyzed by software simulation.
Abstract: In order to restrain partial discharge phenomenon of oil-paper insulation in convertor transformer, the nano montmorillonite (MMT) modified pressboards were developed by nano-doping method. The influence of nano modification on dielectric characteristics of modified pressboards were investigated, and their partial discharge characteristics in pin-plane electrode system were also studied. Meanwhile, the inhibition mechanism to partial discharge of nano-modification was analyzed by software simulation, and its impact to breakdown characteristics was verified as well. The measuring results showed that the conductivity rose with the increase of nano-doping content, while the relative permittivity fell firstly and then rose, which reaching the minimum value at 1%. Also, the breakdown strength of modified pressboard at nano doping ratio of 1% is higher than other ratios under DC and AC voltage. Further, the partial discharge inception voltage increases and maximum PD quantity decreases under same voltage level in oil-paper insulation by using nano modified pressboard at this ratio. Moreover, it helps to homogenize the electric field distribution of insulation structure in PD experiment, and finally contributes to the improvement of breakdown characteristics.

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

Journal ArticleDOI
TL;DR: It is indicated that the polarity of injected space charges is alternated with the decaying rate being raised when the absorbing moisture is aggravated in oil-impregnated pressboards, which accounts for the significantly exacerbated space charge accumulation.
Abstract: Oil-paper insulation as an irreplaceable insulation system in the electrical power transformer is inevitably suffering the water intrusion on working condition, which will make dielectric performance degraded and insulation aging accelerated. In high interest of explaining the deterioration effect of absorbing moisture on the dielectric properties of oil-paper insulation system and corresponding mechanism, the space charge and trap level characteristics are investigated for the specially prepared oil-impregnated pressboards with different moisture contents. The Pulsed Electro Acoustic and Isothermal Surface Potential Decay experiments are performed to test the space charge distributions and evaluate the trap level depths, respectively. It is indicated that the polarity of injected space charges is alternated with the decaying rate being raised when the absorbing moisture is aggravated in oil-impregnated pressboards. The charge carriers are more prone to be captured by the shallow-level traps that have been introduced by water permeation near pressboard surface than the intrinsic deep-level traps inside pressboard. The captured charge carries accounts for the significantly exacerbated space charge accumulation as manifested by the surface charges of oil-impregnated pressboards.

Journal ArticleDOI
TL;DR: In the presence of aged pressboard, the streamers in PO generally propagate further than MO at the same voltage level, and after subjected to ageing, the positive lightning breakdown voltages for PO and MO impregnated pressboards decrease.
Abstract: This paper presents the investigation on the breakdown characteristics and pre-breakdown streamer propagation of Palm Oil (PO) impregnated aged pressboard under positive lightning impulse voltages. The experimental work was carried out under a non-uniform field with needle-plane electrodes configuration. The streamer stopping length and breakdown voltage of 2 types of refined, bleached, and deodorized palm oil were examined in the presence of new and aged pressboards. The pressboard was placed in parallel to the needle-plane electrode at a gap distance of 50 mm. The lightning breakdown voltage was applied to the samples based on 1 shot per step rising voltage method under positive polarity as per IEC 60897. The presence of impregnated pressboard in both PO slightly increases the 50% positive lightning breakdown voltages. PO impregnated pressboards have lower 50% positive lightning breakdown voltages than MO. After subjected to ageing, the positive lightning breakdown voltages for PO and MO impregnated pressboards decrease. In the presence of aged pressboard, the streamers in PO generally propagate further than MO at the same voltage level.

Journal ArticleDOI
TL;DR: In this article, an isoparaffinic oil produced using a gas-to-liquid production process is compared to mineral oil as reference, which is shown to have a very high resistivity, and the resistivity is so high that it is more resistive than impregnated pressboard with low moisture content.
Abstract: Converter transformers in HVDC stations are subject to both AC and DC stresses. With new oils entering the market it is important that these are characterized both for AC and DC. This paper focuses on DC characterization of one such oil, which is an isoparaffinic oil produced using a gas to liquid production process. The results are compared to mineral oil as reference. The isoparaffinic oil is shown to have a very high resistivity. In fact, the resistivity is so high that it is more resistive than impregnated pressboard with low moisture content. This is a dielectric risk which must be considered if the oil is to be used in converter transformers.

