Showing papers on "Pressboard published in 2022"

Comparative Studies on Furans as Aging Markers for Power Transformers

Abstract: The investigation on the aging marker for palm fatty acid ester (PFAE)-filled transformer was performed. Sealed aging stainless steel tanks containing transformer components in the same ratio as a 27-MVA power transformer, were aged at 151 °C–168.5 °C for 48–2880 h in a strictly controlled oxygen-free and low initial moisture environment. The generation of 2-furaldehyde (2FAL) and other four kinds of furans including 5-hydroxymethylfurfural (5HMF), 2-furfuryl alcohol (2FOL), 2-acetylfuran (2ACF), and 5-methyl furfural (5MEF) from cellulosic insulating components impregnated in mineral oil (MO) and PFAE after thermally aging tests are determined and compared. The results indicate that the concentration of 2FAL, 2ACF, and 5MEF maintains an increasing trend with the aging time and temperature for both fluids. The correlations between 2FAL and the degree of polymerization (DP) of high-density pressboard (HPB) in both fluids are established. The results demonstrate that 2ACF and 5MEF can also be used as aging markers for transformer in spite of their much lower concentrations in the aged fluids.

Frequency-Domain Spectroscopy of Oil and Oil-Impregnated Pressboard With DC Bias

Abstract: Dielectric response measurement is widely used to characterize oil/oil-impregnated pressboards in order to evaluate the condition of transformer insulation. Insulation systems with oil can exhibit voltage dependence at low frequencies. This is due to the limited number of mobile charges in the liquid, rather than to the dielectric relaxation processes that may, in some cases, be of more interest to study. This work investigates the voltage-dependent properties of oil/oil-impregnated pressboards under ac voltages with regard to the complex permittivities. Frequency-domain spectroscopy (FDS) measurements are made with a dc voltage added to the ac voltage stimulus. Considered influences are the voltage amplitudes and polarity as well as the dependence on temperature. Properties of the space-charge polarization in the oil and oil-impregnated pressboard are calculated and discussed based on the dc-biased measurements. It is seen that using the dc bias in the FDS measurements of oil/oil-impregnated pressboard can significantly decrease the voltage dependence of the results that are caused by ion drift and consequent depletion of ions from the bulk liquid. Based on the dc-biased measurements, the complex permittivity due to the linear and nonlinear polarization can be separated.

Effects of Thermally Induced Bubbles on the Discharge Characteristics of Oil-Impregnated Pressboard

Abstract: Under temperature and moisture combined, bubbles will be formed in oil-impregnated paper/pressboard (OIP/OIPB). Although many studies have been done to understand the thermo-induced bubble phenomenon in OIPB, the formation mechanism is not, let alone its effect on discharge characteristics and insulation performance of OIPB. In this article, the phenomena and mechanisms of thermally induced bubbles, their effects and mechanisms on the partial discharge (PD), and breakdown characteristics of OIPB are investigated. The results show that the bubble inception temperature (BIT) decreases significantly with the increase of the moisture of OIPB. There is a growth process of bubbles from small to large, and the heating temperature and moisture content of OIPB affect the gas component inside the bubble. Thermal bubbles significantly reduce the PD inception voltage (PDIV) and breakdown voltage (BV), and the PD evolution and dissolved gas components in the oil show significant differences in the presence/absence of bubbles. Interestingly, PDIV and BV start to decrease in a temperature interval before BIT, and we infer that many microbubbles and gaseous channels exist inside OIPB before observable bubbles are formed on the surface. Considering the fibrous porous structure of OIPB and combining it with the bubble nucleation theory, we propose a new model for interpreting bubble formation and insulation degradation. The model suggests that bubbles (vapors) form first in microcavities and holes inside the OIPB and then spill over to the surface to form observable bubbles. The microcavities and pores in the OIPB are filled with gas (vapor) as the main cause of PD inception and insulation degradation. This study contributes to understanding the mechanisms of bubble formation in oil–paper insulation as well as provides a reference for risk assessment during overload operation and diagnosis of bubble-induced related faults of transformers.