Proceedings ArticleDOI
25 Oct 2020
TL;DR: In this article, the authors present the patterns of dissolved gas generation in natural ester under partial discharge conditions and the existence of acidity number is also reported, which can be detected by the dissolved gas analysis technique.
Abstract: The normal operating transformers encounter various kinds of stresses such as thermal stress, electrical stress, mechanical stress, and ambient effect that can expedite the deterioration process. Partial discharge occurs normally in high voltage equipment under high electric field. For a natural-immersed transformer, partial discharge can be detected by the dissolved gas analysis technique. This paper presents the patterns of dissolved gas generation in natural ester under partial discharge conditions. Besides, the existence of acidity number is also reported. Various types of partial discharge, including corona discharge, surface discharge, and the FR3 impregnated pressboard internal discharge, were simulated. A needle-plane electrode was used to simulate corona discharge, a needle-plane electrode with the impregnated pressboard was used to simulate surface discharge, whereas the plane electrode with pressboards was used to simulate the internal discharge. Each experiment was simulated in the test vessel under 24 kV test voltage for 3000 hours. The natural ester and the pressboards were heated at 80°C for 12 hours. before filled in the test vessel. Each test vessel was contained two identical plane electrodes with a 32 mm gap spacing. The impregnated pressboard was fit-inserted between such electrodes. The high voltage electrode was subjected to 24 kV whereas another plane electrode was grounded. Then, the natural ester were sampled from the vessel after operating for 350, 1000, and 3000 hours respectively. Dissolved gas analysis of the natural ester in this work followed ASTM D3612; the acidity number of aging natural ester was also tested according to ASTM D664. From the test results, it was found that different types of partial discharges generated various patterns of dissolved gas generation. Besides, the experiment with non-impregnated pressboards couldn’t survive in this condition.

Proceedings ArticleDOI
06 Sep 2020
TL;DR: In this article, the electric field and interface charge characteristics in both small and large scale oil-pressboard insulation structures were obtained to present the impacting mechanism of scale effect, which could be attributed to the more interface charge accumulation in larger scale model, namely the larger the scale of model, the stronger the weakening effect of more interface charges on electric field in oil.
Abstract: The design of oil-pressboard/paper insulation structure is the key to maintain the safe and reliable operation of power transformers, which mainly relies on RC model simulations and verification tests undertaken with simple or equivalent test models, neglecting the potential scale effect and falling short of the physical entity verification conducted in large-scale oil-pressboard insulation structures. Based on Kerr electro-optic effect, the electric field and interface charge characteristics in both small and large scale oil-pressboard insulation structures were obtained to present the impacting mechanism of scale effect. The coaxial large-scale model is based on a converter transformer outlet device entity, which has three layers of pressboards with the thickness of 1 mm to separate out three 5-mm-wide oil spacings, and the model has an inner diameter of 200 mm and a length of 500 mm. As for the small scale model with parallel-plate electrodes, it was scaling down at the same proportion with the large one on the basis of electric field strength equivalence principle. The experiment results indicated that: 1) in the small-scale model, as the size of pressboard increases, the difference between the measured field strength in oil and the RC model one would increase, which could be attributed to the more interface charge accumulation in larger scale model, namely the larger the scale of model, the stronger the weakening effect of more interface charge on electric field in oil; 2) The spatial-temporal distribution characteristics of electric field in the large-scale model are obtained showed that, in the inner and middle oil-spacing, the measured steady-state field strength is larger than that obtained by RC model, with a maximum increase of 77.3%, while in outer oil-spacing, it is smaller than the RC model calculating one, and the maximum decrease is 36.8%; 3) As for the large scale oil-pressboard test model with multi oil-spacings and multi interfaces, the calculation method for charge density on different interfaces was proposed to quantitatively present the impacting mechanism of scale effect on the electric field and interface charge characteristics.


Journal ArticleDOI
TL;DR: In this paper, the porosity and fiber width of unaged pressboard were calculated by image segmentation to calculate the fiber width, cross-sectional porosity, and carbon-trace area, and it was concluded that the fibre width and porosity could be used as criteria to judge the degradation of pressboard.
Abstract: 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.