Impact of DBDS and Silver Sulfide on the Performance of Thermally Aged Mineral oil Impregnated Pressboard Material

Abstract: The corrosive sulfur compounds present in the mineral oil causes the formation of dibenzyl disulfide (DBDS) complex and it can also erode the on-load tap changing selector contacts leading to generation of silver sulfide (AgS ₂ ).The present work reports the effect of DBDS and Ag ₂ S concentrations in mineral oil on thermally aged pressboard material. The results demonstrate that surface discharge inception voltage (SDIV) is higher for the aged pressboard specimen. The higher concentrations of DBDS showed a reduction of more than 50% in the magnitude of SDIV compared to its effect with the addition of Ag ₂ S. The surface potential analysis indicates a higher initial potential for the aged specimens with an increased half lifetime. In addition, only deep traps observed on the aged specimens with a slight right shift in its trap energy level. The mechanical strength of aged pressboard specimens with DBDS and Ag ₂ S is understood using tensile testing and simultaneous analysis is captured using Digital Image Correlation (DIC) technique to understand its strain percentage at different areas on the pressboard material during its elongation. The laser induced breakdown spectroscopy (LIBS) analysis is performed to identify the elements responsible for thermal ageing such as carbon, copper and sulfur on the surface of pressboard material. The plasma temperature calculated from sulfur peaks is higher for DBDS aged specimen compared to AgS ₂ and a linear correlation is observed on the LIBS intensity with ageing duration. Further, the Principal Component Analysis (PCA) is used for its classification on the degradation of pressboard due to the diffusion of both copper and silver sulfide from the mineral oil. The two principal components (PC1 and PC2) provided a higher variance of 99.8% with a clear classification observed between the unaged and aged pressboard specimens due to the addition of both DBDS and Ag ₂ S in mineral oil.

Partial Discharge Pattern Recognition Based on a Multifrequency F–P Sensing Array, AOK Time–Frequency Representation, and Deep Learning

Abstract: Recognizing the patterns of partial discharge (PD) is the key to assess the hazard level of PD. This article proposes an oil-immersed transformer PD pattern recognition method based on a multifrequency fiber-optic Fabry–Perot (F–P) ultrasound sensing array and deep learning (DL). Three F–P ultrasonic probes with different resonance bands were used to form a sensing array for collecting ultrasonic signals excited by PD. Five types of PD models were prepared, including metal tip (MT) discharge in oil, PD in the air cavity (AC), and surface discharge (SD) on the pressboard, in addition to SD after pressboard blocking or after pressboard plus transformer winding blocking, to study the influence of ultrasonic attenuation on recognition accuracy. The collected PD ultrasound signal time-frequency matrix was obtained using adaptive optimal-kernel time–frequency representation to reinforce the differences in PD patterns. Then, based on the features of the PD ultrasonic signals, the signal time and frequency band for analysis were determined. The intercepted time–frequency matrix of the signals from the three F–P probes was formed into a (3, 300, 375) tensor. A modified ResNet-18 net was used for PD pattern recognition, which achieved a 98% recognition accuracy. The probabilities given by the softmax function were used to study the confidence of the model’s predictions of signals belonging to known and unknown types.

Effect of Graphene Oxide Dispersed Natural Ester Based Insulating Oil on Transformer Solid Insulation

Abstract: Graphene oxide is one of the latest in- trend nanomaterials in the field of nanotechnology which is being studied in various engineering fields. Yet, its effects on the transformer solid insulation remain unexplored. In this research, the transformer pressboard and Kraft paper are impregnated with nonconductive graphene oxide dispersed vegetable ester-based insulating nanofluid and the behavior of impregnated solid insulation is studied under different thermal aging conditions. The results show that after 60 days of aging, the crystallinity of nanofluid impregnated pressboard is 24.9% more than that of the base oil-blend (OB) impregnated pressboard whereas the crystallinity of nanofluid impregnated Kraft paper is 3.8% more than that of the base OB impregnated Kraft paper. In terms of dielectric properties, the ac breakdown voltage (BDV) of nanofluid impregnated pressboard and Kraft paper is 29.5% and 41% more than the OB impregnated pressboard and Kraft paper, respectively. The dielectric stress is more uniform in nanofluid-based insulation system as compared to the base OB-based insulation system leading to a better design clearance. Therefore, graphene oxide dispersed ester oil (EO)-based nanofluid impregnated solid insulation has better mechanical and dielectric properties, thus leading to an increased lifespan of the transformer.

Cellulose Degradation and Transformer Fault Detection by the Application of Integrated Analyses of Gases and Low Molecular Weight Alcohols Dissolved in Mineral Oil

Abstract: This article presents a method for quantification of methanol and ethanol integrated in the same gas-chromatographic run with a quantification of gases dissolved in mineral oil, making it an integrated tool in transformer diagnostics. The results of aging experiments at 120 °C and 60 °C of Kraft paper, copper, barrier, and pressboard immersed in mineral oil, as well as the aging of thermal upgrade paper in mineral and natural ester oil at 140 °C are presented, in order to investigate correlations between different aging markers and to evaluate their partitioning between oil and cellulose at defined conditions. The results of partitioning experiments at 60 °C showed that re-absorption of methanol from oil to the cellulose materials is faster than the re-absorption of furans. This means that methanol is a paper-degradation marker that can be used in diagnostics over shorter equilibrium times and for the detection of developing faults at broader temperature ranges. Furthermore, a statistical overview of methanol concentration from a database and two transformer fault diagnostic cases are presented. Therefore, in addition to an analysis of gases dissolved in oil, the use of methanol and ethanol in transformer fault and failure investigations should be explored and verified through transformer fault investigations and postmortem analyses.

Assessment of Partial Discharge Phenomenon in AC and Fast-rising Square Voltage Using Optical Detection Technique in Needle-Plane Configuration With Pressboard in Mineral Oil

Abstract: The benefits offered by silicon carbide (SiC) switches in terms of repetition rate (> 100 kHz) and switching speed (up to 100 kV/µs) make them a popular choice for the converters of future grid projects. However, the impact of the fast repetitive pulses generated by these devices on the insulator discharge mechanism is yet unknown. This could be a significant obstacle in the future development of power system applications. This paper investigates the partial discharge (PD) phenomenon for pressboard/mineral oil insulation system under unipolar square voltage (USV) waveforms (dV/dt > 50 kV/µs), using a photomultiplier tube (PMT), with a high divergent field. As compared to AC, the partial discharge inception voltage (PDIV) is found to be lower under USV for tested specimens. The phase resolved partial discharge (PRPD) pattern highlights the difference in PD mechanism between AC and USV waveforms. Also, the number of PDs (NPDs) and average charge amplitude (Qavg) obtained under AC and USV waveforms are compared and discussed.

Green Solution for Insulation System of a Medium Frequency High Voltage Transformer for an Offshore Wind Farm

Abstract: High Voltage Direct Current (HVDC) transmission represents the most efficient way for transporting produced electrical energy from remotely located offshore wind farms to the shore. Such systems are implemented today using very expensive and large power transformers and converter stations placed on dedicated platforms. The present study aims at elaborating a compact solution for an energy collections system. The solution allows for a minimum of total transformer weight in the wind turbine nacelle reducing or even eliminating the need for a sea-based platform(s). The heart of the project is a Medium Frequency Transformer (MFT) that has a high DC voltage insulation towards ground. The transformer is employed in a DC/DC converter that delivers the energy into a serial array without additional conversion units. The insulation design methodology of an environmentally friendly HV insulation system for an MFT, based on pressboard and biodegradable oil, is introduced. The measurement method and results of the measurements of electrical conductivities of the transformer oil and Oil Impregnated Pressboard (OIP) are reported. The measurements show that the biodegradable ester oil/OIP conductivities are generally higher than the mineral oil/OIP conductivities. Numerical simulations reveal that the performance of the insulation system is slightly better when ester oil is used. Additionally, a lower temperature dependency for ester oil/OIP conductivities is observed, with the result that the transformer filled with ester oil is less sensitive to temperature variations.