Journal ArticleDOI
TL;DR: In this article, the deterioration of insulation pressboard under needle-plate discharge was tested, and the degradation stage was divided according to observed experimental phenomena, based on the improved Top-hat watershed image segmentation method, the white marks area of the pressboard were tested.
Abstract: The deterioration of insulation pressboard under needle-plate discharge was tested, and the degradation stage was divided according to observed experimental phenomena. Based on the improved Top-hat watershed image segmentation method, the white marks area of the pressboard were tested. Fibres with different discharge states were extracted from SEM images. The fibre width was calculated, and the porosity of the surface of the insulation pressboard at different degradation stages was calculated. Simultaneously, the 3D reconstruction technique was used to observe the 3D morphology of fibres at different discharge stages. The study found that with the deepening of the discharge process, the discharge of the pressboard increased, and the white marks of the pressboard continued to expand from point to surface. In addition, the diameter of the fibre of the insulating pressboard decreased obviously with the increase of the pressing time, and the fibre diameter was 89.6% after the breakdown. Moreover, the electrical stress had a great effect on the expansion of the interlayer pores of the pressboard; the cross-section porosity of the insulating pressboard gradually increased with the deepening of the discharge process, and after breakdown, the interlayer porosity reached 12.5%.

Proceedings ArticleDOI
25 Oct 2020
TL;DR: In this article, the polarization and depolarization current (PDC) characteristics of natural ester impregnated pressboards with different impregnation period were investigated. And the results showed that the PDC values of the pressboards experienced with longer electric field stress period (3 months) were higher than these of the the pressboard experienced with shorter electric field pressure period (1 month).
Abstract: This paper represents the polarization and depolarization current (PDC) characteristics of natural ester impregnated pressboards with different impregnation period. The pressboard specimen were prepared as a rectangular shape (the length and the width are 120 mm as well) and thickness dimension of the pressboard in the experiment was approximately 1.6 mm. The pressboards were dried up in the vacuum oven with temperature at 80°C for 12 hours under 200 mbar pressure. In addition, these pressboards were divided into 3 groups according to the impregnation periods. The 1st group, the pressboard was non-impregnated with natural ester. The 2nd and 3rd group, the pressboards were impregnated at 60°C for 8 and 16 hours, respectively, under 200 mbar pressure-controlled condition. The plane-plane electrodes inserted with the specified number of the pressboard specimens were prepared. These electrodes were installed in the test cell filled with the natural ester. A 24 kV AC Voltage was applied to the test cell for 3 months. At the end of the first month, half of the pressboard samples was token from the test cells. Then, the PDC characteristics of these natural ester impregnated pressboards were investigated. At the end of the third month, another half of the pressboard samples was token from the test cells to perform PDC measurement also. It was found that the PDC values of the pressboards with longer period impregnation process were lower than these of the pressboards with longer period impregnation process. Besides, the PDC values of the pressboards experienced with longer electric field stress period (3 months) were higher than these of the pressboards experienced with shorter electric field stress period (1 month).

Proceedings ArticleDOI
01 Jun 2020
TL;DR: In this paper, the authors investigated the role of partial discharge (PD) in the degradation process of the oil-cellulose insulation system in power transformers and found that the inner wall of the channels is carbonized by the PDs, such that the carbonized tracks and trees propagate along the gas channels.
Abstract: Oil-cellulose insulation systems are widely used in power transformers. Partial discharge (PD) is considered to play an important role in the deterioration process of the oil-cellulose insulation system. There has been little research in the past on the process and mechanism of PD destroying the oil-impregnated paper and pressboard. Through two sets of high voltage experiments on oil-impregnated pressboards, the phenomena about carbonized tracks, and trees were observed and analysed to understand the formation and development mechanism of the carbonized trees inside the pressboards. Furthermore, this analysis was also performed to reveal the mechanism and the process of PDs destroying the pressboards. From the results, we found that PDs occur in the gas channels. The inner wall of the channels is carbonized by the PDs, such that the carbonized tracks and trees propagate along the gas channels. The propagation of the carbonized tree is the process of the thickening and elongating of the tree branches. In addition, in the later stages of the thickening, the energy for carbonization comes from the leakage current and the chemical energy from the combustion of paper.