The AC measurmets of the composite of cellulose - bio-based hydrocarbon transformer oil – water nanodrops

Abstract: The article presents a comparison of the currently used solutions in the field of insulating oils with the new Nynas NYTRO BIO 300X bio-carbon oil. The paper shows the results of measurements of an electrotechnical pressboard sample impregnated with bio-carbon oil, the moisture content of which was 4% by weight. Measurements made using the FDS method in the frequency range from 10 mHz to 5 kHz at the temperatures of 293.15 K, 301.15 K, 309.15 K, 317.15 K, 325.15 K and 333.15 K, made it possible to determine the frequency-temperature dependences of the following electrical parameters: admittance, phase shift angle, loss tangent, conductivity and permittivity.

DC and AC Tests of Moisture Electrical Pressboard Impregnated with Mineral Oil or Synthetic Ester—Determination of Water Status in Power Transformer Insulation

Abstract: In this study, the conductivity and permittivity of electrical pressboard—insulating liquid—water composites were investigated, and the electrical properties of the composites and water were analysed comparatively. Mineral oil and synthetic ester were used as insulating liquids. It was found that the presence of water caused an increase in the permeability of the composite in the frequency range below 100 Hz. The value of static permittivity determined by water in the content of 5 wt. % was approximately 15. To obtain this value caused by liquid water, its volume should be approximately five (oil) and four times (ester) higher than its actual content, respectively. The determined values of the activation energy of the DC conductivity of the composites were several times higher than the values of the activation energy of the conductivity of the liquid water. The experimental values of the dielectric relaxation times were many orders of magnitude higher than the dielectric relaxation times of water. This means that the experimental results obtained for the dielectric permittivity, the activation energy of conductivity and the dielectric relaxation times for moisture electrical pressboard impregnated by mineral oil or synthetic ester exclude the possibility of the presence of liquid water in the composites. It was found that the conductivity of the composites increased exponentially with increasing water content. Such dependencies are characteristic of hopping conductivity, caused by the quantum phenomenon of electron tunnelling between nanometre-sized potential wells. As the increase in conductivity is determined by the presence of water in the composites, therefore, the nanometre potential wells were single-water molecules or nanodrops.

Prediction Model of Bubble Formation in Oil-Paper Insulation Based on the ITBE Envelope

Abstract: Due to rapid temperature rise and insulation moisture, bubbles can generate in the oil-paper insulation of oil-immersed power transformers, which can reduce the dielectric strength of oil-paper insulation and even bring about the risk of insulation breakdown. In order to accurately evaluate the conditions of bubble formation, the physical process of bubble formation in oil-paper insulation is studied in this article. Considering the effects of water vaporization, gas dissolution in oil, and moisture migration on bubble formation, a bubble formation prediction model is constructed. Based on Henry’s law, the dissolution equilibrium of gas in oil is taken into consideration and the upper boundary of the initial temperature of bubble effect (ITBE) envelope is obtained in this model. Based on the moisture equilibrium in oil-paper insulation, the bubble formation process caused by the emergence and rapid evaporation of free water is analyzed, and then, the lower boundary of the ITBE envelope is obtained in this model. The model results show that the moisture content of the pressboard and the presence of free water have a great impact on ITBE. Specifically, the higher the moisture content, the lower the ITBE value. In addition, with the further increase of moisture content, the upper and lower boundaries of the ITBE envelope gradually tend to be the same. However, once free water is present on the surface of cellulose, ITBE decreases rapidly to below 120 °C. The model built in this article is of great significance to study the temperature limit of the power transformer. Hence the ITBE envelope can provide an important theoretical reference for reducing the risk of bubble formation during power transformer operation.

Analysis of Discharge Image and Particle Characteristics of Needle Plate in Transformer Oil Under AC Voltage

Abstract: For the development of converter transformer applications, research on the discharge characteristics in transformer oil is required. This article presents an analysis of the streamer discharge characteristics caused by a needle-plate electrode in oil-pressboard insulation under ac voltage and discharge images on the time scale of microseconds. Based on the discharge image, a Gaussian filter, image segmentation of the Otsu algorithm, and Canny edge detection are used to detect the edge of the discharge image. By extracting the discharge profile image, the discharge characteristics are effectively identified. The size and quantity of particles in transformer oil during the discharge are analyzed. Research on the characteristics of streamer discharge propagation is not only helpful for understanding the process of discharge but also has important significance for converter transformer design from the perspective of inhibiting streamer discharge.

Experimental Study on Bubble Effect of Oil–Paper Insulation in Natural Ester

Abstract: The rapid temperature rise brought by short-term overload may cause the oil–paper insulation of transformer to produce bubbles. The existence of bubbles will reduce the electrical strength of transformer insulation and may lead to partial discharge or even breakdown of the oil–paper insulation in the transformer. In this article, the bubble effect of oil–paper insulation in natural ester is studied experimentally. A bubble effect test platform is designed and built according to the temperature conditions of the transformer in operation, and bubbling tests are carried out for natural ester-cellulose insulation. Initial temperature of bubble escape (ITBE) is obtained for different winding insulation paper layers, temperature rise rate, and pressboard aging time, respectively. The experimental results show that the bubbles aggregate in the gap between the paper layers before bubbles escape into oil, and ITBE is affected by winding insulation paper layers. Besides, a higher temperature rise rate corresponds to a lower ITBE. With the thermal aging of pressboard, ITBE increases, because the number and diameter of pores decrease in fibers of pressboard.

Diagnostics on the Basis of the Frequency-Temperature Dependences of the Loss Angle Tangent of Heavily Moistured Oil-Impregnated Pressboard

Abstract: The aim of this study was to perform precision measurements of the frequency-temperature dependences of the loss angle tangent of the liquid-solid composite with the FDS Dirana meter. The composite consisted of heavily moistered oil-impregnated pressboard. The moisturization of the pressboard occurred in a manner as close as possible to the process of wetting the insulation in power transformers to a moisture content of (5.0 ± 0.2) wt. %. This value of moisture content was chosen because exceeding this value can lead to transformer failure. The measuring temperature range was from 293.15 K (20 °C) to 333.15 K (60 °C), with a step of 8 K. The measuring frequency range was 0.0001 Hz to 5000 Hz. It was observed that the shape of the frequency dependence of the loss angle tangent for a moisture content of 5.0 wt. % does not depend on the value of the measuring temperature. An increase in temperature leads to a shift of the waveforms into the higher frequency region. This is associated with a decrease in the relaxation time, and its value depends on the activation energy. It was found that a good fit of the waveforms, simulated by Dirana, to the actual tgδ waveforms obtained at temperatures between 293.15 K (20 °C) and 333.15 K (60 °C) requires the introduction of temperatures, higher than the actual insulation temperatures, into the program. It was found that estimating the moisture content for different temperatures using Dirana soft-ware for insulating an oil-impregnated pressboard produced large discrepancies from the actual content. Better results were obtained after an adjustment requiring manual temperature correction towards higher, compared to measured, temperatures. The moisture content estimated after correction by the Dirana meter ranges from of 4.5 wt. % to 5.7 wt. % and increases almost linearly with increasing measuring temperature. The average moisture content estimated by the Dirana meter for all measuring temperatures is 5.1 wt. % and is close to the actual content (5.0 ± 0.2) wt. %. The uncertainty of the estimate is ±0.43 wt. % and is more than twice as high as the true value.

Molecular Dynamics on Cracking Mechanism of Oil-Pressboard Insulation Under Electric–Thermal Coupling

Abstract: Transformer oil and insulation paper are subject to the combined effects of the electric field and temperature operations, which significantly impact insulation deterioration. This article designs a comparative test of the electro-thermal combined aging to obtain several characteristic parameters for the aging degree of oil-pressboard insulation. Molecular dynamics are used to establish a mixed molecular model of oil-pressboard insulation to simulate the pyrolysis reactions at high temperatures using the ReaxFF force field. The microscopic effects of the electric field on the aging of oil-pressboard insulation are studied. The rationale of the simulation model is verified from experimental results, and the trend of the reactants and products in the reaction process was considered to determine the influence mechanism of electric–thermal coupling on oil-pressboard insulation. The experimental results show that thermal stress remains the most important factor toward cracking in insulation paper. The acid value and furfural formation rate in oil increase, and the polymerization degree of the insulation paper decreases rapidly after the combined electric and thermal aging. In addition, the molecular dynamics simulations show that pyrolysis increases the bond energy, promotes repulsion between atoms, and leads to fractured bonds. The addition of the electric field increases the susceptibility of the chemical bonds in cellulose molecules to breakage. The morphology of the cellulose chain then changes, and the diffusion of small molecules intensifies. This study reveals the mechanism of insulation aging under electro-thermal coupling through combined macroscopic testing and molecular dynamics simulations.

Activation energy of DC conductivity of high moisture content electrical pressboard impregnated with mineral oil of natural origin NYNAS NYTRO BIO300X

Abstract: The paper presents the results of testing the polarization currents and DC conductivity of electrical pressboard produced by Weidman company, containing water nanoparticles with content of 4 wt. % and impregnated with mineral oil of natural origin NYNAS NYTRO BIO 300X. The time dependence of polarization currents and DC conductivity was determined for measurement temperatures in the range of 20 - 70°C. Arrhenius plot was prepared for 6 temperatures from the whole range of measurement temperatures. Linear approximations of the experimental results were made on the Arrhenius plot. The activation energy of the DC conductivity and the values of the coefficient of determination R ² were determined. The values of the coefficient of determination close to unity testify to the high accuracy of the measurements and the stability of the temperature. It was found that the value of the activation energy of the conductivity for the pressboard impregnated with mineral oil of natural origin NYNAS NYTRO BIO 300X is ΔE = 1.117 eV.

The Generation and Migration of Bubbles in Oil-Pressboard Insulation Needle-Plate System

Abstract: Bubbles in transformer oil can easily lead to partial discharge (PD), which can deteriorate the transformer oil and even breakdown the transformer insulation. To clarify the migration process and the characteristics of bubbles generated in an oil-immersed power transformer exposed to an extremely uneven electric field, we experimentally monitor these phenomena under an extremely nonuniform ac electric field and numerically simulate the migration distance and the migration speed of bubbles with different initial positions and sizes. The results show that the streamer discharge channel formed by a PD in oil is gasified into a bubble channel. After it collides with the surface of the pressboard, its morphology is transformed into approximately spherical bubbles due to the surface tension of the gas–liquid interface. After bubbles are generated in the oil, they move away from areas with a strong electric field due to the electric-field force and gradually approach the oil surface due to the buoyancy force. The experimental results are consistent with the simulation results, which verify the rationality of the simulation model.

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

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

Identification of Sulphide Content in Pressboard Insulation in Transformer Adopting LIBS and Wavelet Technique

Abstract: In transformers, silver sulfide and copper sulfide can form and cause a significant threat to its life. Identification of sulfur content formation in transformer insulation and diagnosing the level of its migration in pressboard material is a major challenge. In the present study, methodical experimental studies were carried out for sulfide formation and for its diffusion in pressboard material. The level of diffusion of sulfur content in the pressboard was diagnosed by adopting laser-induced breakdown spectroscopy (LIBS). The wavelet technique was adopted to enhance the LIBS signal thereby enhancing the efficiency of the technique for identification of the level of diffusion of sulfide contents.

PD characteristics and defect type recognition of “rapidly-developing” fault in oil-pressboard insulation

Abstract: With the high voltage grade of UHV transformers and wide area of high field intensity, the discharge phenomenon that the average apparent discharge and the number of discharge pulses of per second increase rapidly often occurs within the transformer oil-pressboard insulation. In order to effectively analyze the discharge characteristics of this type of discharge faults and enhance the recognition technology of discharge types, three kinds of simulated defects were prepared and a small simulation experimental platform was built in this paper. Based on the power frequency data of PD, PRPD spectrum was drawn to analyze the discharge characteristics of “rapidly-developing” faults. According to improved D-S evidence theory, the features information between high frequency pulse and UHF pulse are fused to achieve the high accuracy recognition of defect types.

Distribution laws of partial discharge and surface flashover of insulation cushion blocks for transformers

Abstract: With the rapid development of HVDC, the converter transformer, which is a key component of HVDC, often has insulation failures. The oil-pressboard insulation structure is the main insulation of the converter transformer, and the insulation performance of the insulation cushion block is a key parameter required in the insulation design. Therefore, it is necessary to study the discharge characteristics of insulation cushion block. In this paper, the partial discharge initiation characteristics and surface flashover characteristics of three kinds of insulation cushion blocks produced by three manufacturers under AC voltage were tested. The test results show that under the parallel electric field component, the initial field strength of partial discharge of rectangular and T-shaped single slot cushion blocks is relatively close, both of which are about 1.7kV/mm. The minimum initial field strength of the rectangular T-shaped double slot cushion blocks is 1.3kv/mm, and the maximum difference between the three manufacturers is 21.2%. Under the vertical electric field component, the starting characteristic trend of partial discharge is the same as that under the parallel component, in which the rectangular and T-shaped single slot cushion blocks are between 4.63kV/mm and 5.54kV/mm. The rectangular T-shaped double slot cushion block has an average of 4.16kV/mm, which is still the lowest, with a maximum difference of 20.7%. For the surface flashover characteristics, the creepage flashover field strengths of the three kinds of cushion blocks are not significantly different, which are 2.3kV/mm, 2.32kV/mm and 2.19kV/m respectively.

Electrical conductivity of pressboard and the influence of moisture content

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

Research on the Statistical Characteristics of the Discharge Inception Electric Field Strength of Gap Defect in Insulation pressboard Based on Three-parameter Weibull Model

Abstract: Partial discharge (PD) has been paid much attention as an essential factor of insulation deterioration of various large equipment in power system. Partial discharges are prone to occur when there are air gaps in insulation pressboard. Based on different sizes of air-gap of air-gap discharge models, this paper proposes a three-parameter Weibull distribution to describe the probability distribution of partial discharge onset field strength in the air-gap model. The results show that the internal air-gap discharge of the pressboard has an obvious size effect. When the air gap diameter increases, the position parameter E 0 and the scale parameter a decrease due to increased inhomogeneity and area effect, while the initial voltage data concentration increases and shape parameter $\beta$ increases. When the separation of the gap defect inside the insulation pressboard is small, the separation of the gap defect is not the main factor affecting the partial discharge. The cross-sectional area of the air gap in the direction perpendicular to the electric field largely determines the initial field strength of the air gap.

An Investigation of the Electrical Properties of Pressboard Impregnated with Mineral Oil-Based Nanofluids at Different Concentrations of Fe3O4 Magnetic Nanoparticles

Abstract: The purpose of this study was to investigate the electrical properties of pressboard impregnated with mineral oil-based nanomaterial. The nanomaterial focused on in this study was magnetite (Fe3O4) at ratios of 0.01%, 0.03%, and 0.05% by mineral oil volume. The electrical tests were performed on the AC breakdown strengths and positive-negative lightning impulse breakdown strengths on impregnated pressboard were carried out in compliance with IEC 60641 and IEC 60243, respectively. Scanning electron microscopy (SEM) and X-ray Diffraction (X-RD) were used to study the micro surface and show that Fe3O4 nanoparticles of nanoscale size were adhered to the pressboard surface and exist in an amorphous state. The results found that the AC breakdown strengths of pressboard impregnated with mineral oil-based Fe3O4 nanofluids at 0.03 wt% were increased the most. Moreover, the lightning impulse breakdown strengths of pressboard impregnated with mineral oil-based Fe3O4 nanofluids at 0.03 wt% were increased the most in both positive and negative polarities. The results, thus, showed promising directions for applications of Fe3O4 nanomaterials to improve the electrical properties of pressboard.

Identification Of Partial Discharge Sources In Oil-Pressboard Insulation With Single Type Defect Through TF Mapping

Abstract: One of the major insulation degradation processes that enhances the risk of equipment failure is partial discharges (PD). The severity of the damage caused by PD will depend on the type of PD source present in the insulation system, ie. Corona discharges, voids, surface discharges, etc. Therefore, early detection and identification of PD sources are very useful for the proper assessment of insulation health. The insulation material investigated in the present case is an oil-impregnated pressboard. Three types of PD sources, void, corona, and surface discharges, were identified from PD signals measured using a high-frequency current transformer (HFCT). It was observed that Time-Frequency mapping is an efficient technique to discriminate corona, surface discharges, and void discharges when they are present standalone in the insulation.

Numerical and experimental evaluation of dielectric properties of thermally aged insulating paper used in power transformers

Abstract: Due to the relevant role of the cellulosic insulation in the dielectric system of a power transformer, it is necessary to know its dielectric properties and its evolution as insulating material. The dependence of the dielectric properties of solid insulation on the moisture content, temperature, or pressure is well-defined. However, it is not clear the effect of the paper degradation on its insulating capacity. To study the impact of ageing on the dielectric properties, samples of Kraft paper were subjected to an accelerated hygrothermal ageing. Degree of polymerization (DP) value was reduced from 1000 to 200, with intermediate study points. All samples were conditioned to the same moisture content in order to only evaluate the effect of the paper degradation. Different dielectric properties, such as loss factor $(\tan\delta)$ and complex permittivity ($\varepsilon$) were measured using the dielectric spectroscopy technique. These experimental results were used to develop a numerical model with the finite element-based tool Comsol Multiphysics. This model provided some extra information about the material properties, such as the electric field distribution. Results showed that the degradation of Kraft paper modifies its dielectric response.

FDS Response of Insulating Papers Impregnated with Various Fluids

Abstract: A preliminary investigation on the dielectric response at different frequencies of the solid insulating system of transformers in mineral oil and natural ester is presented. FDS measurements have been performed on new scale model winding specimens with vapor-phase dried solid insulation when impregnated in three different insulating oils: an uninhibited naphthenic mineral oil and two typical natural esters for transformers. Two different types of insulating paper (standard Kraft paper and a Thermally Upgraded Kraft paper) were used as solid material. Comparative FDS results indicate that the dielectric response curves of Kraft and TUK paper-specimens overlap, when impregnated in with the same type of insulating oil. This confirms that new papers show the same dielectric behavior when the same drying pre-treatment is applied. Instead, for the same solid insulation in different insulating oil, the greater affinity for water of esters with respect to mineral oil heavily influences the dielectric dissipation loss at low frequency.